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SubjectRe: [PATCH 5/6] kernel/locking: Prevent slowpath writers getting held up by fastpath
On Thu, Oct 05, 2017 at 01:54:56PM +0100, Will Deacon wrote:
> When a prospective writer takes the qrwlock locking slowpath due to the
> lock being held, it attempts to cmpxchg the wmode field from 0 to
> _QW_WAITING so that concurrent lockers also take the slowpath and queue
> on the spinlock accordingly, allowing the lockers to drain.
>
> Unfortunately, this isn't fair, because a fastpath writer that comes in
> after the lock is made available but before the _QW_WAITING flag is set
> can effectively jump the queue. If there is a steady stream of prospective
> writers, then the waiter will be held off indefinitely.
>
> This patch restores fairness by separating _QW_WAITING and _QW_LOCKED
> into two bits in the wmode byte and having the waiter set _QW_WAITING
> unconditionally. This then forces the slow-path for concurrent lockers,
> but requires that a writer unlock operation performs an
> atomic_sub_release instead of a store_release so that the waiting status
> is preserved.

> diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
> index 02c0a768e6b0..8b7edef500e5 100644
> --- a/include/asm-generic/qrwlock.h
> +++ b/include/asm-generic/qrwlock.h
> @@ -41,7 +41,7 @@
> * +----+----+----+----+
> */
> #define _QW_WAITING 1 /* A writer is waiting */
> -#define _QW_LOCKED 0xff /* A writer holds the lock */
> +#define _QW_LOCKED 2 /* A writer holds the lock */
> #define _QW_WMASK 0xff /* Writer mask */
> #define _QR_SHIFT 8 /* Reader count shift */
> #define _QR_BIAS (1U << _QR_SHIFT)
> @@ -134,7 +134,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
> */
> static inline void queued_write_unlock(struct qrwlock *lock)
> {
> - smp_store_release(&lock->wmode, 0);
> + (void)atomic_sub_return_release(_QW_LOCKED, &lock->cnts);
> }

That is a fairly painful hit on x86. Changes a regular store into an
"LOCK XADD" +20 cycles right there.

Can't we steal one of the reader bits for waiting?

diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h
index 7d026bf27713..5524801a02a5 100644
--- a/include/asm-generic/qrwlock.h
+++ b/include/asm-generic/qrwlock.h
@@ -40,10 +40,10 @@
* | rd | wr |
* +----+----+----+----+
*/
-#define _QW_WAITING 1 /* A writer is waiting */
-#define _QW_LOCKED 0xff /* A writer holds the lock */
-#define _QW_WMASK 0xff /* Writer mask */
-#define _QR_SHIFT 8 /* Reader count shift */
+#define _QW_WAITING 0x100 /* A writer is waiting */
+#define _QW_LOCKED 0x0ff /* A writer holds the lock */
+#define _QW_WMASK 0x1ff /* Writer mask */
+#define _QR_SHIFT 9 /* Reader count shift */
#define _QR_BIAS (1U << _QR_SHIFT)

/*
diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c
index 2655f26ec882..5f75caea97e0 100644
--- a/kernel/locking/qrwlock.c
+++ b/kernel/locking/qrwlock.c
@@ -54,7 +54,7 @@ struct __qrwlock {
static __always_inline void
rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts)
{
- while ((cnts & _QW_WMASK) == _QW_LOCKED) {
+ while ((cnts & _QW_LOCKED)) {
cpu_relax();
cnts = atomic_read_acquire(&lock->cnts);
}
@@ -120,21 +120,10 @@ void queued_write_lock_slowpath(struct qrwlock *lock)
(atomic_cmpxchg_acquire(&lock->cnts, 0, _QW_LOCKED) == 0))
goto unlock;

- /*
- * Set the waiting flag to notify readers that a writer is pending,
- * or wait for a previous writer to go away.
- */
- for (;;) {
- struct __qrwlock *l = (struct __qrwlock *)lock;
-
- if (!READ_ONCE(l->wmode) &&
- (cmpxchg_relaxed(&l->wmode, 0, _QW_WAITING) == 0))
- break;
-
- cpu_relax();
- }
+ /* Set the waiting flag to notify readers that a writer is pending */
+ atomic_add(_QW_WAITING, &lock->cnts);

- /* When no more readers, set the locked flag */
+ /* When no more readers or writers, set the locked flag */
for (;;) {
cnts = atomic_read(&lock->cnts);
if ((cnts == _QW_WAITING) &&
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