提交 d89e588c 编写于 作者: P Peter Zijlstra 提交者: Ingo Molnar

locking: Introduce smp_mb__after_spinlock()

Since its inception, our understanding of ACQUIRE, esp. as applied to
spinlocks, has changed somewhat. Also, I wonder if, with a simple
change, we cannot make it provide more.

The problem with the comment is that the STORE done by spin_lock isn't
itself ordered by the ACQUIRE, and therefore a later LOAD can pass over
it and cross with any prior STORE, rendering the default WMB
insufficient (pointed out by Alan).

Now, this is only really a problem on PowerPC and ARM64, both of
which already defined smp_mb__before_spinlock() as a smp_mb().

At the same time, we can get a much stronger construct if we place
that same barrier _inside_ the spin_lock(). In that case we upgrade
the RCpc spinlock to an RCsc.  That would make all schedule() calls
fully transitive against one another.
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: NWill Deacon <will.deacon@arm.com>
Cc: Alan Stern <stern@rowland.harvard.edu>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Nicholas Piggin <npiggin@gmail.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: NIngo Molnar <mingo@kernel.org>
上级 ff7a5fb0
......@@ -367,5 +367,7 @@ static inline int arch_read_trylock(arch_rwlock_t *rw)
* smp_mb__before_spinlock() can restore the required ordering.
*/
#define smp_mb__before_spinlock() smp_mb()
/* See include/linux/spinlock.h */
#define smp_mb__after_spinlock() smp_mb()
#endif /* __ASM_SPINLOCK_H */
......@@ -342,5 +342,8 @@ static inline void arch_write_unlock(arch_rwlock_t *rw)
#define arch_read_relax(lock) __rw_yield(lock)
#define arch_write_relax(lock) __rw_yield(lock)
/* See include/linux/spinlock.h */
#define smp_mb__after_spinlock() smp_mb()
#endif /* __KERNEL__ */
#endif /* __ASM_SPINLOCK_H */
......@@ -38,6 +38,9 @@
* Besides, if an arch has a special barrier for acquire/release, it could
* implement its own __atomic_op_* and use the same framework for building
* variants
*
* If an architecture overrides __atomic_op_acquire() it will probably want
* to define smp_mb__after_spinlock().
*/
#ifndef __atomic_op_acquire
#define __atomic_op_acquire(op, args...) \
......
......@@ -130,6 +130,42 @@ do { \
#define smp_mb__before_spinlock() smp_wmb()
#endif
/*
* This barrier must provide two things:
*
* - it must guarantee a STORE before the spin_lock() is ordered against a
* LOAD after it, see the comments at its two usage sites.
*
* - it must ensure the critical section is RCsc.
*
* The latter is important for cases where we observe values written by other
* CPUs in spin-loops, without barriers, while being subject to scheduling.
*
* CPU0 CPU1 CPU2
*
* for (;;) {
* if (READ_ONCE(X))
* break;
* }
* X=1
* <sched-out>
* <sched-in>
* r = X;
*
* without transitivity it could be that CPU1 observes X!=0 breaks the loop,
* we get migrated and CPU2 sees X==0.
*
* Since most load-store architectures implement ACQUIRE with an smp_mb() after
* the LL/SC loop, they need no further barriers. Similarly all our TSO
* architectures imply an smp_mb() for each atomic instruction and equally don't
* need more.
*
* Architectures that can implement ACQUIRE better need to take care.
*/
#ifndef smp_mb__after_spinlock
#define smp_mb__after_spinlock() do { } while (0)
#endif
/**
* raw_spin_unlock_wait - wait until the spinlock gets unlocked
* @lock: the spinlock in question.
......
......@@ -1967,8 +1967,8 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
* reordered with p->state check below. This pairs with mb() in
* set_current_state() the waiting thread does.
*/
smp_mb__before_spinlock();
raw_spin_lock_irqsave(&p->pi_lock, flags);
smp_mb__after_spinlock();
if (!(p->state & state))
goto out;
......@@ -3281,8 +3281,8 @@ static void __sched notrace __schedule(bool preempt)
* can't be reordered with __set_current_state(TASK_INTERRUPTIBLE)
* done by the caller to avoid the race with signal_wake_up().
*/
smp_mb__before_spinlock();
rq_lock(rq, &rf);
smp_mb__after_spinlock();
/* Promote REQ to ACT */
rq->clock_update_flags <<= 1;
......
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