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Will Deacon authored
commit 6512276d upstream. If a locker taking the qspinlock slowpath reads a lock value indicating that only the pending bit is set, then it will spin whilst the concurrent pending->locked transition takes effect. Unfortunately, there is no guarantee that such a transition will ever be observed since concurrent lockers could continuously set pending and hand over the lock amongst themselves, leading to starvation. Whilst this would probably resolve in practice, it means that it is not possible to prove liveness properties about the lock and means that lock acquisition time is unbounded. Rather than removing the pending->locked spinning from the slowpath altogether (which has been shown to heavily penalise a 2-threaded locking stress test on x86), this patch replaces the explicit spinning with a call to atomic_cond_read_relaxed and allows the architecture to provide a bound on the number of spins. For architectures that can respond to changes in cacheline state in their smp_cond_load implementation, it should be sufficient to use the default bound of 1. Suggested-by:
Waiman Long <longman@redhat.com> Signed-off-by:
Will Deacon <will.deacon@arm.com> Acked-by:
Peter Zijlstra (Intel) <peterz@infradead.org> Acked-by:
Ingo Molnar <mingo@kernel.org> Signed-off-by:
Sebastian Andrzej Siewior <bigeasy@linutronix.de> Signed-off-by:
Sasha Levin <sashal@kernel.org>
3dab30f3
