Make the code more readable by replacing the atomic_cmpxchg_acquire()
by an equivalent atomic_try_cmpxchg_acquire() and change atomic_add()
to atomic_or().
For architectures that use qrwlock, I do not find one that has an
atomic_add() defined but not an atomic_or(). I guess it should be fine
by changing atomic_add() to atomic_or(). I add a comment to state that
we can change it back to atomic_add() if there is an architecture that
has a more performant atomic_add() than an atomic_or().
Suggested-by: Linus Torvalds <[email protected]>
Signed-off-by: Waiman Long <[email protected]>
---
kernel/locking/qrwlock.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index b94f3831e963..a1fa0f636b07 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -60,18 +60,26 @@ EXPORT_SYMBOL(queued_read_lock_slowpath);
*/
void queued_write_lock_slowpath(struct qrwlock *lock)
{
- int cnts;
+ int cnts = 0;
/* Put the writer into the wait queue */
arch_spin_lock(&lock->wait_lock);
/* Try to acquire the lock directly if no reader is present */
if (!atomic_read(&lock->cnts) &&
- (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
+ atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED))
goto unlock;
- /* Set the waiting flag to notify readers that a writer is pending */
- atomic_add(_QW_WAITING, &lock->cnts);
+ /*
+ * Set the waiting flag to notify readers that a writer is pending
+ *
+ * As only one writer who is the wait_lock owner can set the waiting
+ * flag which will be cleared later on when acquiring the write lock,
+ * we can easily replace atomic_or() by an atomic_add() if there is
+ * an architecture where an atomic_add() performs better than an
+ * atomic_or().
+ */
+ atomic_or(_QW_WAITING, &lock->cnts);
/* When no more readers or writers, set the locked flag */
do {
--
2.18.1
On Sun, Apr 25, 2021 at 04:06:37PM -0400, Waiman Long wrote:
> void queued_write_lock_slowpath(struct qrwlock *lock)
> {
> - int cnts;
> + int cnts = 0;
>
> /* Put the writer into the wait queue */
> arch_spin_lock(&lock->wait_lock);
>
> /* Try to acquire the lock directly if no reader is present */
> if (!atomic_read(&lock->cnts) &&
> - (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
> + atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED))
> goto unlock;
Would not something like:
if (!(cnts = atomic_read(&lock->cnts)) &&
atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)
goto unlock;
Be clearer?
>
> - /* Set the waiting flag to notify readers that a writer is pending */
> - atomic_add(_QW_WAITING, &lock->cnts);
> + /*
> + * Set the waiting flag to notify readers that a writer is pending
> + *
> + * As only one writer who is the wait_lock owner can set the waiting
> + * flag which will be cleared later on when acquiring the write lock,
> + * we can easily replace atomic_or() by an atomic_add() if there is
> + * an architecture where an atomic_add() performs better than an
> + * atomic_or().
That might be a little overboard on the comment, but sure :-) I don't
think there's any arch that doesn't have atomic_or(), like I wrote
elsewhere, the one that's often an issue is atomic_fetch_or().
> + */
> + atomic_or(_QW_WAITING, &lock->cnts);
On 4/26/21 4:08 AM, Peter Zijlstra wrote:
> On Sun, Apr 25, 2021 at 04:06:37PM -0400, Waiman Long wrote:
>
>> void queued_write_lock_slowpath(struct qrwlock *lock)
>> {
>> - int cnts;
>> + int cnts = 0;
>>
>> /* Put the writer into the wait queue */
>> arch_spin_lock(&lock->wait_lock);
>>
>> /* Try to acquire the lock directly if no reader is present */
>> if (!atomic_read(&lock->cnts) &&
>> - (atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
>> + atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED))
>> goto unlock;
> Would not something like:
>
> if (!(cnts = atomic_read(&lock->cnts)) &&
> atomic_try_cmpxchg_acquire(&lock->cnts, &cnts, _QW_LOCKED)
> goto unlock;
>
> Be clearer?
That works for me too. It is equivalent anyway.
>
>>
>> - /* Set the waiting flag to notify readers that a writer is pending */
>> - atomic_add(_QW_WAITING, &lock->cnts);
>> + /*
>> + * Set the waiting flag to notify readers that a writer is pending
>> + *
>> + * As only one writer who is the wait_lock owner can set the waiting
>> + * flag which will be cleared later on when acquiring the write lock,
>> + * we can easily replace atomic_or() by an atomic_add() if there is
>> + * an architecture where an atomic_add() performs better than an
>> + * atomic_or().
> That might be a little overboard on the comment, but sure :-) I don't
> think there's any arch that doesn't have atomic_or(), like I wrote
> elsewhere, the one that's often an issue is atomic_fetch_or().
>
I was not sure as I didn't look at other archs that hadn't used qrwlock
yet. Given what you said, I will remove the comment.
Cheers,
Longman