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Signed-off-by: Charles Han Signed-off-by: Paul E. McKenney Signed-off-by: Neeraj Upadhyay (AMD) --- Documentation/RCU/torture.rst | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/Documentation/RCU/torture.rst b/Documentation/RCU/torture.rst index b3b6dfa85248..49e7beea6ae1 100644 --- a/Documentation/RCU/torture.rst +++ b/Documentation/RCU/torture.rst @@ -185,7 +185,7 @@ argument. Not all changes require that all scenarios be run. For example, a change to Tree SRCU might run only the SRCU-N and SRCU-P scenarios using the --configs argument to kvm.sh as follows: "--configs 'SRCU-N SRCU-P'". -Large systems can run multiple copies of of the full set of scenarios, +Large systems can run multiple copies of the full set of scenarios, for example, a system with 448 hardware threads can run five instances of the full set concurrently. 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Change the docstring so it describes what the function actually does. Signed-off-by: Philipp Stanner Signed-off-by: Paul E. McKenney Signed-off-by: Neeraj Upadhyay (AMD) --- include/linux/rculist.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/linux/rculist.h b/include/linux/rculist.h index d29740be4833..3dc1e58865f7 100644 --- a/include/linux/rculist.h +++ b/include/linux/rculist.h @@ -355,7 +355,7 @@ static inline void list_splice_tail_init_rcu(struct list_head *list, }) /** - * list_next_or_null_rcu - get the first element from a list + * list_next_or_null_rcu - get the next element from a list * @head: the head for the list. * @ptr: the list head to take the next element from. * @type: the type of the struct this is embedded in. 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McKenney" Currently, the reader/updater compatibility rules for the three RCU Tasks flavors are squished together in a single paragraph, which can result in confusion. This commit therefore splits them out into a list, clearly showing the distinction between these flavors. Link: https://lore.kernel.org/all/20231002211936.5948253e@gandalf.local.home/ Reported-by: Steven Rostedt Signed-off-by: Paul E. McKenney Reviewed-by: Mathieu Desnoyers Reviewed-by: Steven Rostedt (Google) Signed-off-by: Neeraj Upadhyay (AMD) --- Documentation/RCU/checklist.rst | 25 ++++++++++++++++--------- 1 file changed, 16 insertions(+), 9 deletions(-) diff --git a/Documentation/RCU/checklist.rst b/Documentation/RCU/checklist.rst index bd3c58c44bef..c432899aff22 100644 --- a/Documentation/RCU/checklist.rst +++ b/Documentation/RCU/checklist.rst @@ -241,15 +241,22 @@ over a rather long period of time, but improvements are always welcome! srcu_struct. The rules for the expedited RCU grace-period-wait primitives are the same as for their non-expedited counterparts. - If the updater uses call_rcu_tasks() or synchronize_rcu_tasks(), - then the readers must refrain from executing voluntary - context switches, that is, from blocking. If the updater uses - call_rcu_tasks_trace() or synchronize_rcu_tasks_trace(), then - the corresponding readers must use rcu_read_lock_trace() and - rcu_read_unlock_trace(). If an updater uses call_rcu_tasks_rude() - or synchronize_rcu_tasks_rude(), then the corresponding readers - must use anything that disables preemption, for example, - preempt_disable() and preempt_enable(). + Similarly, it is necssary to correctly use the RCU Tasks flavors: + + a. If the updater uses synchronize_rcu_tasks() or + call_rcu_tasks(), then the readers must refrain from + executing voluntary context switches, that is, from + blocking. + + b. If the updater uses call_rcu_tasks_trace() + or synchronize_rcu_tasks_trace(), then the + corresponding readers must use rcu_read_lock_trace() + and rcu_read_unlock_trace(). + + c. If an updater uses call_rcu_tasks_rude() or + synchronize_rcu_tasks_rude(), then the corresponding + readers must use anything that disables preemption, + for example, preempt_disable() and preempt_enable(). Mixing things up will result in confusion and broken kernels, and has even resulted in an exploitable security issue. Therefore, From patchwork Tue Dec 12 17:26:52 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Neeraj Upadhyay (AMD)" X-Patchwork-Id: 177489 Return-Path: Delivered-To: ouuuleilei@gmail.com Received: by 2002:a59:bcd1:0:b0:403:3b70:6f57 with SMTP id r17csp7878302vqy; Tue, 12 Dec 2023 09:27:52 -0800 (PST) X-Google-Smtp-Source: AGHT+IGvEjS62+C0gXgOebaON/BdFWKrwGHTBZ2uD02c9w2u6DTBl02Fr6u90CJll+fWfB6uFDNZ X-Received: by 2002:a17:90b:e0e:b0:28a:dcda:a101 with SMTP id ge14-20020a17090b0e0e00b0028adcdaa101mr72981pjb.47.1702402072353; Tue, 12 Dec 2023 09:27:52 -0800 (PST) ARC-Seal: i=1; a=rsa-sha256; t=1702402072; cv=none; d=google.com; s=arc-20160816; b=blkCmC5I2XFajstKS+E8GXy7qttrvE01uwwq7EigGkB7AfwgcPQWCA10ZPsdsK0z7O Y5ONXIreS7WOtizQTP5myUr54uRh6Dr2lD9lj+OSZImeY2TJM6QhVbKWfVnnT/5mskSI sNt4Wdz7osI6fylcxiYRKHQbZdMeCEDhe+5wocL51yZpO33o4fK8+0frLvc1DEdLZOaz K7FxrSNYvJtJJH42s7ohlpOzS4IZCjRc5q7zf5DJT32jXSs9K6fHwxQI66M7GmvXrry2 cIYmt9ie+N8k4pS7cDpPzTuFn04Z7JJl152Fb/hwLbeX/VscS7OT6VQios46NQGqLzja 0Z2w== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:content-transfer-encoding:mime-version :references:in-reply-to:message-id:date:subject:cc:to:from :dkim-signature; bh=cn4VwDYjNkD85ZVWjPPzSfZ086ShlP6kMAwWhA80/9Q=; fh=iHAPmcU+8zYxunk60y1JfkcGFukNs9v6EaWP7739dtk=; b=rGueFC16CcRysY22KVL5o/iAhpj60Lrd62rR1IhvsSulFYMuKLy1VIHQ+665NysMmQ Fg4loVtYQk+5BptIUf5O32Qs8eV1pk5f7LfB3AgIrxpuKtzBa2pKap7wbltZOKNlj7uz N4gytzM+j8OapleRPX0q6PvigJ+tVbciSFAr6xDO9Cb628RhOPLJ/uQEFXsZsyZY+RJ6 cI7FAZOhe8Jn0Y9Y9FnZzjsNay/4C4nYTnyPK0GEw1ywRJ8DpH6v1H0w4rp1hey01c8S 4jsKAAgInytoFQLLDbnC1ekYZmVdBvC4ajvIx3INiNCrJgZNuyh5UevPVv8LpT3jYyMZ 6IdA== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@gmail.com header.s=20230601 header.b=hwjbdDt1; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:1 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Received: from morse.vger.email (morse.vger.email. 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McKenney" This commit adds discussion of address and data dependencies to the beginning of rcu_dereference.rst in order to enable readers to more easily make the connection to the Linux-kernel memory model in general and to memory-barriers.txt in particular. Reported-by: Jonas Oberhauser Reported-by: Akira Yokosawa Signed-off-by: Paul E. McKenney Signed-off-by: Neeraj Upadhyay (AMD) --- Documentation/RCU/rcu_dereference.rst | 27 ++++++++++++++++++++------- 1 file changed, 20 insertions(+), 7 deletions(-) diff --git a/Documentation/RCU/rcu_dereference.rst b/Documentation/RCU/rcu_dereference.rst index 3b739f6243c8..659d5913784d 100644 --- a/Documentation/RCU/rcu_dereference.rst +++ b/Documentation/RCU/rcu_dereference.rst @@ -3,13 +3,26 @@ PROPER CARE AND FEEDING OF RETURN VALUES FROM rcu_dereference() =============================================================== -Most of the time, you can use values from rcu_dereference() or one of -the similar primitives without worries. Dereferencing (prefix "*"), -field selection ("->"), assignment ("="), address-of ("&"), addition and -subtraction of constants, and casts all work quite naturally and safely. - -It is nevertheless possible to get into trouble with other operations. -Follow these rules to keep your RCU code working properly: +Proper care and feeding of address and data dependencies is critically +important to correct use of things like RCU. To this end, the pointers +returned from the rcu_dereference() family of primitives carry address and +data dependencies. These dependencies extend from the rcu_dereference() +macro's load of the pointer to the later use of that pointer to compute +either the address of a later memory access (representing an address +dependency) or the value written by a later memory access (representing +a data dependency). + +Most of the time, these dependencies are preserved, permitting you to +freely use values from rcu_dereference(). For example, dereferencing +(prefix "*"), field selection ("->"), assignment ("="), address-of +("&"), casts, and addition or subtraction of constants all work quite +naturally and safely. However, because current compilers do not take +either address or data dependencies into account it is still possible +to get into trouble. + +Follow these rules to preserve the address and data dependencies emanating +from your calls to rcu_dereference() and friends, thus keeping your RCU +readers working properly: - You must use one of the rcu_dereference() family of primitives to load an RCU-protected pointer, otherwise CONFIG_PROVE_RCU From patchwork Tue Dec 12 17:26:53 2023 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: "Neeraj Upadhyay (AMD)" X-Patchwork-Id: 177490 Return-Path: Delivered-To: ouuuleilei@gmail.com Received: by 2002:a59:bcd1:0:b0:403:3b70:6f57 with SMTP id r17csp7878373vqy; Tue, 12 Dec 2023 09:27:59 -0800 (PST) X-Google-Smtp-Source: AGHT+IHH7PrHkiMrojWKcECJKLq9xkCS/BZhdGNtsqR8F9UpAkEG5jYZokRV8gxedVu08iud5GCl X-Received: by 2002:a17:90b:148c:b0:286:6cc1:77f2 with SMTP id js12-20020a17090b148c00b002866cc177f2mr4868145pjb.53.1702402078981; Tue, 12 Dec 2023 09:27:58 -0800 (PST) ARC-Seal: i=1; a=rsa-sha256; t=1702402078; cv=none; d=google.com; s=arc-20160816; b=wi81sf7R2jOniiWAT48vy2j+HxadzW/hne4iFJ0g5gVdUHcOwT/STADKvfLbFyv6IG 4fOM+fPKuhu9k3xJl1joSfs+0SLcfA/PV0bhsYI618zUEn7rxOB5MiUyZPr8crm48SnK SsFjJEhop2kf7hxFLhpWNNQwqpaHwUz9IRWTU/nUkoPmTKofJTIVBhnq07y+DuTLynQk 9sw9cru1sR6c7VnBYdB7Gtu0vM55aXpT31qCw3qh0GxFa5h1m8SxhIA3lLLtuQJRDwtx SNxDGAgMZAJbJ1S7ildm9a8y/J4/zo6JeyHYL/BPjF870U4elp8MZACth7Xh33x14fIJ dYnw== ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=arc-20160816; h=list-id:precedence:content-transfer-encoding:mime-version :references:in-reply-to:message-id:date:subject:cc:to:from :dkim-signature; bh=jynICPKoLwHaNl/nb1iCaIX3mL0xnnXkT4vQVCsRZhA=; fh=gzWlCm/zIbXaxBhhpYoWBFizlq3RvMkmJZBwhHjnDUQ=; b=HzxpnsDX7YfM1mkSpZpYqYplb2HACaO3vDK2fXj0VQkG9vrlGufF634ateo3VeUWQt FIxZOwvHRxIQr+KXAyeoa7OJ0by50nF4MO/GYmgAH3+YNKoCyYygcqe7V625wBCt/0Bb l61/QZCMx2HuHbcBl2DSBzFB6GQqjKczVisljqWu5N5MyXwZmCNRlXtP33SuPPMRu160 F7Dr8iOENt10iGK+fQto5kAagnSJ3Ok0+PnaCZW2De3EowYOH3SEyubVkHh3o5MdXQJe /lz3Hqij9145SmEflb9z+6vb8j2rmfFEPeo+AM5sM3/XnMbEhPdAoh/j91GXXHcjyU3t YhfQ== ARC-Authentication-Results: i=1; mx.google.com; dkim=pass header.i=@gmail.com header.s=20230601 header.b=ePqER2dE; spf=pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.32 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=QUARANTINE dis=NONE) header.from=gmail.com Received: from agentk.vger.email (agentk.vger.email. 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Tue, 12 Dec 2023 09:27:49 -0800 (PST) From: "Neeraj Upadhyay (AMD)" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@meta.com, rostedt@goodmis.org, paulmck@kernel.org, Neeraj.Upadhyay@amd.com, Jonas Oberhauser , Alan Stern , Andrea Parri , Will Deacon , Peter Zijlstra , Boqun Feng , Nicholas Piggin , David Howells , Jade Alglave , Luc Maranget , Akira Yokosawa , Daniel Lustig , Joel Fernandes , Jonathan Corbet , linux-arch@vger.kernel.org, linux-doc@vger.kernel.org, Neeraj Upadhyay Subject: [PATCH rcu 5/5] doc: Clarify historical disclaimers in memory-barriers.txt Date: Tue, 12 Dec 2023 22:56:53 +0530 Message-Id: <20231212172653.11485-5-neeraj.iitr10@gmail.com> X-Mailer: git-send-email 2.40.1 In-Reply-To: <20231212172343.GA11383@neeraj.linux> References: <20231212172343.GA11383@neeraj.linux> MIME-Version: 1.0 X-Spam-Status: No, score=-0.6 required=5.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,FREEMAIL_FORGED_FROMDOMAIN,FREEMAIL_FROM, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE, SPF_PASS,T_SCC_BODY_TEXT_LINE autolearn=unavailable autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on agentk.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 (agentk.vger.email [0.0.0.0]); Tue, 12 Dec 2023 09:27:56 -0800 (PST) X-getmail-retrieved-from-mailbox: INBOX X-GMAIL-THRID: 1785097962176255158 X-GMAIL-MSGID: 1785097962176255158 From: "Paul E. McKenney" This commit makes it clear that the reason that these sections are historical is that smp_read_barrier_depends() is no more. It also removes the point about comparison operations, given that there are other optimizations that can break address dependencies. Suggested-by: Jonas Oberhauser Signed-off-by: Paul E. McKenney Cc: Alan Stern Cc: Andrea Parri Cc: Will Deacon Cc: Peter Zijlstra Cc: Boqun Feng Cc: Nicholas Piggin Cc: David Howells Cc: Jade Alglave Cc: Luc Maranget Cc: Akira Yokosawa Cc: Daniel Lustig Cc: Joel Fernandes Cc: Jonathan Corbet Cc: Cc: Signed-off-by: Neeraj Upadhyay (AMD) --- Documentation/memory-barriers.txt | 17 ++++++++++------- 1 file changed, 10 insertions(+), 7 deletions(-) diff --git a/Documentation/memory-barriers.txt b/Documentation/memory-barriers.txt index d414e145f912..4202174a6262 100644 --- a/Documentation/memory-barriers.txt +++ b/Documentation/memory-barriers.txt @@ -396,10 +396,11 @@ Memory barriers come in four basic varieties: (2) Address-dependency barriers (historical). - [!] This section is marked as HISTORICAL: For more up-to-date - information, including how compiler transformations related to pointer - comparisons can sometimes cause problems, see - Documentation/RCU/rcu_dereference.rst. + [!] This section is marked as HISTORICAL: it covers the long-obsolete + smp_read_barrier_depends() macro, the semantics of which are now + implicit in all marked accesses. For more up-to-date information, + including how compiler transformations can sometimes break address + dependencies, see Documentation/RCU/rcu_dereference.rst. An address-dependency barrier is a weaker form of read barrier. In the case where two loads are performed such that the second depends on the @@ -560,9 +561,11 @@ There are certain things that the Linux kernel memory barriers do not guarantee: ADDRESS-DEPENDENCY BARRIERS (HISTORICAL) ---------------------------------------- -[!] This section is marked as HISTORICAL: For more up-to-date information, -including how compiler transformations related to pointer comparisons can -sometimes cause problems, see Documentation/RCU/rcu_dereference.rst. +[!] This section is marked as HISTORICAL: it covers the long-obsolete +smp_read_barrier_depends() macro, the semantics of which are now implicit +in all marked accesses. For more up-to-date information, including +how compiler transformations can sometimes break address dependencies, +see Documentation/RCU/rcu_dereference.rst. As of v4.15 of the Linux kernel, an smp_mb() was added to READ_ONCE() for DEC Alpha, which means that about the only people who need to pay attention