Outside of the locking code itself, {read,spin,write}_can_lock() have no
users in tree. Apparmor (the last remaining user of write_can_lock()) got
moved over to lockdep by the previous patch.

This patch removes the use of {read,spin,write}_can_lock() from the
BUILD_LOCK_OPS macro, deferring to the trylock operation for testing the
lock status, and subsequently removes the unused macros altogether. They
aren't guaranteed to work in a concurrent environment and can give
incorrect results in the case of qrwlock.
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1507055129-12300-2-git-send-email-will.deacon@arm.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

This patch aims to get rid of endianness in queued_write_unlock(). We
want to set  __qrwlock->wmode to NULL, however the address is not
&lock->cnts in big endian machine. That causes queued_write_unlock()
write NULL to the wrong field of __qrwlock.

So implement __qrwlock_write_byte() which returns the correct
__qrwlock->wmode address.
Suggested-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: NPan Xinhui <xinhui.pan@linux.vnet.ibm.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman.Long@hpe.com
Cc: arnd@arndb.de
Cc: boqun.feng@gmail.com
Cc: will.deacon@arm.com
Link: http://lkml.kernel.org/r/1468835259-4486-1-git-send-email-xinhui.pan@linux.vnet.ibm.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

The qrwlock implementation is slightly heavy in its use of memory
barriers, mainly through the use of _cmpxchg() and _return() atomics, which
imply full barrier semantics.

This patch modifies the qrwlock code to use the more relaxed atomic
routines so that we can reduce the unnecessary barrier overhead on
weakly-ordered architectures.
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Waiman.Long@hp.com
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1438880084-18856-7-git-send-email-will.deacon@arm.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

Since the following commit:

  536fa402 ("compiler: Allow 1- and 2-byte smp_load_acquire() and smp_store_release()")

smp_store_release() supports byte accesses, so use that in writer unlock
and remove the conditional macro override.
Signed-off-by: NWill Deacon <will.deacon@arm.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: NWaiman Long <Waiman.Long@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1438880084-18856-6-git-send-email-will.deacon@arm.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

The qrwlock is fair in the process context, but becoming unfair when
in the interrupt context to support use cases like the tasklist_lock.

The current code isn't that well-documented on what happens when
in the interrupt context. The rspin_until_writer_unlock() will only
spin if the writer has gotten the lock. If the writer is still in the
waiting state, the increment in the reader count will cause the writer
to remain in the waiting state and the new interrupt context reader
will get the lock and return immediately. The current code, however,
does an additional read of the lock value which is not necessary as
the information has already been there in the fast path. This may
sometime cause an additional cacheline transfer when the lock is
highly contended.

This patch passes the lock value information gotten in the fast path
to the slow path to eliminate the additional read. It also documents
the action for the interrupt context readers more clearly.
Signed-off-by: NWaiman Long <Waiman.Long@hp.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: NWill Deacon <will.deacon@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Douglas Hatch <doug.hatch@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1434729002-57724-3-git-send-email-Waiman.Long@hp.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

To sync up with the naming convention used in qspinlock, all the
qrwlock functions were renamed to started with "queued" instead of
"queue".
Signed-off-by: NWaiman Long <Waiman.Long@hp.com>
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Douglas Hatch <doug.hatch@hp.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Scott J Norton <scott.norton@hp.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Link: http://lkml.kernel.org/r/1434729002-57724-2-git-send-email-Waiman.Long@hp.comSigned-off-by: NIngo Molnar <mingo@kernel.org>

This rwlock uses the arch_spin_lock_t as a waitqueue, and assuming the
arch_spin_lock_t is a fair lock (ticket,mcs etc..) the resulting
rwlock is a fair lock.

It fits in the same 8 bytes as the regular rwlock_t by folding the
reader and writer count into a single integer, using the remaining 4
bytes for the arch_spinlock_t.

Architectures that can single-copy adress bytes can optimize
queue_write_unlock() with a 0 write to the LSB (the write count).

Performance as measured by Davidlohr Bueso (rwlock_t -> qrwlock_t):

 +--------------+-------------+---------------+
 |   Workload   |   #users    |     delta     |
 +--------------+-------------+---------------+
 | alltests     | > 1400      | -4.83%        |
 | custom       | 0-100,> 100 | +1.43%,-1.57% |
 | high_systime | > 1000      | -2.61         |
 | shared       | all         | +0.32         |
 +--------------+-------------+---------------+

http://www.stgolabs.net/qrwlock-stuff/aim7-results-vs-rwsem_optsin/Signed-off-by: NWaiman Long <Waiman.Long@hp.com>
[peterz: near complete rewrite]
Signed-off-by: NPeter Zijlstra <peterz@infradead.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Paul E.McKenney" <paulmck@linux.vnet.ibm.com>
Cc: linux-arch@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Link: http://lkml.kernel.org/n/tip-gac1nnl3wvs2ij87zv2xkdzq@git.kernel.orgSigned-off-by: NIngo Molnar <mingo@kernel.org>