| Message ID | 1522947547-24081-2-git-send-email-will.deacon@arm.com |
|---|---|
| State | New |
| Headers | show |
| Series | kernel/locking: qspinlock improvements | expand |
diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index d880296245c5..a192af2fe378 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -306,16 +306,6 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) return; /* - * wait for in-progress pending->locked hand-overs - * - * 0,1,0 -> 0,0,1 - */ - if (val == _Q_PENDING_VAL) { - while ((val = atomic_read(&lock->val)) == _Q_PENDING_VAL) - cpu_relax(); - } - - /* * trylock || pending * * 0,0,0 -> 0,0,1 ; trylock
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. Remove the pending->locked spinning from the slowpath and instead queue explicitly if pending is set. Cc: Peter Zijlstra <peterz@infradead.org> Cc: Ingo Molnar <mingo@kernel.org> Signed-off-by: Will Deacon <will.deacon@arm.com> --- kernel/locking/qspinlock.c | 10 ---------- 1 file changed, 10 deletions(-) -- 2.1.4