Messages in this thread Patch in this message |  | | From | Waiman Long <> | Subject | [PATCH 2/2] locking/qspinlock: Limit # of spins in _Q_PENDING_VAL wait loop | Date | Mon, 9 Apr 2018 14:08:52 -0400 |
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A locker in the pending code path is doing an infinite number of spins when waiting for the _Q_PENDING_VAL to _Q_LOCK_VAL transition. There is a concern that lock starvation can happen concurrent lockers are able to take the lock in the cmpxchg loop without queuing and pass it around amongst themselves.
To ensure forward progress while still taking advantage of using the pending code path without queuing, the code is now modified to do a limited number of spins before aborting the effort and going to queue itself.
Ideally, the spinning times should be at least a few times the typical cacheline load time from memory which I think can be down to 100ns or so for each cacheline load with the newest systems or up to several hundreds ns for older systems.
Signed-off-by: Waiman Long <longman@redhat.com> --- kernel/locking/qspinlock.c | 19 +++++++++++++++++-- 1 file changed, 17 insertions(+), 2 deletions(-)
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index 634a49b..35367cc 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -82,6 +82,15 @@ #endif /* + * The pending bit spinning loop count. + * This parameter can be overridden by another architecture specific + * constant. Default is 512. + */ +#ifndef _Q_PENDING_LOOP +#define _Q_PENDING_LOOP (1 << 9) +#endif + +/* * Per-CPU queue node structures; we can never have more than 4 nested * contexts: task, softirq, hardirq, nmi. * @@ -311,13 +320,19 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) return; /* - * wait for in-progress pending->locked hand-overs + * wait for in-progress pending->locked hand-overs with a + * limited number of spins. * * 0,1,0 -> 0,0,1 */ if (val == _Q_PENDING_VAL) { - while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL) + int cnt = _Q_PENDING_LOOP; + + while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL) { + if (!--cnt) + goto queue; cpu_relax(); + } } /* -- 1.8.3.1
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