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Darwish" , Boqun Feng , Jonathan Corbet , Waiman Long , Will Deacon , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org Subject: [PATCH 1/2] seqlock: fix the wrong read_seqbegin_or_lock/need_seqretry documentation Message-ID: <20231024120808.GA15382@redhat.com> MIME-Version: 1.0 Content-Disposition: inline User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.10 X-Spam-Status: No, score=-0.9 required=5.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on fry.vger.email Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-Greylist: Sender passed SPF test, not delayed by milter-greylist-4.6.4 (fry.vger.email [0.0.0.0]); Tue, 24 Oct 2023 05:10:15 -0700 (PDT) X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1780638744518789379 X-GMAIL-MSGID: 1780638744518789379 Half of the read_seqbegin_or_lock's users are buggy (I'll send the fixes), and I guess this is because the documentation and the pseudo code in Documentation/locking/seqlock.rst are wrong. Pseudo code: int seq = 0; do { read_seqbegin_or_lock(&foo_seqlock, &seq); /* ... [[read-side critical section]] ... */ } while (need_seqretry(&foo_seqlock, seq)); read_seqbegin_or_lock() returns with the even seq, need_seqretry() doesn't change this counter. This means that seq is always even and thus the locking pass is simply impossible. IOW, "_or_lock" has no effect and this code doesn't differ from do { seq = read_seqbegin(&foo_seqlock); /* ... [[read-side critical section]] ... */ } while (read_seqretry(&foo_seqlock, seq)); Signed-off-by: Oleg Nesterov --- Documentation/locking/seqlock.rst | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/Documentation/locking/seqlock.rst b/Documentation/locking/seqlock.rst index bfda1a5fecad..4bdf8d4ed2a2 100644 --- a/Documentation/locking/seqlock.rst +++ b/Documentation/locking/seqlock.rst @@ -218,13 +218,14 @@ Read path, three categories: according to a passed marker. This is used to avoid lockless readers starvation (too much retry loops) in case of a sharp spike in write activity. First, a lockless read is tried (even marker passed). If - that trial fails (odd sequence counter is returned, which is used as - the next iteration marker), the lockless read is transformed to a - full locking read and no retry loop is necessary:: + that trial fails (sequence counter doesn't match), make the marker + odd for the next iteration, the lockless read is transformed to a + full locking read and no retry loop is necessary, for example:: /* marker; even initialization */ - int seq = 0; + int seq = 1; do { + seq++; /* 2 on the 1st/lockless path, otherwise odd */ read_seqbegin_or_lock(&foo_seqlock, &seq); /* ... [[read-side critical section]] ... */ From patchwork Tue Oct 24 12:08:40 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Oleg Nesterov X-Patchwork-Id: 157402 Return-Path: Delivered-To: ouuuleilei@gmail.com Received: by 2002:a59:ce89:0:b0:403:3b70:6f57 with SMTP id p9csp1894477vqx; Tue, 24 Oct 2023 05:10:41 -0700 (PDT) X-Google-Smtp-Source: AGHT+IHr0l59LS5Uv2eXGtWreltwdmQs3MIidbpPUMVHaWUBXAmZMYIq2/okk27XZypEhR4LneLR X-Received: by 2002:a17:902:ea07:b0:1c9:f6df:8a88 with SMTP id s7-20020a170902ea0700b001c9f6df8a88mr24449043plg.26.1698149441061; Tue, 24 Oct 2023 05:10:41 -0700 (PDT) ARC-Seal: i=1; a=rsa-sha256; t=1698149441; cv=none; d=google.com; s=arc-20160816; b=XwgCKBgF5KHaAM0l7wXBOAPGsM6Oat9DypqcoRRMLIoxJxsXIz2xUL0qxs9Evn/De+ 9cBcCAjCVqn1y2OoXwXSz5Zr5iOE8/QkzzVial/ldhHcQukfBqIsiexWqCtvaSL0Udh+ QAFmLaohTnX1TRDtNRv1Y3qIlYVc5KVVKbqNkGFcYXXjdl+Tr3v148JgUo+Z0LYdQge/ m/agmEqZZsBqbMS/Op+970rrej3pfJnVGZbKpdUPjKt61jZgu6zqNGpkFgxGncVuMlYk f26I4tdAkdICpT59ggJMMRjAM9ygxuKzs9cpTljUhKg+yT8tPJMBm+f2mVjEdUD2O5xO 4khA== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:user-agent:in-reply-to:content-disposition :mime-version:references:message-id:subject:cc:to:from:date :dkim-signature; bh=efDOfqBOY7+QTlTqGFMqCfejaDDmVYLxZ1lQTyV6fFc=; fh=+Q0A7yH/24szLnVJ2ZJNg7LKA0/TnmLjGytBQxPStmI=; b=M40G4COH16SYbSD6zN6wg5Ux39SsqG6IrelnHTyFfZVJhUqq8hOZOwEfNyvng6R6Dt 6iMrbCMwV7vyUFmaaSlAlOnm9S6uD554dop5q+olxcF97rA9i83gXaHL1GKK+z7cqkOW 7kGmrTF8MmN3y6UY8OyP2CCEEKpv2OurVfsqD//mrstWYP8vkuRrW5Pya59VpNTvwUHb t3SV53Lkc7CMayZCRczkqj+8xp0S7QnVpoSSoOwC93fXxHCFIUHMp6euVSuURZ6l4NJ6 3yuDkYMS9A7+WA1LgXZUKOW0ee04MY3i+EvoyD+vdU53yqNU5DPdOy2DE837FHfgFW1+ dtYQ== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@redhat.com header.s=mimecast20190719 header.b=U6LL56Do; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:7 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=redhat.com Received: from snail.vger.email (snail.vger.email. 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Darwish" , Boqun Feng , Jonathan Corbet , Waiman Long , Will Deacon , linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org Subject: [RFC PATCH 2/2] seqlock: introduce need_seqretry_xxx() Message-ID: <20231024120840.GA15391@redhat.com> References: <20231024120808.GA15382@redhat.com> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20231024120808.GA15382@redhat.com> User-Agent: Mutt/1.5.24 (2015-08-30) X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.5 X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H3,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_NONE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-Greylist: Sender passed SPF test, not delayed by milter-greylist-4.6.4 (snail.vger.email [0.0.0.0]); Tue, 24 Oct 2023 05:10:39 -0700 (PDT) X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1780638748617133342 X-GMAIL-MSGID: 1780638748617133342 Not for inclusion, just for discussion... Modulo naming, do you think the new need_seqretry_xxx() makes sense? Simpler to use and less error prone. thread_group_cputime() is changed as an example. --- include/linux/seqlock.h | 10 ++++++++++ kernel/sched/cputime.c | 9 +++------ 2 files changed, 13 insertions(+), 6 deletions(-) diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index d0050c889f26..9b3bc4ce3332 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -1165,6 +1165,16 @@ static inline int need_seqretry(seqlock_t *lock, int seq) return !(seq & 1) && read_seqretry(lock, seq); } +static inline int need_seqretry_xxx(seqlock_t *lock, int *seq) +{ + int ret = !(*seq & 1) && read_seqretry(lock, *seq); + + if (ret) + ++*seq; /* make this counter odd */ + + return ret; +} + /** * done_seqretry() - end seqlock_t "locking or lockless" reader section * @lock: Pointer to seqlock_t diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c index af7952f12e6c..45704a84baec 100644 --- a/kernel/sched/cputime.c +++ b/kernel/sched/cputime.c @@ -314,7 +314,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times) struct signal_struct *sig = tsk->signal; u64 utime, stime; struct task_struct *t; - unsigned int seq, nextseq; + unsigned int seq; unsigned long flags; /* @@ -330,9 +330,8 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times) rcu_read_lock(); /* Attempt a lockless read on the first round. */ - nextseq = 0; + seq = 0; do { - seq = nextseq; flags = read_seqbegin_or_lock_irqsave(&sig->stats_lock, &seq); times->utime = sig->utime; times->stime = sig->stime; @@ -344,9 +343,7 @@ void thread_group_cputime(struct task_struct *tsk, struct task_cputime *times) times->stime += stime; times->sum_exec_runtime += read_sum_exec_runtime(t); } - /* If lockless access failed, take the lock. */ - nextseq = 1; - } while (need_seqretry(&sig->stats_lock, seq)); + } while (need_seqretry_xxx(&sig->stats_lock, &seq)); done_seqretry_irqrestore(&sig->stats_lock, seq, flags); rcu_read_unlock(); }