When transitioning from unlocked - locked (for write), we notify readers
of our presence by setting the _QW_WAITING flag using a cmpxchg.
Given that we use another cmpxchg to transition to the _QW_LOCKED state,
we can drop the barriers from the intermediate step by offering
architectures the chance to implement cmpxchg_relaxed, in a similar
manner to the cmpxchg64_relaxed macro in the lockref code.
Signed-off-by: Will Deacon <will.deacon@xxxxxxx>
---
kernel/locking/qrwlock.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index a1b63df19a04..e3c51c4635d3 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -113,6 +113,10 @@ EXPORT_SYMBOL(queued_read_lock_slowpath);
* queued_write_lock_slowpath - acquire write lock of a queue rwlock
* @lock : Pointer to queue rwlock structure
*/
+#ifndef cmpxchg_relaxed
+# define cmpxchg_relaxed cmpxchg
+#endif
+
void queued_write_lock_slowpath(struct qrwlock *lock)
{
u32 cnts;
@@ -133,7 +137,7 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
struct __qrwlock *l = (struct __qrwlock *)lock;
if (!READ_ONCE(l->wmode) &&
- (cmpxchg(&l->wmode, 0, _QW_WAITING) == 0))
+ (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
break;
cpu_relax_lowlatency();
--
2.1.4
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