Re: [PATCH V4] x86 spinlock: Fix memory corruption on completing completions
From: Oleg Nesterov <oleg@redhat.com>
Date: 2015-02-13 15:35:44
Also in:
kvm, lkml
On 02/13, Raghavendra K T wrote:
quoted hunk ↗ jump to hunk
@@ -164,7 +161,7 @@ static inline int arch_spin_is_locked(arch_spinlock_t *lock) { struct __raw_tickets tmp = READ_ONCE(lock->tickets); - return tmp.tail != tmp.head; + return tmp.tail != (tmp.head & ~TICKET_SLOWPATH_FLAG); }
Well, this can probably use __tickets_equal() too. But this is cosmetic. It seems that arch_spin_is_contended() should be fixed with this change, (__ticket_t)(tmp.tail - tmp.head) > TICKET_LOCK_INC can be true because of TICKET_SLOWPATH_FLAG in .head, even if it is actually unlocked. And the "(__ticket_t)" typecast looks unnecessary, it only adds more confusuin, but this is cosmetic too.
quoted hunk ↗ jump to hunk
@@ -772,7 +773,8 @@ __visible void kvm_lock_spinning(struct arch_spinlock *lock, __ticket_t want) * check again make sure it didn't become free while * we weren't looking. */ - if (ACCESS_ONCE(lock->tickets.head) == want) { + head = READ_ONCE(lock->tickets.head); + if (__tickets_equal(head, want)) { add_stats(TAKEN_SLOW_PICKUP, 1); goto out;
This is off-topic, but with or without this change perhaps it makes sense to add smp_mb__after_atomic(). It is nop on x86, just to make this code more understandable for those (for me ;) who can never remember even the x86 rules. Oleg.