From: Linus Torvalds torvalds@linux-foundation.org
[ Upstream commit 125b0bb95dd6bec81b806b997a4ccb026eeecf8f ]
We really don't want to do atomic_read() or anything like that, since we already have the value, not the lock. The whole point of this is that we've loaded the lock from memory, and we want to check whether the value we loaded was a locked one or not.
The main use of this is the lockref code, which loads both the lock and the reference count in one atomic operation, and then works on that combined value. With the atomic_read(), the compiler would pointlessly spill the value to the stack, in order to then be able to read it back "atomically".
This is the qspinlock version of commit c6f4a9002252 ("asm-generic: ticket-lock: Optimize arch_spin_value_unlocked()") which fixed this same bug for ticket locks.
Cc: Guo Ren guoren@kernel.org Cc: Ingo Molnar mingo@kernel.org Cc: Waiman Long longman@redhat.com Link: https://lore.kernel.org/all/CAHk-=whNRv0v6kQiV5QO6DJhjH4KEL36vWQ6Re8Csrnh4zb... Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Sasha Levin sashal@kernel.org --- include/asm-generic/qspinlock.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/asm-generic/qspinlock.h b/include/asm-generic/qspinlock.h index 4fe7fd0fe834d..d1ddbcf71791d 100644 --- a/include/asm-generic/qspinlock.h +++ b/include/asm-generic/qspinlock.h @@ -41,7 +41,7 @@ static __always_inline int queued_spin_is_locked(struct qspinlock *lock) */ static __always_inline int queued_spin_value_unlocked(struct qspinlock lock) { - return !atomic_read(&lock.val); + return !lock.val.counter; }
/**