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authorWill Deacon <will.deacon@arm.com>2018-04-26 11:34:17 +0100
committerIngo Molnar <mingo@kernel.org>2018-04-27 09:48:46 +0200
commit6512276d97b160d90b53285bd06f7f201459a7e3 (patch)
tree92bb4729c58a40b2983a47c20cf01bc56bc8d511 /kernel/locking
parent625e88be1f41b53cec55827c984e4a89ea8ee9f9 (diff)
locking/qspinlock: Bound spinning on pending->locked transition in slowpath
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>
Diffstat (limited to 'kernel/locking')
-rw-r--r--kernel/locking/qspinlock.c20
1 files changed, 17 insertions, 3 deletions
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c
index f5b0e59f6d14..a0f7976348f8 100644
--- a/kernel/locking/qspinlock.c
+++ b/kernel/locking/qspinlock.c
@@ -77,6 +77,18 @@
#endif
/*
+ * The pending bit spinning loop count.
+ * This heuristic is used to limit the number of lockword accesses
+ * made by atomic_cond_read_relaxed when waiting for the lock to
+ * transition out of the "== _Q_PENDING_VAL" state. We don't spin
+ * indefinitely because there's no guarantee that we'll make forward
+ * progress.
+ */
+#ifndef _Q_PENDING_LOOPS
+#define _Q_PENDING_LOOPS 1
+#endif
+
+/*
* Per-CPU queue node structures; we can never have more than 4 nested
* contexts: task, softirq, hardirq, nmi.
*
@@ -266,13 +278,15 @@ 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 bounded
+ * number of spins so that we guarantee forward progress.
*
* 0,1,0 -> 0,0,1
*/
if (val == _Q_PENDING_VAL) {
- while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL)
- cpu_relax();
+ int cnt = _Q_PENDING_LOOPS;
+ val = atomic_cond_read_relaxed(&lock->val,
+ (VAL != _Q_PENDING_VAL) || !cnt--);
}
/*