Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933955AbbDIUIu (ORCPT ); Thu, 9 Apr 2015 16:08:50 -0400 Received: from g2t2352.austin.hp.com ([15.217.128.51]:41806 "EHLO g2t2352.austin.hp.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S933502AbbDIUIq (ORCPT ); Thu, 9 Apr 2015 16:08:46 -0400 From: Waiman Long To: Arnd Bergmann , Ingo Molnar , Peter Zijlstra , linux-arch@vger.kernel.org Cc: linux-kernel@vger.kernel.org, Linus Torvalds , Scott J Norton , Douglas Hatch , Waiman Long Subject: [PATCH] qrwlock: Fix bug in interrupt handling code Date: Thu, 9 Apr 2015 16:07:55 -0400 Message-Id: <1428610075-38957-1-git-send-email-Waiman.Long@hp.com> X-Mailer: git-send-email 1.7.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3124 Lines: 89 The qrwlock is fair in the process context, but becoming unfair when in the interrupt context to support use cases like the tasklist_lock. However, the unfair code in the interrupt context has problem that may cause deadlock. The fast path increments the reader count. In the interrupt context, the reader in the slowpath will wait until the writer release the lock. However, if other readers have the lock and the writer is just in the waiting mode. It will never get the write lock because the that interrupt context reader has increment the count. This will cause deadlock. This patch fixes this problem by checking the state of the reader/writer count retrieved at the fast path. If the writer is in waiting mode, the reader will get the lock immediately and return. Otherwise, it will wait until the writer release the lock like before. Signed-off-by: Waiman Long --- include/asm-generic/qrwlock.h | 4 ++-- kernel/locking/qrwlock.c | 14 ++++++++------ 2 files changed, 10 insertions(+), 8 deletions(-) diff --git a/include/asm-generic/qrwlock.h b/include/asm-generic/qrwlock.h index 6383d54..865d021 100644 --- a/include/asm-generic/qrwlock.h +++ b/include/asm-generic/qrwlock.h @@ -36,7 +36,7 @@ /* * External function declarations */ -extern void queue_read_lock_slowpath(struct qrwlock *lock); +extern void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts); extern void queue_write_lock_slowpath(struct qrwlock *lock); /** @@ -105,7 +105,7 @@ static inline void queue_read_lock(struct qrwlock *lock) return; /* The slowpath will decrement the reader count, if necessary. */ - queue_read_lock_slowpath(lock); + queue_read_lock_slowpath(lock, cnts); } /** diff --git a/kernel/locking/qrwlock.c b/kernel/locking/qrwlock.c index f956ede..3fa4af2 100644 --- a/kernel/locking/qrwlock.c +++ b/kernel/locking/qrwlock.c @@ -43,22 +43,24 @@ rspin_until_writer_unlock(struct qrwlock *lock, u32 cnts) * queue_read_lock_slowpath - acquire read lock of a queue rwlock * @lock: Pointer to queue rwlock structure */ -void queue_read_lock_slowpath(struct qrwlock *lock) +void queue_read_lock_slowpath(struct qrwlock *lock, u32 cnts) { - u32 cnts; - /* * Readers come here when they cannot get the lock without waiting */ if (unlikely(in_interrupt())) { /* - * Readers in interrupt context will spin until the lock is - * available without waiting in the queue. + * Readers in interrupt context will get the lock immediately + * if the writer is just waiting (not holding the lock yet) + * or they will spin until the lock is available without + * waiting in the queue. */ - cnts = smp_load_acquire((u32 *)&lock->cnts); + if ((cnts & _QW_WMASK) != _QW_LOCKED) + return; rspin_until_writer_unlock(lock, cnts); return; } + atomic_sub(_QR_BIAS, &lock->cnts); /* -- 1.7.1 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/