From patchwork Tue Dec 18 17:13:55 2018 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Sebastian Andrzej Siewior X-Patchwork-Id: 10736053 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork-2.web.codeaurora.org (Postfix) with ESMTP id 255D86C5 for ; Tue, 18 Dec 2018 17:15:38 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 0DF022434C for ; Tue, 18 Dec 2018 17:15:38 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 01D0D2AB65; Tue, 18 Dec 2018 17:15:37 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-5.2 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,MAILING_LIST_MULTI,RCVD_IN_DNSWL_MED autolearn=ham version=3.3.1 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id 3E3642434C for ; Tue, 18 Dec 2018 17:15:37 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:Cc:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:References:In-Reply-To: Message-Id:Date:Subject:To:From:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=MUCnvcErj3EUKPsR1En8Q0FgF4ffSkpqijCQSY/yjXI=; b=EqYaE1xwVKsVjh QcGW6WV56SlFbDUIhvKslhTzcjKO2NhcO61yqw4F8Ezi3AafRBYi9cKqJSw6xQ2OvEvh/IC+FJsMk +kQBe+rnXrSKlcKDqkopZV7kbLLrowltrc+qBOLGVDWgLegl7IqXQK1KZHU1y1iTiTn66WKUM0tDW N138ZaMeVuMh/G5LzP3GFGc23zNqW4hlxB/HrJYRjJqxJKqMLTfMWTDk98V6Ez1xt+gCNVBTI6F/3 qsK0wC1oQEMXZW1tE69f/DQD9z1KQoU6F6IC+O0ulMb8fpiMS5P6QYl6qO6WZsxOPKEuFiL9Vca50 68avR/+MXJkOi/22/F9Q==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.90_1 #2 (Red Hat Linux)) id 1gZIxy-0008Cy-2h; Tue, 18 Dec 2018 17:15:34 +0000 Received: from galois.linutronix.de ([2a01:7a0:2:106d:700::1]) by bombadil.infradead.org with esmtps (Exim 4.90_1 #2 (Red Hat Linux)) id 1gZIxD-0006I5-2X for linux-arm-kernel@lists.infradead.org; Tue, 18 Dec 2018 17:14:52 +0000 Received: from localhost ([127.0.0.1] helo=bazinga.breakpoint.cc) by Galois.linutronix.de with esmtp (Exim 4.80) (envelope-from ) id 1gZIwi-0006QU-S8; Tue, 18 Dec 2018 18:14:17 +0100 From: Sebastian Andrzej Siewior To: stable@vger.kernel.org Subject: [PATCH STABLE v4.14 05/10] locking/qspinlock: Remove unbounded cmpxchg() loop from locking slowpath Date: Tue, 18 Dec 2018 18:13:55 +0100 Message-Id: <20181218171400.22711-6-bigeasy@linutronix.de> X-Mailer: git-send-email 2.20.1 In-Reply-To: <20181218171400.22711-1-bigeasy@linutronix.de> References: <20181218171400.22711-1-bigeasy@linutronix.de> MIME-Version: 1.0 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20181218_091448_240407_53D57A1C X-CRM114-Status: GOOD ( 21.72 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Peter Zijlstra , bigeasy@linutronix.de, Will Deacon , Ingo Molnar , Daniel Wagner , Waiman Long , Thomas Gleixner , paulmck@linux.vnet.ibm.com, Linus Torvalds , boqun.feng@gmail.com, linux-arm-kernel@lists.infradead.org Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org X-Virus-Scanned: ClamAV using ClamSMTP From: Will Deacon commit 59fb586b4a07b4e1a0ee577140ab4842ba451acd upstream. The qspinlock locking slowpath utilises a "pending" bit as a simple form of an embedded test-and-set lock that can avoid the overhead of explicit queuing in cases where the lock is held but uncontended. This bit is managed using a cmpxchg() loop which tries to transition the uncontended lock word from (0,0,0) -> (0,0,1) or (0,0,1) -> (0,1,1). Unfortunately, the cmpxchg() loop is unbounded and lockers can be starved indefinitely if the lock word is seen to oscillate between unlocked (0,0,0) and locked (0,0,1). This could happen if concurrent lockers are able to take the lock in the cmpxchg() loop without queuing and pass it around amongst themselves. This patch fixes the problem by unconditionally setting _Q_PENDING_VAL using atomic_fetch_or, and then inspecting the old value to see whether we need to spin on the current lock owner, or whether we now effectively hold the lock. The tricky scenario is when concurrent lockers end up queuing on the lock and the lock becomes available, causing us to see a lockword of (n,0,0). With pending now set, simply queuing could lead to deadlock as the head of the queue may not have observed the pending flag being cleared. Conversely, if the head of the queue did observe pending being cleared, then it could transition the lock from (n,0,0) -> (0,0,1) meaning that any attempt to "undo" our setting of the pending bit could race with a concurrent locker trying to set it. We handle this race by preserving the pending bit when taking the lock after reaching the head of the queue and leaving the tail entry intact if we saw pending set, because we know that the tail is going to be updated shortly. Signed-off-by: Will Deacon Acked-by: Peter Zijlstra (Intel) Acked-by: Waiman Long Cc: Linus Torvalds Cc: Thomas Gleixner Cc: boqun.feng@gmail.com Cc: linux-arm-kernel@lists.infradead.org Cc: paulmck@linux.vnet.ibm.com Link: http://lkml.kernel.org/r/1524738868-31318-6-git-send-email-will.deacon@arm.com Signed-off-by: Ingo Molnar Signed-off-by: Sebastian Andrzej Siewior --- kernel/locking/qspinlock.c | 102 ++++++++++++++++------------ kernel/locking/qspinlock_paravirt.h | 5 -- 2 files changed, 58 insertions(+), 49 deletions(-) diff --git a/kernel/locking/qspinlock.c b/kernel/locking/qspinlock.c index e60e618287b49..7bd053e528c22 100644 --- a/kernel/locking/qspinlock.c +++ b/kernel/locking/qspinlock.c @@ -127,6 +127,17 @@ static inline __pure struct mcs_spinlock *decode_tail(u32 tail) #define _Q_LOCKED_PENDING_MASK (_Q_LOCKED_MASK | _Q_PENDING_MASK) #if _Q_PENDING_BITS == 8 +/** + * clear_pending - clear the pending bit. + * @lock: Pointer to queued spinlock structure + * + * *,1,* -> *,0,* + */ +static __always_inline void clear_pending(struct qspinlock *lock) +{ + WRITE_ONCE(lock->pending, 0); +} + /** * clear_pending_set_locked - take ownership and clear the pending bit. * @lock: Pointer to queued spinlock structure @@ -162,6 +173,17 @@ static __always_inline u32 xchg_tail(struct qspinlock *lock, u32 tail) #else /* _Q_PENDING_BITS == 8 */ +/** + * clear_pending - clear the pending bit. + * @lock: Pointer to queued spinlock structure + * + * *,1,* -> *,0,* + */ +static __always_inline void clear_pending(struct qspinlock *lock) +{ + atomic_andnot(_Q_PENDING_VAL, &lock->val); +} + /** * clear_pending_set_locked - take ownership and clear the pending bit. * @lock: Pointer to queued spinlock structure @@ -266,7 +288,7 @@ static __always_inline u32 __pv_wait_head_or_lock(struct qspinlock *lock, void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) { struct mcs_spinlock *prev, *next, *node; - u32 new, old, tail; + u32 old, tail; int idx; BUILD_BUG_ON(CONFIG_NR_CPUS >= (1U << _Q_TAIL_CPU_BITS)); @@ -289,59 +311,51 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) (VAL != _Q_PENDING_VAL) || !cnt--); } + /* + * If we observe any contention; queue. + */ + if (val & ~_Q_LOCKED_MASK) + goto queue; + /* * trylock || pending * * 0,0,0 -> 0,0,1 ; trylock * 0,0,1 -> 0,1,1 ; pending */ - for (;;) { + val = atomic_fetch_or_acquire(_Q_PENDING_VAL, &lock->val); + if (!(val & ~_Q_LOCKED_MASK)) { /* - * If we observe any contention; queue. + * We're pending, wait for the owner to go away. + * + * *,1,1 -> *,1,0 + * + * this wait loop must be a load-acquire such that we match the + * store-release that clears the locked bit and create lock + * sequentiality; this is because not all + * clear_pending_set_locked() implementations imply full + * barriers. */ - if (val & ~_Q_LOCKED_MASK) - goto queue; - - new = _Q_LOCKED_VAL; - if (val == new) - new |= _Q_PENDING_VAL; + if (val & _Q_LOCKED_MASK) { + smp_cond_load_acquire(&lock->val.counter, + !(VAL & _Q_LOCKED_MASK)); + } /* - * Acquire semantic is required here as the function may - * return immediately if the lock was free. + * take ownership and clear the pending bit. + * + * *,1,0 -> *,0,1 */ - old = atomic_cmpxchg_acquire(&lock->val, val, new); - if (old == val) - break; - - val = old; + clear_pending_set_locked(lock); + return; } /* - * we won the trylock + * If pending was clear but there are waiters in the queue, then + * we need to undo our setting of pending before we queue ourselves. */ - if (new == _Q_LOCKED_VAL) - return; - - /* - * we're pending, wait for the owner to go away. - * - * *,1,1 -> *,1,0 - * - * this wait loop must be a load-acquire such that we match the - * store-release that clears the locked bit and create lock - * sequentiality; this is because not all clear_pending_set_locked() - * implementations imply full barriers. - */ - smp_cond_load_acquire(&lock->val.counter, !(VAL & _Q_LOCKED_MASK)); - - /* - * take ownership and clear the pending bit. - * - * *,1,0 -> *,0,1 - */ - clear_pending_set_locked(lock); - return; + if (!(val & _Q_PENDING_MASK)) + clear_pending(lock); /* * End of pending bit optimistic spinning and beginning of MCS @@ -445,15 +459,15 @@ void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val) * claim the lock: * * n,0,0 -> 0,0,1 : lock, uncontended - * *,0,0 -> *,0,1 : lock, contended + * *,*,0 -> *,*,1 : lock, contended * - * If the queue head is the only one in the queue (lock value == tail), - * clear the tail code and grab the lock. Otherwise, we only need - * to grab the lock. + * If the queue head is the only one in the queue (lock value == tail) + * and nobody is pending, clear the tail code and grab the lock. + * Otherwise, we only need to grab the lock. */ for (;;) { /* In the PV case we might already have _Q_LOCKED_VAL set */ - if ((val & _Q_TAIL_MASK) != tail) { + if ((val & _Q_TAIL_MASK) != tail || (val & _Q_PENDING_MASK)) { set_locked(lock); break; } diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h index 1435ba7954c3d..854443f7b60b6 100644 --- a/kernel/locking/qspinlock_paravirt.h +++ b/kernel/locking/qspinlock_paravirt.h @@ -89,11 +89,6 @@ static __always_inline void set_pending(struct qspinlock *lock) WRITE_ONCE(lock->pending, 1); } -static __always_inline void clear_pending(struct qspinlock *lock) -{ - WRITE_ONCE(lock->pending, 0); -} - /* * The pending bit check in pv_queued_spin_steal_lock() isn't a memory * barrier. Therefore, an atomic cmpxchg_acquire() is used to acquire the