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[209.132.180.67]) by mx.google.com with ESMTP id y10-v6si1906969pgf.312.2018.08.28.13.14.03; Tue, 28 Aug 2018 13:14:03 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of stable-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@linaro.org header.s=google header.b="Fi/NIJOk"; spf=pass (google.com: best guess record for domain of stable-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=stable-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=linaro.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727179AbeH2AHQ (ORCPT + 13 others); Tue, 28 Aug 2018 20:07:16 -0400 Received: from mail-pg1-f193.google.com ([209.85.215.193]:43619 "EHLO mail-pg1-f193.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726961AbeH2AHQ (ORCPT ); Tue, 28 Aug 2018 20:07:16 -0400 Received: by mail-pg1-f193.google.com with SMTP id v66-v6so1232140pgb.10 for ; Tue, 28 Aug 2018 13:14:02 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linaro.org; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references; bh=pManMXc5BRtQsqH+JUzfaDh+UG1RtWI46x4cSZ2txCQ=; b=Fi/NIJOkEE20r7nuFFBNqdvr/1e7zDoVi7CCzaNzpa8i0feXJIiFZOngYxee/NZ717 HxvVH01UF7JDG3nOLLMKP89dlG1PU6RNCpx3NDhXQ0oh9r8HRl/DjpWWydnG1dUuu+MJ M1nzF6Lv/hy2zWKVeOKmyUXc89FpOxM5tBnKw= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references; bh=pManMXc5BRtQsqH+JUzfaDh+UG1RtWI46x4cSZ2txCQ=; b=j2QASP3QskwCqmo9tQG033FyeOeXlCdeCaA/nDDWjg9h8j8I7nfxIjF+imILVvetPH pna2AzCRx5VMatTd8IZv09sktpR08kEImt48+YO6AIKDu/pGwHXtErb98bwScjIAQ/pa QP3UVXPcEOlDfWzL9aZQQUkbMmkqo4DU0TU/MLQSU2NHOMx2NoU14cZZ470TVNrJ5IUg qW4SAzgpf219/0+J5uotn18USctCqaJ9tBmPvjwNLGb98wEoLUMAjE0vIwht+BUOxArR 4sYWcXN64zP0eV4+FbV4EnHBAE3O2Q8mXwi9Gz1NWSOkPjcF9MmyphErEB3XCZYdW5Zq gUtw== X-Gm-Message-State: APzg51A08iDnxnjnLl6XLMpwQN+6jhW1lqPeK/ZpP/QrGYykmC5eRriq fk6gkp5ZZEUyAr9SyxVCe+ytgg== X-Received: by 2002:a62:9e08:: with SMTP id s8-v6mr3004788pfd.23.1535487241709; Tue, 28 Aug 2018 13:14:01 -0700 (PDT) Received: from localhost.localdomain ([49.207.48.21]) by smtp.gmail.com with ESMTPSA id t86-v6sm3098181pfe.109.2018.08.28.13.13.57 (version=TLS1_2 cipher=ECDHE-RSA-AES128-SHA bits=128/128); Tue, 28 Aug 2018 13:14:00 -0700 (PDT) From: Amit Pundir To: Greg KH Cc: Stable , Prateek Sood , Peter Zijlstra , Linus Torvalds , Thomas Gleixner , dave@stgolabs.net, longman@redhat.com, parri.andrea@gmail.com, sramana@codeaurora.org, Ingo Molnar Subject: [PATCH for-4.9.y 10/14] locking/rwsem-xadd: Fix missed wakeup due to reordering of load Date: Wed, 29 Aug 2018 01:43:21 +0530 Message-Id: <1535487205-26280-11-git-send-email-amit.pundir@linaro.org> X-Mailer: git-send-email 2.7.4 In-Reply-To: <1535487205-26280-1-git-send-email-amit.pundir@linaro.org> References: <1535487205-26280-1-git-send-email-amit.pundir@linaro.org> Sender: stable-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: stable@vger.kernel.org From: Prateek Sood commit 9c29c31830a4eca724e137a9339137204bbb31be upstream. If a spinner is present, there is a chance that the load of rwsem_has_spinner() in rwsem_wake() can be reordered with respect to decrement of rwsem count in __up_write() leading to wakeup being missed: spinning writer up_write caller --------------- ----------------------- [S] osq_unlock() [L] osq spin_lock(wait_lock) sem->count=0xFFFFFFFF00000001 +0xFFFFFFFF00000000 count=sem->count MB sem->count=0xFFFFFFFE00000001 -0xFFFFFFFF00000001 spin_trylock(wait_lock) return rwsem_try_write_lock(count) spin_unlock(wait_lock) schedule() Reordering of atomic_long_sub_return_release() in __up_write() and rwsem_has_spinner() in rwsem_wake() can cause missing of wakeup in up_write() context. In spinning writer, sem->count and local variable count is 0XFFFFFFFE00000001. It would result in rwsem_try_write_lock() failing to acquire rwsem and spinning writer going to sleep in rwsem_down_write_failed(). The smp_rmb() will make sure that the spinner state is consulted after sem->count is updated in up_write context. Signed-off-by: Prateek Sood Signed-off-by: Peter Zijlstra (Intel) Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner Cc: dave@stgolabs.net Cc: longman@redhat.com Cc: parri.andrea@gmail.com Cc: sramana@codeaurora.org Link: http://lkml.kernel.org/r/1504794658-15397-1-git-send-email-prsood@codeaurora.org Signed-off-by: Ingo Molnar Signed-off-by: Amit Pundir --- To be applied on 4.4.y as well. Build tested on v4.4.153. kernel/locking/rwsem-xadd.c | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) -- 2.7.4 diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index 2337b4bb2366..a4112dfcd0fb 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -574,6 +574,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) WAKE_Q(wake_q); /* + * __rwsem_down_write_failed_common(sem) + * rwsem_optimistic_spin(sem) + * osq_unlock(sem->osq) + * ... + * atomic_long_add_return(&sem->count) + * + * - VS - + * + * __up_write() + * if (atomic_long_sub_return_release(&sem->count) < 0) + * rwsem_wake(sem) + * osq_is_locked(&sem->osq) + * + * And __up_write() must observe !osq_is_locked() when it observes the + * atomic_long_add_return() in order to not miss a wakeup. + * + * This boils down to: + * + * [S.rel] X = 1 [RmW] r0 = (Y += 0) + * MB RMB + * [RmW] Y += 1 [L] r1 = X + * + * exists (r0=1 /\ r1=0) + */ + smp_rmb(); + + /* * If a spinner is present, it is not necessary to do the wakeup. * Try to do wakeup only if the trylock succeeds to minimize * spinlock contention which may introduce too much delay in the