From d133166146333e1f13fc81c0e6c43c8d99290a8a Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Thu, 12 Oct 2017 13:20:51 +0100 Subject: locking/qrwlock: Prevent slowpath writers getting held up by fastpath 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 Tested-by: Jeremy Linton Tested-by: Adam Wallis Tested-by: Jan Glauber Signed-off-by: Will Deacon Acked-by: Peter Zijlstra Cc: Boqun Feng Cc: Jeremy.Linton@arm.com Cc: Linus Torvalds Cc: Paul E. McKenney Cc: Thomas Gleixner 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 --- include/asm-generic/qrwlock.h | 23 +++++------------------ 1 file changed, 5 insertions(+), 18 deletions(-) (limited to 'include/asm-generic/qrwlock.h') diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h index c716b02e8fd7..0f7062bd55e5 100644 --- a/include/asm-generic/qrwlock.h +++ b/include/asm-generic/qrwlock.h @@ -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); } /* -- cgit v1.2.3