Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 81D16C433EF for ; Tue, 16 Nov 2021 01:38:08 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 618EF615E1 for ; Tue, 16 Nov 2021 01:38:08 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1381379AbhKPBlA (ORCPT ); Mon, 15 Nov 2021 20:41:00 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:48879 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1355350AbhKPBch (ORCPT ); Mon, 15 Nov 2021 20:32:37 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1637026176; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding; bh=44iJ0uJRz66CtzXF/j2CkWWwF7Y4VvxAidMpL99o60k=; b=ZeG0wE89KKKmZnESN7PwCxct/ObCzLmoMn5Fws8zs2tGMYzKBZvKHFd8J7HbkWKMKKNS1g bGy2qHkzXrv9w64XCV+ENbj96ROeiIF46WGllhPCqHpuXkIMXIpSj66GjdCpS/STLC9Ups eHVlDxt0tX4hozFC7lDdF9Um2EnzXXM= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-250-i5R33VBAMG-NejlDo0nxkw-1; Mon, 15 Nov 2021 20:29:34 -0500 X-MC-Unique: i5R33VBAMG-NejlDo0nxkw-1 Received: from smtp.corp.redhat.com (int-mx08.intmail.prod.int.phx2.redhat.com [10.5.11.23]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id 6A16E8042E6; Tue, 16 Nov 2021 01:29:33 +0000 (UTC) Received: from llong.com (unknown [10.22.16.105]) by smtp.corp.redhat.com (Postfix) with ESMTP id DEC2219729; Tue, 16 Nov 2021 01:29:24 +0000 (UTC) From: Waiman Long To: Peter Zijlstra , Ingo Molnar , Will Deacon Cc: linux-kernel@vger.kernel.org, Davidlohr Bueso , mazhenhua , Hillf Danton , Maria Yu , Waiman Long Subject: [PATCH v5] locking/rwsem: Make handoff bit handling more consistent Date: Mon, 15 Nov 2021 20:29:12 -0500 Message-Id: <20211116012912.723980-1-longman@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org There are some inconsistency in the way that the handoff bit is being handled in readers and writers. Firstly, when a queue head writer set the handoff bit, it will clear it when the writer is being killed or interrupted on its way out without acquiring the lock. That is not the case for a queue head reader. The handoff bit will simply be inherited by the next waiter. Secondly, in the out_nolock path of rwsem_down_read_slowpath(), both the waiter and handoff bits are cleared if the wait queue becomes empty. For rwsem_down_write_slowpath(), however, the handoff bit is not checked and cleared if the wait queue is empty. This can potentially make the handoff bit set with empty wait queue. To make the handoff bit handling more consistent and robust, extract out handoff bit clearing code into the new rwsem_del_waiter() helper function. The common function will only use atomic_long_andnot() to clear bits when the wait queue is empty to avoid possible race condition. If the first waiter with handoff bit set is killed or interrupted to exit the slowpath without acquiring the lock, the next waiter will inherit the handoff bit. While at it, simplify the trylock for loop in rwsem_down_write_slowpath() to make it easier to read. Fixes: 4f23dbc1e657 ("locking/rwsem: Implement lock handoff to prevent lock starvation") Suggested-by: Peter Zijlstra Signed-off-by: Waiman Long --- kernel/locking/rwsem.c | 171 ++++++++++++++++++++--------------------- 1 file changed, 85 insertions(+), 86 deletions(-) diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index c51387a43265..e039cf1605af 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -105,9 +105,9 @@ * atomic_long_cmpxchg() will be used to obtain writer lock. * * There are three places where the lock handoff bit may be set or cleared. - * 1) rwsem_mark_wake() for readers. - * 2) rwsem_try_write_lock() for writers. - * 3) Error path of rwsem_down_write_slowpath(). + * 1) rwsem_mark_wake() for readers -- set, clear + * 2) rwsem_try_write_lock() for writers -- set, clear + * 3) rwsem_del_waiter() -- clear * * For all the above cases, wait_lock will be held. A writer must also * be the first one in the wait_list to be eligible for setting the handoff @@ -334,6 +334,9 @@ struct rwsem_waiter { struct task_struct *task; enum rwsem_waiter_type type; unsigned long timeout; + + /* Writer only, not initialized in reader */ + bool handoff_set; }; #define rwsem_first_waiter(sem) \ list_first_entry(&sem->wait_list, struct rwsem_waiter, list) @@ -344,12 +347,6 @@ enum rwsem_wake_type { RWSEM_WAKE_READ_OWNED /* Waker thread holds the read lock */ }; -enum writer_wait_state { - WRITER_NOT_FIRST, /* Writer is not first in wait list */ - WRITER_FIRST, /* Writer is first in wait list */ - WRITER_HANDOFF /* Writer is first & handoff needed */ -}; - /* * The typical HZ value is either 250 or 1000. So set the minimum waiting * time to at least 4ms or 1 jiffy (if it is higher than 4ms) in the wait @@ -365,6 +362,31 @@ enum writer_wait_state { */ #define MAX_READERS_WAKEUP 0x100 +static inline void +rwsem_add_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter) +{ + lockdep_assert_held(&sem->wait_lock); + list_add_tail(&waiter->list, &sem->wait_list); + /* caller will set RWSEM_FLAG_WAITERS */ +} + +/* + * Remove a waiter from the wait_list and clear flags. + * + * Both rwsem_mark_wake() and rwsem_try_write_lock() contain a full 'copy' of + * this function. Modify with care. + */ +static inline void +rwsem_del_waiter(struct rw_semaphore *sem, struct rwsem_waiter *waiter) +{ + lockdep_assert_held(&sem->wait_lock); + list_del(&waiter->list); + if (likely(!list_empty(&sem->wait_list))) + return; + + atomic_long_andnot(RWSEM_FLAG_HANDOFF | RWSEM_FLAG_WAITERS, &sem->count); +} + /* * handle the lock release when processes blocked on it that can now run * - if we come here from up_xxxx(), then the RWSEM_FLAG_WAITERS bit must @@ -376,6 +398,8 @@ enum writer_wait_state { * preferably when the wait_lock is released * - woken process blocks are discarded from the list after having task zeroed * - writers are only marked woken if downgrading is false + * + * Implies rwsem_del_waiter() for all woken readers. */ static void rwsem_mark_wake(struct rw_semaphore *sem, enum rwsem_wake_type wake_type, @@ -490,18 +514,25 @@ static void rwsem_mark_wake(struct rw_semaphore *sem, adjustment = woken * RWSEM_READER_BIAS - adjustment; lockevent_cond_inc(rwsem_wake_reader, woken); + + oldcount = atomic_long_read(&sem->count); if (list_empty(&sem->wait_list)) { - /* hit end of list above */ + /* + * Combined with list_move_tail() above, this implies + * rwsem_del_waiter(). + */ adjustment -= RWSEM_FLAG_WAITERS; + if (oldcount & RWSEM_FLAG_HANDOFF) + adjustment -= RWSEM_FLAG_HANDOFF; + } else if (woken) { + /* + * When we've woken a reader, we no longer need to force + * writers to give up the lock and we can clear HANDOFF. + */ + if (oldcount & RWSEM_FLAG_HANDOFF) + adjustment -= RWSEM_FLAG_HANDOFF; } - /* - * When we've woken a reader, we no longer need to force writers - * to give up the lock and we can clear HANDOFF. - */ - if (woken && (atomic_long_read(&sem->count) & RWSEM_FLAG_HANDOFF)) - adjustment -= RWSEM_FLAG_HANDOFF; - if (adjustment) atomic_long_add(adjustment, &sem->count); @@ -532,12 +563,12 @@ static void rwsem_mark_wake(struct rw_semaphore *sem, * race conditions between checking the rwsem wait list and setting the * sem->count accordingly. * - * If wstate is WRITER_HANDOFF, it will make sure that either the handoff - * bit is set or the lock is acquired with handoff bit cleared. + * Implies rwsem_del_waiter() on success. */ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, - enum writer_wait_state wstate) + struct rwsem_waiter *waiter) { + bool first = rwsem_first_waiter(sem) == waiter; long count, new; lockdep_assert_held(&sem->wait_lock); @@ -546,13 +577,19 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, do { bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF); - if (has_handoff && wstate == WRITER_NOT_FIRST) - return false; + if (has_handoff) { + if (!first) + return false; + + /* First waiter inherits a previously set handoff bit */ + waiter->handoff_set = true; + } new = count; if (count & RWSEM_LOCK_MASK) { - if (has_handoff || (wstate != WRITER_HANDOFF)) + if (has_handoff || (!rt_task(waiter->task) && + !time_after(jiffies, waiter->timeout))) return false; new |= RWSEM_FLAG_HANDOFF; @@ -569,9 +606,17 @@ static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, * We have either acquired the lock with handoff bit cleared or * set the handoff bit. */ - if (new & RWSEM_FLAG_HANDOFF) + if (new & RWSEM_FLAG_HANDOFF) { + waiter->handoff_set = true; + lockevent_inc(rwsem_wlock_handoff); return false; + } + /* + * Have rwsem_try_write_lock() fully imply rwsem_del_waiter() on + * success. + */ + list_del(&waiter->list); rwsem_set_owner(sem); return true; } @@ -956,7 +1001,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat } adjustment += RWSEM_FLAG_WAITERS; } - list_add_tail(&waiter.list, &sem->wait_list); + rwsem_add_waiter(sem, &waiter); /* we're now waiting on the lock, but no longer actively locking */ count = atomic_long_add_return(adjustment, &sem->count); @@ -1002,11 +1047,7 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat return sem; out_nolock: - list_del(&waiter.list); - if (list_empty(&sem->wait_list)) { - atomic_long_andnot(RWSEM_FLAG_WAITERS|RWSEM_FLAG_HANDOFF, - &sem->count); - } + rwsem_del_waiter(sem, &waiter); raw_spin_unlock_irq(&sem->wait_lock); __set_current_state(TASK_RUNNING); lockevent_inc(rwsem_rlock_fail); @@ -1020,9 +1061,7 @@ static struct rw_semaphore * rwsem_down_write_slowpath(struct rw_semaphore *sem, int state) { long count; - enum writer_wait_state wstate; struct rwsem_waiter waiter; - struct rw_semaphore *ret = sem; DEFINE_WAKE_Q(wake_q); /* do optimistic spinning and steal lock if possible */ @@ -1038,16 +1077,13 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state) waiter.task = current; waiter.type = RWSEM_WAITING_FOR_WRITE; waiter.timeout = jiffies + RWSEM_WAIT_TIMEOUT; + waiter.handoff_set = false; raw_spin_lock_irq(&sem->wait_lock); - - /* account for this before adding a new element to the list */ - wstate = list_empty(&sem->wait_list) ? WRITER_FIRST : WRITER_NOT_FIRST; - - list_add_tail(&waiter.list, &sem->wait_list); + rwsem_add_waiter(sem, &waiter); /* we're now waiting on the lock */ - if (wstate == WRITER_NOT_FIRST) { + if (rwsem_first_waiter(sem) != &waiter) { count = atomic_long_read(&sem->count); /* @@ -1083,13 +1119,16 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state) /* wait until we successfully acquire the lock */ set_current_state(state); for (;;) { - if (rwsem_try_write_lock(sem, wstate)) { + if (rwsem_try_write_lock(sem, &waiter)) { /* rwsem_try_write_lock() implies ACQUIRE on success */ break; } raw_spin_unlock_irq(&sem->wait_lock); + if (signal_pending_state(state, current)) + goto out_nolock; + /* * After setting the handoff bit and failing to acquire * the lock, attempt to spin on owner to accelerate lock @@ -1098,7 +1137,7 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state) * In this case, we attempt to acquire the lock again * without sleeping. */ - if (wstate == WRITER_HANDOFF) { + if (waiter.handoff_set) { enum owner_state owner_state; preempt_disable(); @@ -1109,66 +1148,26 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state) goto trylock_again; } - /* Block until there are no active lockers. */ - for (;;) { - if (signal_pending_state(state, current)) - goto out_nolock; - - schedule(); - lockevent_inc(rwsem_sleep_writer); - set_current_state(state); - /* - * If HANDOFF bit is set, unconditionally do - * a trylock. - */ - if (wstate == WRITER_HANDOFF) - break; - - if ((wstate == WRITER_NOT_FIRST) && - (rwsem_first_waiter(sem) == &waiter)) - wstate = WRITER_FIRST; - - count = atomic_long_read(&sem->count); - if (!(count & RWSEM_LOCK_MASK)) - break; - - /* - * The setting of the handoff bit is deferred - * until rwsem_try_write_lock() is called. - */ - if ((wstate == WRITER_FIRST) && (rt_task(current) || - time_after(jiffies, waiter.timeout))) { - wstate = WRITER_HANDOFF; - lockevent_inc(rwsem_wlock_handoff); - break; - } - } + schedule(); + lockevent_inc(rwsem_sleep_writer); + set_current_state(state); trylock_again: raw_spin_lock_irq(&sem->wait_lock); } __set_current_state(TASK_RUNNING); - list_del(&waiter.list); raw_spin_unlock_irq(&sem->wait_lock); lockevent_inc(rwsem_wlock); - - return ret; + return sem; out_nolock: __set_current_state(TASK_RUNNING); raw_spin_lock_irq(&sem->wait_lock); - list_del(&waiter.list); - - if (unlikely(wstate == WRITER_HANDOFF)) - atomic_long_add(-RWSEM_FLAG_HANDOFF, &sem->count); - - if (list_empty(&sem->wait_list)) - atomic_long_andnot(RWSEM_FLAG_WAITERS, &sem->count); - else + rwsem_del_waiter(sem, &waiter); + if (!list_empty(&sem->wait_list)) rwsem_mark_wake(sem, RWSEM_WAKE_ANY, &wake_q); raw_spin_unlock_irq(&sem->wait_lock); wake_up_q(&wake_q); lockevent_inc(rwsem_wlock_fail); - return ERR_PTR(-EINTR); } -- 2.27.0