locking/qspinlock: Bound spinning on pending->locked transition in slowpath
authorWill Deacon <will.deacon@arm.com>
Tue, 18 Dec 2018 17:13:53 +0000 (18:13 +0100)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Fri, 21 Dec 2018 13:13:08 +0000 (14:13 +0100)
commit3dab30f33814dc0b97c001921d3d724004c09b5f
treee071cec7cb22a96f97e27dea39860674c8b915d2
parent13f14c36323d71e2178d7d97dc65808c0e5c872e
locking/qspinlock: Bound spinning on pending->locked transition in slowpath

commit 6512276d97b160d90b53285bd06f7f201459a7e3 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: Waiman Long <longman@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: boqun.feng@gmail.com
Cc: linux-arm-kernel@lists.infradead.org
Cc: paulmck@linux.vnet.ibm.com
Link: http://lkml.kernel.org/r/1524738868-31318-4-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Sasha Levin <sashal@kernel.org>
kernel/locking/qspinlock.c