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locking/qrwlock: Prevent slowpath writers getting held up by fastpath

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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 distinct fields: _QW_LOCKED continues to occupy the bottom byte
of the lockword so that it can be cleared unconditionally when unlocking,
but _QW_WAITING now occupies what used to be the bottom bit of the reader
count. This then forces the slow-path for concurrent lockers.

Tested-by: Waiman Long <longman@redhat.com>
Tested-by: Jeremy Linton <jeremy.linton@arm.com>
Tested-by: Adam Wallis <awallis@codeaurora.org>
Tested-by: Jan Glauber <jglauber@cavium.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Jeremy.Linton@arm.com
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arm-kernel@lists.infradead.org
Link: http://lkml.kernel.org/r/1507810851-306-6-git-send-email-will.deacon@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Will Deacon
2017-10-12 13:20:51 +01:00
committed by Ingo Molnar
parent 087133ac90
commit d133166146
3 changed files with 14 additions and 37 deletions
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23
include/asm-generic/qrwlock.h
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@@ -26,24 +26,11 @@
/*
* Writer states & reader shift and bias.
*
* | +0 | +1 | +2 | +3 |
* ----+----+----+----+----+
* LE | 78 | 56 | 34 | 12 | 0x12345678
* ----+----+----+----+----+
* | wr | rd |
* +----+----+----+----+
*
* ----+----+----+----+----+
* BE | 12 | 34 | 56 | 78 | 0x12345678
* ----+----+----+----+----+
* | 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)
/*
@@ -134,7 +121,7 @@ static inline void queued_read_unlock(struct qrwlock *lock)
*/
static inline void queued_write_unlock(struct qrwlock *lock)
{
smp_store_release(&lock->wmode, 0);
smp_store_release(&lock->wlocked, 0);
}
/*