Dave Jones got the following lockdep splat:

>  ======================================================
>  [ INFO: possible circular locking dependency detected ]
>  3.12.0-rc3+ #92 Not tainted
>  -------------------------------------------------------
>  trinity-child2/15191 is trying to acquire lock:
>   (&rdp->nocb_wq){......}, at: [<ffffffff8108ff43>] __wake_up+0x23/0x50
>
> but task is already holding lock:
>   (&ctx->lock){-.-...}, at: [<ffffffff81154c19>] perf_event_exit_task+0x109/0x230
>
> which lock already depends on the new lock.
>
>
> the existing dependency chain (in reverse order) is:
>
> -> #3 (&ctx->lock){-.-...}:
>         [<ffffffff810cc243>] lock_acquire+0x93/0x200
>         [<ffffffff81733f90>] _raw_spin_lock+0x40/0x80
>         [<ffffffff811500ff>] __perf_event_task_sched_out+0x2df/0x5e0
>         [<ffffffff81091b83>] perf_event_task_sched_out+0x93/0xa0
>         [<ffffffff81732052>] __schedule+0x1d2/0xa20
>         [<ffffffff81732f30>] preempt_schedule_irq+0x50/0xb0
>         [<ffffffff817352b6>] retint_kernel+0x26/0x30
>         [<ffffffff813eed04>] tty_flip_buffer_push+0x34/0x50
>         [<ffffffff813f0504>] pty_write+0x54/0x60
>         [<ffffffff813e900d>] n_tty_write+0x32d/0x4e0
>         [<ffffffff813e5838>] tty_write+0x158/0x2d0
>         [<ffffffff811c4850>] vfs_write+0xc0/0x1f0
>         [<ffffffff811c52cc>] SyS_write+0x4c/0xa0
>         [<ffffffff8173d4e4>] tracesys+0xdd/0xe2
>
> -> #2 (&rq->lock){-.-.-.}:
>         [<ffffffff810cc243>] lock_acquire+0x93/0x200
>         [<ffffffff81733f90>] _raw_spin_lock+0x40/0x80
>         [<ffffffff810980b2>] wake_up_new_task+0xc2/0x2e0
>         [<ffffffff81054336>] do_fork+0x126/0x460
>         [<ffffffff81054696>] kernel_thread+0x26/0x30
>         [<ffffffff8171ff93>] rest_init+0x23/0x140
>         [<ffffffff81ee1e4b>] start_kernel+0x3f6/0x403
>         [<ffffffff81ee1571>] x86_64_start_reservations+0x2a/0x2c
>         [<ffffffff81ee1664>] x86_64_start_kernel+0xf1/0xf4
>
> -> #1 (&p->pi_lock){-.-.-.}:
>         [<ffffffff810cc243>] lock_acquire+0x93/0x200
>         [<ffffffff8173419b>] _raw_spin_lock_irqsave+0x4b/0x90
>         [<ffffffff810979d1>] try_to_wake_up+0x31/0x350
>         [<ffffffff81097d62>] default_wake_function+0x12/0x20
>         [<ffffffff81084af8>] autoremove_wake_function+0x18/0x40
>         [<ffffffff8108ea38>] __wake_up_common+0x58/0x90
>         [<ffffffff8108ff59>] __wake_up+0x39/0x50
>         [<ffffffff8110d4f8>] __call_rcu_nocb_enqueue+0xa8/0xc0
>         [<ffffffff81111450>] __call_rcu+0x140/0x820
>         [<ffffffff81111b8d>] call_rcu+0x1d/0x20
>         [<ffffffff81093697>] cpu_attach_domain+0x287/0x360
>         [<ffffffff81099d7e>] build_sched_domains+0xe5e/0x10a0
>         [<ffffffff81efa7fc>] sched_init_smp+0x3b7/0x47a
>         [<ffffffff81ee1f4e>] kernel_init_freeable+0xf6/0x202
>         [<ffffffff817200be>] kernel_init+0xe/0x190
>         [<ffffffff8173d22c>] ret_from_fork+0x7c/0xb0
>
> -> #0 (&rdp->nocb_wq){......}:
>         [<ffffffff810cb7ca>] __lock_acquire+0x191a/0x1be0
>         [<ffffffff810cc243>] lock_acquire+0x93/0x200
>         [<ffffffff8173419b>] _raw_spin_lock_irqsave+0x4b/0x90
>         [<ffffffff8108ff43>] __wake_up+0x23/0x50
>         [<ffffffff8110d4f8>] __call_rcu_nocb_enqueue+0xa8/0xc0
>         [<ffffffff81111450>] __call_rcu+0x140/0x820
>         [<ffffffff81111bb0>] kfree_call_rcu+0x20/0x30
>         [<ffffffff81149abf>] put_ctx+0x4f/0x70
>         [<ffffffff81154c3e>] perf_event_exit_task+0x12e/0x230
>         [<ffffffff81056b8d>] do_exit+0x30d/0xcc0
>         [<ffffffff8105893c>] do_group_exit+0x4c/0xc0
>         [<ffffffff810589c4>] SyS_exit_group+0x14/0x20
>         [<ffffffff8173d4e4>] tracesys+0xdd/0xe2
>
> other info that might help us debug this:
>
> Chain exists of:
>   &rdp->nocb_wq --> &rq->lock --> &ctx->lock
>
>   Possible unsafe locking scenario:
>
>         CPU0                    CPU1
>         ----                    ----
>    lock(&ctx->lock);
>                                 lock(&rq->lock);
>                                 lock(&ctx->lock);
>    lock(&rdp->nocb_wq);
>
>  *** DEADLOCK ***
>
> 1 lock held by trinity-child2/15191:
>  #0:  (&ctx->lock){-.-...}, at: [<ffffffff81154c19>] perf_event_exit_task+0x109/0x230
>
> stack backtrace:
> CPU: 2 PID: 15191 Comm: trinity-child2 Not tainted 3.12.0-rc3+ #92
>  ffffffff82565b70 ffff880070c2dbf8 ffffffff8172a363 ffffffff824edf40
>  ffff880070c2dc38 ffffffff81726741 ffff880070c2dc90 ffff88022383b1c0
>  ffff88022383aac0 0000000000000000 ffff88022383b188 ffff88022383b1c0
> Call Trace:
>  [<ffffffff8172a363>] dump_stack+0x4e/0x82
>  [<ffffffff81726741>] print_circular_bug+0x200/0x20f
>  [<ffffffff810cb7ca>] __lock_acquire+0x191a/0x1be0
>  [<ffffffff810c6439>] ? get_lock_stats+0x19/0x60
>  [<ffffffff8100b2f4>] ? native_sched_clock+0x24/0x80
>  [<ffffffff810cc243>] lock_acquire+0x93/0x200
>  [<ffffffff8108ff43>] ? __wake_up+0x23/0x50
>  [<ffffffff8173419b>] _raw_spin_lock_irqsave+0x4b/0x90
>  [<ffffffff8108ff43>] ? __wake_up+0x23/0x50
>  [<ffffffff8108ff43>] __wake_up+0x23/0x50
>  [<ffffffff8110d4f8>] __call_rcu_nocb_enqueue+0xa8/0xc0
>  [<ffffffff81111450>] __call_rcu+0x140/0x820
>  [<ffffffff8109bc8f>] ? local_clock+0x3f/0x50
>  [<ffffffff81111bb0>] kfree_call_rcu+0x20/0x30
>  [<ffffffff81149abf>] put_ctx+0x4f/0x70
>  [<ffffffff81154c3e>] perf_event_exit_task+0x12e/0x230
>  [<ffffffff81056b8d>] do_exit+0x30d/0xcc0
>  [<ffffffff810c9af5>] ? trace_hardirqs_on_caller+0x115/0x1e0
>  [<ffffffff810c9bcd>] ? trace_hardirqs_on+0xd/0x10
>  [<ffffffff8105893c>] do_group_exit+0x4c/0xc0
>  [<ffffffff810589c4>] SyS_exit_group+0x14/0x20
>  [<ffffffff8173d4e4>] tracesys+0xdd/0xe2

The underlying problem is that perf is invoking call_rcu() with the
scheduler locks held, but in NOCB mode, call_rcu() will with high
probability invoke the scheduler -- which just might want to use its
locks.  The reason that call_rcu() needs to invoke the scheduler is
to wake up the corresponding rcuo callback-offload kthread, which
does the job of starting up a grace period and invoking the callbacks
afterwards.

One solution (championed on a related problem by Lai Jiangshan) is to
simply defer the wakeup to some point where scheduler locks are no longer
held.  Since we don't want to unnecessarily incur the cost of such
deferral, the task before us is threefold:

1.	Determine when it is likely that a relevant scheduler lock is held.

2.	Defer the wakeup in such cases.

3.	Ensure that all deferred wakeups eventually happen, preferably
	sooner rather than later.

We use irqs_disabled_flags() as a proxy for relevant scheduler locks
being held.  This works because the relevant locks are always acquired
with interrupts disabled.  We may defer more often than needed, but that
is at least safe.

The wakeup deferral is tracked via a new field in the per-CPU and
per-RCU-flavor rcu_data structure, namely ->nocb_defer_wakeup.

This flag is checked by the RCU core processing.  The __rcu_pending()
function now checks this flag, which causes rcu_check_callbacks()
to initiate RCU core processing at each scheduling-clock interrupt
where this flag is set.  Of course this is not sufficient because
scheduling-clock interrupts are often turned off (the things we used to
be able to count on!).  So the flags are also checked on entry to any
state that RCU considers to be idle, which includes both NO_HZ_IDLE idle
state and NO_HZ_FULL user-mode-execution state.

This approach should allow call_rcu() to be invoked regardless of what
locks you might be holding, the key word being "should".
Reported-by: NDave Jones <davej@redhat.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>

It is all too easy to forget that wait_event() does not necessarily
imply a full memory barrier.  The case where it does not is where the
condition transitions to true just as wait_event() starts execution.
This is actually a feature: The standard use of wait_event() involves
locking, in which case the locks provide the needed ordering (you hold a
lock across the wake_up() and acquire that same lock after wait_event()
returns).

Given that I did forget that wait_event() does not necessarily imply a
full memory barrier in one case, this commit fixes that case.  This commit
also adds comments calling out the placement of existing memory barriers
relied on by wait_event() calls.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

When an RCU CPU stall warning occurs, the CPU invokes resched_cpu() on
itself.  This can help move the grace period forward in some situations,
but it would be even better to do this -before- the RCU CPU stall warning.
This commit therefore causes resched_cpu() to be called every five jiffies
once the system is halfway to an RCU CPU stall warning.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NIngo Molnar <mingo@kernel.org>

The old rcu_is_cpu_idle() function is just __rcu_is_watching() with
preemption disabled.  This commit therefore renames rcu_is_cpu_idle()
to rcu_is_watching.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Commit e6b80a3b (rcu: Detect illegal rcu dereference in extended
quiescent state) exported the pre-existing rcu_is_cpu_idle() function
using EXPORT_SYMBOL().  However, this is inconsistent with the remaining
exports from RCU, which are all EXPORT_SYMBOL_GPL().  The current state
of affairs means that a non-GPL module could use rcu_is_cpu_idle(),
but in a CONFIG_TREE_PREEMPT_RCU=y kernel would be unable to invoke
rcu_read_lock() and rcu_read_unlock().

This commit therefore makes rcu_is_cpu_idle()'s export be consistent
with the rest of RCU, namely EXPORT_SYMBOL_GPL().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

There is currently no way for kernel code to determine whether it
is safe to enter an RCU read-side critical section, in other words,
whether or not RCU is paying attention to the currently running CPU.
Given the large and increasing quantity of code shared by the idle loop
and non-idle code, the this shortcoming is becoming increasingly painful.

This commit therefore adds __rcu_is_watching(), which returns true if
it is safe to enter an RCU read-side critical section on the currently
running CPU.  This function is quite fast, using only a __this_cpu_read().
However, the caller must disable preemption.
Reported-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

We're going to deprecate and remove set_need_resched() for it will do
the wrong thing. Make an exception for RCU and allow it to use
resched_cpu() which will do the right thing.
Signed-off-by: NPeter Zijlstra <peterz@infradead.org>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Link: http://lkml.kernel.org/n/tip-2eywnacjl1nllctl1nszqa5w@git.kernel.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>

This commit adds tracing to the normal grace-period request points.
These are rcu_gp_cleanup(), which checks for the need for another
grace period at the end of the previous grace period, and
rcu_start_gp_advanced(), which restarts RCU's state machine after
an idle period.  These trace events are intended to help track down
bugs where RCU remains idle despite there being work for it to do.
Reported-by: NClark Williams <williams@redhat.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit adds tracing to the rcu_gp_kthread() function in order to
help trace down hangs potentially involving this kthread.
Reported-by: NClark Williams <williams@redhat.com>
Reported-by: NCarsten Emde <C.Emde@osadl.org>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit applies ACCESS_ONCE() to an outside-of-lock access to
->gp_flags.  Although it is hard to imagine any sane compiler messing
this particular case up, the documentation benefits are substantial.
Plus the definition of "sane compiler" grows ever looser.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Spurious wakeups in the force-quiescent-state loop in rcu_gp_kthread()
cause the timeout to be recalculated, which would prevent rcu_gp_fqs()
from ever being called.  This would in turn would prevent the grace period
from ever ending for as long as there was at least one CPU in an extended
quiescent state that had not yet passed through a quiescent state.

This commit therefore avoids recalculating the timeout unless the
previous pass's call to wait_event_interruptible_timeout() actually
did time out, thus preventing the above scenario.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit improves grace-period start logic by checking ->gp_flags
under the lock and by issuing a warning if a grace period is already
in progress.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

This commit extends the work done in f7f7bac9 (rcu: Have the RCU
tracepoints use the tracepoint_string infrastructure) to cover rcutiny.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>

If a system is idle from an RCU perspective for longer than specified
by CONFIG_RCU_CPU_STALL_TIMEOUT, and if one CPU starts a grace period
just as a second checks for CPU stalls, and if this second CPU happens
to see the old value of rsp->jiffies_stall, it will incorrectly report a
CPU stall.  This is quite rare, but apparently occurs deterministically
on systems with about 6TB of memory.

This commit therefore orders accesses to the data used to determine
whether or not a CPU stall is in progress.  Grace-period initialization
and cleanup first increments rsp->completed to mark the end of the
previous grace period, then records the current jiffies in rsp->gp_start,
then records the jiffies at which a stall can be expected to occur in
rsp->jiffies_stall, and finally increments rsp->gpnum to mark the start
of the new grace period.  Now, this ordering by itself does not prevent
false positives.  For example, if grace-period initialization was delayed
between recording rsp->gp_start and rsp->jiffies_stall, the CPU stall
warning code might still see an old value of rsp->jiffies_stall.

Therefore, this commit also orders the CPU stall warning accesses as
well, loading rsp->gpnum and jiffies, then rsp->jiffies_stall, then
rsp->gp_start, and finally rsp->completed.  This ordering means that
the false-positive scenario in the previous paragraph would result
in rsp->completed being greater than or equal to rsp->gpnum, which is
never valid for a CPU stall, allowing the false positive to be rejected.
Furthermore, any fetch that gets an old value of rsp->jiffies_stall
must also get an old value of rsp->gpnum, which will again be rejected
by the comparison of rsp->gpnum and rsp->completed.  Situations where
rsp->gp_start is later than rsp->jiffies_stall are also rejected, as
are situations where jiffies is less than rsp->jiffies_stall.

Although use of unsynchronized accesses means that there are likely
still some false-positive scenarios (synchronization has proven to be
a very bad idea on large systems), this should get rid of a large class
of these scenarios.
Reported-by: NFabian Herschel <fabian.herschel@suse.com>
Reported-by: NMichal Hocko <mhocko@suse.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NMichal Hocko <mhocko@suse.cz>
Tested-by: NJochen Striepe <jochen@tolot.escape.de>

The for_each_rcu_flavor() loop unconditionally scans all flavors, even
when the first flavor might have some non-lazy callbacks.  Once the
loop has seen a non-lazy callback, further passes through the loop
cannot change the state.  This is not a huge problem, given that there
can be at most three RCU flavors (RCU-bh, RCU-preempt, and RCU-sched),
but this code is on the path to idle, so speeding it up even a small
amount would have some benefit.

This commit therefore does two things:

1.	Rearranges the order of the list of RCU flavors in order to
	place the most active flavor first in the list.  The most active
	RCU flavor is RCU-preempt, or, if there is no RCU-preempt,
	RCU-sched.

2.	Reworks the for_each_rcu_flavor() to exit early when the first
	non-lazy callback is seen, or, in the case where the caller
	does not care about non-lazy callbacks (RCU_FAST_NO_HZ=n),
	when the first callback is seen.
Reported-by: NChen Gang <gang.chen@asianux.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The "idle" variable in both rcu_eqs_enter_common() and
rcu_eqs_exit_common() is only used in a WARN_ON_ONCE().  If the kernel
is built disabling WARN_ON_ONCE(), the compiler will complain (rightly)
that "idle" is unused.  This commit therefore adds a __maybe_unused to
the declaration of both variables.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

__get_cpu_var() is used for multiple purposes in the kernel source. One
of them is address calculation via the form &__get_cpu_var(x). This
calculates the address for the instance of the percpu variable of the
current processor based on an offset.

Other use cases are for storing and retrieving data from the current
processors percpu area.  __get_cpu_var() can be used as an lvalue when
writing data or on the right side of an assignment.

__get_cpu_var() is defined as :

__get_cpu_var() always only does an address determination. However,
store and retrieve operations could use a segment prefix (or global
register on other platforms) to avoid the address calculation.

this_cpu_write() and this_cpu_read() can directly take an offset into
a percpu area and use optimized assembly code to read and write per
cpu variables.

This patch converts __get_cpu_var into either an explicit address
calculation using this_cpu_ptr() or into a use of this_cpu operations
that use the offset. Thereby address calcualtions are avoided and less
registers are used when code is generated.

At the end of the patchset all uses of __get_cpu_var have been removed
so the macro is removed too.

The patchset includes passes over all arches as well. Once these
operations are used throughout then specialized macros can be defined in
non -x86 arches as well in order to optimize per cpu access by f.e. using
a global register that may be set to the per cpu base.

Transformations done to __get_cpu_var()

1. Determine the address of the percpu instance of the current processor.

	DEFINE_PER_CPU(int, y);
	int *x = &__get_cpu_var(y);

    Converts to

	int *x = this_cpu_ptr(&y);

2. Same as #1 but this time an array structure is involved.

	DEFINE_PER_CPU(int, y[20]);
	int *x = __get_cpu_var(y);

    Converts to

	int *x = this_cpu_ptr(y);

3. Retrieve the content of the current processors instance of a per cpu
   variable.

	DEFINE_PER_CPU(int, u);
	int x = __get_cpu_var(y)

   Converts to

	int x = __this_cpu_read(y);

4. Retrieve the content of a percpu struct

	DEFINE_PER_CPU(struct mystruct, y);
	struct mystruct x = __get_cpu_var(y);

   Converts to

	memcpy(this_cpu_ptr(&x), y, sizeof(x));

5. Assignment to a per cpu variable

	DEFINE_PER_CPU(int, y)
	__get_cpu_var(y) = x;

   Converts to

	this_cpu_write(y, x);

6. Increment/Decrement etc of a per cpu variable

	DEFINE_PER_CPU(int, y);
	__get_cpu_var(y)++

   Converts to

	this_cpu_inc(y)
Signed-off-by: NChristoph Lameter <cl@linux.com>
[ paulmck: Address conflicts. ]
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The rcu_cpu_stall_timeout kernel parameter, the rcu_dynticks per-CPU
variable, and the rcu_gp_fqs() function are used only locally.  This
commit therefore marks them as static.
Reported-by: Nkbuild test robot <fengguang.wu@intel.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

One of the ->gp_flags assignments used a raw number rather than the
cpp macro that was intended for this purpose, which this commit fixes.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

Because RCU's quiescent-state-forcing mechanism is used to drive the
full-system-idle state machine, and because this mechanism is executed
by RCU's grace-period kthreads, this commit forces these kthreads to
run on the timekeeping CPU (tick_do_timer_cpu).  To do otherwise would
mean that the RCU grace-period kthreads would force the system into
non-idle state every time they drove the state machine, which would
be just a bit on the futile side.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit adds the state machine that takes the per-CPU idle data
as input and produces a full-system-idle indication as output.  This
state machine is driven out of RCU's quiescent-state-forcing
mechanism, which invokes rcu_sysidle_check_cpu() to collect per-CPU
idle state and then rcu_sysidle_report() to drive the state machine.

The full-system-idle state is sampled using rcu_sys_is_idle(), which
also drives the state machine if RCU is idle (and does so by forcing
RCU to become non-idle).  This function returns true if all but the
timekeeping CPU (tick_do_timer_cpu) are idle and have been idle long
enough to avoid memory contention on the full_sysidle_state state
variable.  The rcu_sysidle_force_exit() may be called externally
to reset the state machine back into non-idle state.

For large systems the state machine is driven out of RCU's
force-quiescent-state logic, which provides good scalability at the price
of millisecond-scale latencies on the transition to full-system-idle
state.  This is not so good for battery-powered systems, which are usually
small enough that they don't need to care about scalability, but which
do care deeply about energy efficiency.  Small systems therefore drive
the state machine directly out of the idle-entry code.  The number of
CPUs in a "small" system is defined by a new NO_HZ_FULL_SYSIDLE_SMALL
Kconfig parameter, which defaults to 8.  Note that this is a build-time
definition.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
[ paulmck: Use true and false for boolean constants per Lai Jiangshan. ]
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
[ paulmck: Simplify logic and provide better comments for memory barriers,
  based on review comments and questions by Lai Jiangshan. ]

This commit drops an unneeded ACCESS_ONCE() and simplifies an "our work
is done" check in _rcu_barrier().  This applies feedback from Linus
(https://lkml.org/lkml/2013/7/26/777) that he gave to similar code
in an unrelated patch.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
[ paulmck: Fix comment to match code, reported by Lai Jiangshan. ]

This commit adds an isidle and jiffies argument to force_qs_rnp(),
dyntick_save_progress_counter(), and rcu_implicit_dynticks_qs() to enable
RCU's force-quiescent-state process to check for full-system idle.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
[ paulmck: Use true and false for boolean constants per Lai Jiangshan. ]
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit adds the code that updates the rcu_dyntick structure's
new fields to track the per-CPU idle state based on interrupts and
transitions into and out of the idle loop (NMIs are ignored because NMI
handlers cannot cleanly read out the time anyway).  This code is similar
to the code that maintains RCU's idea of per-CPU idleness, but differs
in that RCU treats CPUs running in user mode as idle, where this new
code does not.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit adds fields to the rcu_dyntick structure that are used to
detect idle CPUs.  These new fields differ from the existing ones in
that the existing ones consider a CPU executing in user mode to be idle,
where the new ones consider CPUs executing in user mode to be busy.
The handling of these new fields is otherwise quite similar to that for
the exiting fields.  This commit also adds the initialization required
for these fields.

So, why is usermode execution treated differently, with RCU considering
it a quiescent state equivalent to idle, while in contrast the new
full-system idle state detection considers usermode execution to be
non-idle?

It turns out that although one of RCU's quiescent states is usermode
execution, it is not a full-system idle state.  This is because the
purpose of the full-system idle state is not RCU, but rather determining
when accurate timekeeping can safely be disabled.  Whenever accurate
timekeeping is required in a CONFIG_NO_HZ_FULL kernel, at least one
CPU must keep the scheduling-clock tick going.  If even one CPU is
executing in user mode, accurate timekeeping is requires, particularly for
architectures where gettimeofday() and friends do not enter the kernel.
Only when all CPUs are really and truly idle can accurate timekeeping be
disabled, allowing all CPUs to turn off the scheduling clock interrupt,
thus greatly improving energy efficiency.

This naturally raises the question "Why is this code in RCU rather than in
timekeeping?", and the answer is that RCU has the data and infrastructure
to efficiently make this determination.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: NFrederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

The rcu_user_enter_after_irq() and rcu_user_exit_after_irq()
functions were intended for use by adaptive ticks, but changes
in implementation have rendered them unnecessary.  This commit
therefore removes them.
Reported-by: NFrederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

When setting up an in-the-future "advanced" grace period, the code needs
to wake up the relevant grace-period kthread, which it currently does
unconditionally.  However, this results in needless wakeups in the case
where the advanced grace period is being set up by the grace-period
kthread itself, which is a non-uncommon situation.  This commit therefore
checks to see if the running thread is the grace-period kthread, and
avoids doing the irq_work_queue()-mediated wakeup in that case.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

If someone does a duplicate call_rcu(), the worst thing the second
call_rcu() could do would be to actually queue the callback the second
time because doing so corrupts whatever list the callback was already
queued on.  This commit therefore makes __call_rcu() check the new
return value from debug-objects and leak the callback upon error.
This commit also substitutes rcu_leak_callback() for whatever callback
function was previously in place in order to avoid freeing the callback
out from under any readers that might still be referencing it.

These changes increase the probability that the debug-objects error
messages will actually make it somewhere visible.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Davidlohr Bueso <davidlohr.bueso@hp.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Tested-by: NSedat Dilek <sedat.dilek@gmail.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

CONFIG_RCU_FAST_NO_HZ can increase grace-period durations by up to
a factor of four, which can result in long suspend and resume times.
Thus, this commit temporarily switches to expedited grace periods when
suspending the box and return to normal settings when resuming.  Similar
logic is applied to hibernation.

Because expedited grace periods are of dubious benefit on very large
systems, so this commit restricts their automated use during suspend
and resume to systems of 256 or fewer CPUs.  (Some day a number of
Linux-kernel facilities, including RCU's expedited grace periods,
will be more scalable, but I need to see bug reports first.)

[ paulmck: This also papers over an audio/irq bug, but hopefully that will
  be fixed soon. ]
Signed-off-by: NBorislav Petkov <bp@suse.de>
Signed-off-by: NBjørn Mork <bjorn@mork.no>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Currently, RCU tracepoints save only a pointer to strings in the
ring buffer. When displayed via the /sys/kernel/debug/tracing/trace file
they are referenced like the printf "%s" that looks at the address
in the ring buffer and prints out the string it points too. This requires
that the strings are constant and persistent in the kernel.

The problem with this is for tools like trace-cmd and perf that read the
binary data from the buffers but have no access to the kernel memory to
find out what string is represented by the address in the buffer.

By using the tracepoint_string infrastructure, the RCU tracepoint strings
can be exported such that userspace tools can map the addresses to
the strings.

 # cat /sys/kernel/debug/tracing/printk_formats
0xffffffff81a4a0e8 : "rcu_preempt"
0xffffffff81a4a0f4 : "rcu_bh"
0xffffffff81a4a100 : "rcu_sched"
0xffffffff818437a0 : "cpuqs"
0xffffffff818437a6 : "rcu_sched"
0xffffffff818437a0 : "cpuqs"
0xffffffff818437b0 : "rcu_bh"
0xffffffff818437b7 : "Start context switch"
0xffffffff818437cc : "End context switch"
0xffffffff818437a0 : "cpuqs"
[...]

Now userspaces tools can display:

 rcu_utilization:      Start context switch
 rcu_dyntick:          Start 1 0
 rcu_utilization:      End context switch
 rcu_batch_start:      rcu_preempt CBs=0/5 bl=10
 rcu_dyntick:          End 0 140000000000000
 rcu_invoke_callback:  rcu_preempt rhp=0xffff880071c0d600 func=proc_i_callback
 rcu_invoke_callback:  rcu_preempt rhp=0xffff880077b5b230 func=__d_free
 rcu_dyntick:          Start 140000000000000 0
 rcu_invoke_callback:  rcu_preempt rhp=0xffff880077563980 func=file_free_rcu
 rcu_batch_end:        rcu_preempt CBs-invoked=3 idle=>c<>c<>c<>c<
 rcu_utilization:      End RCU core
 rcu_grace_period:     rcu_preempt 9741 start
 rcu_dyntick:          Start 1 0
 rcu_dyntick:          End 0 140000000000000
 rcu_dyntick:          Start 140000000000000 0

Instead of:

 rcu_utilization:      ffffffff81843110
 rcu_future_grace_period: ffffffff81842f1d 9939 9939 9940 0 0 3 ffffffff81842f32
 rcu_batch_start:      ffffffff81842f1d CBs=0/4 bl=10
 rcu_future_grace_period: ffffffff81842f1d 9939 9939 9940 0 0 3 ffffffff81842f3c
 rcu_grace_period:     ffffffff81842f1d 9939 ffffffff81842f80
 rcu_invoke_callback:  ffffffff81842f1d rhp=0xffff88007888aac0 func=file_free_rcu
 rcu_grace_period:     ffffffff81842f1d 9939 ffffffff81842f95
 rcu_invoke_callback:  ffffffff81842f1d rhp=0xffff88006aeb4600 func=proc_i_callback
 rcu_future_grace_period: ffffffff81842f1d 9939 9939 9940 0 0 3 ffffffff81842f32
 rcu_future_grace_period: ffffffff81842f1d 9939 9939 9940 0 0 3 ffffffff81842f3c
 rcu_invoke_callback:  ffffffff81842f1d rhp=0xffff880071cb9fc0 func=__d_free
 rcu_grace_period:     ffffffff81842f1d 9939 ffffffff81842f80
 rcu_invoke_callback:  ffffffff81842f1d rhp=0xffff88007888ae80 func=file_free_rcu
 rcu_batch_end:        ffffffff81842f1d CBs-invoked=4 idle=>c<>c<>c<>c<
 rcu_utilization:      ffffffff8184311f
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The RCU_STATE_INITIALIZER() macro is used only in the rcutree.c file
as well as the rcutree_plugin.h file. It is passed as a rvalue to
a variable of a similar name. A per_cpu variable is also created
with a similar name as well.

The uses of RCU_STATE_INITIALIZER() can be simplified to remove some
of the duplicate code that is done. Currently the three users of this
macro has this format:

struct rcu_state rcu_sched_state =
	RCU_STATE_INITIALIZER(rcu_sched, call_rcu_sched);
DEFINE_PER_CPU(struct rcu_data, rcu_sched_data);

Notice that "rcu_sched" is called three times. This is the same with
the other two users. This can be condensed to just:

RCU_STATE_INITIALIZER(rcu_sched, call_rcu_sched);

by moving the rest into the macro itself.

This also opens the door to allow the RCU tracepoint strings and
their addresses to be exported so that userspace tracing tools can
translate the contents of the pointers of the RCU tracepoints.
The change will allow for helper code to be placed in the
RCU_STATE_INITIALIZER() macro to export the name that is used.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

All the RCU tracepoints and functions that reference char pointers do
so with just 'char *' even though they do not modify the contents of
the string itself. This will cause warnings if a const char * is used
in one of these functions.

The RCU tracepoints store the pointer to the string to refer back to them
when the trace output is displayed. As this can be minutes, hours or
even days later, those strings had better be constant.

This change also opens the door to allow the RCU tracepoint strings and
their addresses to be exported so that userspace tracing tools can
translate the contents of the pointers of the RCU tracepoints.
Signed-off-by: NSteven Rostedt <rostedt@goodmis.org>

The __cpuinit type of throwaway sections might have made sense
some time ago when RAM was more constrained, but now the savings
do not offset the cost and complications.  For example, the fix in
commit 5e427ec2 ("x86: Fix bit corruption at CPU resume time")
is a good example of the nasty type of bugs that can be created
with improper use of the various __init prefixes.

After a discussion on LKML[1] it was decided that cpuinit should go
the way of devinit and be phased out.  Once all the users are gone,
we can then finally remove the macros themselves from linux/init.h.

This removes all the drivers/rcu uses of the __cpuinit macros
from all C files.

[1] https://lkml.org/lkml/2013/5/20/589

Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Josh Triplett <josh@freedesktop.org>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>
Signed-off-by: NPaul Gortmaker <paul.gortmaker@windriver.com>

Calling kthread_run with a single name parameter causes it to be handled
as a format string. Many callers are passing potentially dynamic string
content, so use "%s" in those cases to avoid any potential accidents.
Signed-off-by: NKees Cook <keescook@chromium.org>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

Systems with HZ=100 can have slow bootup times due to the default
three-jiffy delays between quiescent-state forcing attempts.  This
commit therefore auto-tunes the RCU_JIFFIES_TILL_FORCE_QS value based
on the value of HZ.  However, this would break very large systems that
require more time between quiescent-state forcing attempts.  This
commit therefore also ups the default delay by one jiffy for each
256 CPUs that might be on the system (based off of nr_cpu_ids at
runtime, -not- NR_CPUS at build time).

Updated to collapse #ifdefs for RCU_JIFFIES_TILL_FORCE_QS into a
step-function definition as suggested by Josh Triplett.
Reported-by: NPaul Mackerras <paulus@au1.ibm.com>
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>

The __rcu_process_callbacks() invokes note_gp_changes() immediately
before invoking rcu_check_quiescent_state(), which conditionally
invokes that same function.  This commit therefore eliminates the
call to note_gp_changes() in __rcu_process_callbacks() in favor of
making unconditional to call from rcu_check_quiescent_state() to
note_gp_changes().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

Given the changes that introduce note_gp_change(), rcu_start_gp_per_cpu()
is now a trivial wrapper function with only one caller.  This commit
therefore inlines it into its sole call site.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

One of the calls to check_for_new_grace_period() is now redundant due to
an immediately preceding call to note_gp_changes().  Eliminating this
redundant call leaves a single caller, which is simpler if inlined.
This commit therefore eliminates the redundant call and inlines the
body of check_for_new_grace_period() into the single remaining call site.
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>

This commit eliminates some duplicated code by merging
__rcu_process_gp_end() into __note_gp_changes().
Signed-off-by: NPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: NJosh Triplett <josh@joshtriplett.org>