From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-11.8 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, MENTIONS_GIT_HOSTING,SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 69260C432C0 for ; Fri, 22 Nov 2019 11:18:57 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 41DB720708 for ; Fri, 22 Nov 2019 11:18:57 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1729507AbfKVLS4 (ORCPT ); Fri, 22 Nov 2019 06:18:56 -0500 Received: from Galois.linutronix.de ([193.142.43.55]:34681 "EHLO Galois.linutronix.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729861AbfKVKsO (ORCPT ); Fri, 22 Nov 2019 05:48:14 -0500 Received: from [5.158.153.53] (helo=tip-bot2.lab.linutronix.de) by Galois.linutronix.de with esmtpsa (TLS1.2:DHE_RSA_AES_256_CBC_SHA256:256) (Exim 4.80) (envelope-from ) id 1iY6Tv-00034B-0r; Fri, 22 Nov 2019 11:48:07 +0100 Received: from [127.0.1.1] (localhost [IPv6:::1]) by tip-bot2.lab.linutronix.de (Postfix) with ESMTP id B3FBF1C1A6B; Fri, 22 Nov 2019 11:48:06 +0100 (CET) Date: Fri, 22 Nov 2019 10:48:06 -0000 From: "tip-bot2 for Marco Elver" Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: locking/kcsan] seqlock, kcsan: Add annotations for KCSAN Cc: Marco Elver , "Paul E. McKenney" , x86 , LKML MIME-Version: 1.0 Message-ID: <157441968666.21853.9839482924858141298.tip-bot2@tip-bot2> X-Mailer: tip-git-log-daemon Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit X-Linutronix-Spam-Score: -1.0 X-Linutronix-Spam-Level: - X-Linutronix-Spam-Status: No , -1.0 points, 5.0 required, ALL_TRUSTED=-1,SHORTCIRCUIT=-0.0001 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org The following commit has been merged into the locking/kcsan branch of tip: Commit-ID: 88ecd153be9519f259b87a9f6f4c8383a8b3bbf1 Gitweb: https://git.kernel.org/tip/88ecd153be9519f259b87a9f6f4c8383a8b3bbf1 Author: Marco Elver AuthorDate: Thu, 14 Nov 2019 19:02:59 +01:00 Committer: Paul E. McKenney CommitterDate: Sat, 16 Nov 2019 07:23:15 -08:00 seqlock, kcsan: Add annotations for KCSAN Since seqlocks in the Linux kernel do not require the use of marked atomic accesses in critical sections, we teach KCSAN to assume such accesses are atomic. KCSAN currently also pretends that writes to `sequence` are atomic, although currently plain writes are used (their corresponding reads are READ_ONCE). Further, to avoid false positives in the absence of clear ending of a seqlock reader critical section (only when using the raw interface), KCSAN assumes a fixed number of accesses after start of a seqlock critical section are atomic. === Commentary on design around absence of clear begin/end markings === Seqlock usage via seqlock_t follows a predictable usage pattern, where clear critical section begin/end is enforced. With subtle special cases for readers needing to be flat atomic regions, e.g. because usage such as in: - fs/namespace.c:__legitimize_mnt - unbalanced read_seqretry - fs/dcache.c:d_walk - unbalanced need_seqretry But, anything directly accessing seqcount_t seems to be unpredictable. Filtering for usage of read_seqcount_retry not following 'do { .. } while (read_seqcount_retry(..));': $ git grep 'read_seqcount_retry' | grep -Ev 'while \(|seqlock.h|Doc|\* ' => about 1/3 of the total read_seqcount_retry usage. Just looking at fs/namei.c, we conclude that it is non-trivial to prescribe and migrate to an interface that would force clear begin/end seqlock markings for critical sections. As such, we concluded that the best design currently, is to simply ensure that KCSAN works well with the existing code. Signed-off-by: Marco Elver Acked-by: Paul E. McKenney Signed-off-by: Paul E. McKenney --- include/linux/seqlock.h | 40 ++++++++++++++++++++++++++++++++++++++-- 1 file changed, 38 insertions(+), 2 deletions(-) diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index bcf4cf2..61232bc 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -37,9 +37,25 @@ #include #include #include +#include #include /* + * The seqlock interface does not prescribe a precise sequence of read + * begin/retry/end. For readers, typically there is a call to + * read_seqcount_begin() and read_seqcount_retry(), however, there are more + * esoteric cases which do not follow this pattern. + * + * As a consequence, we take the following best-effort approach for raw usage + * via seqcount_t under KCSAN: upon beginning a seq-reader critical section, + * pessimistically mark then next KCSAN_SEQLOCK_REGION_MAX memory accesses as + * atomics; if there is a matching read_seqcount_retry() call, no following + * memory operations are considered atomic. Usage of seqlocks via seqlock_t + * interface is not affected. + */ +#define KCSAN_SEQLOCK_REGION_MAX 1000 + +/* * Version using sequence counter only. * This can be used when code has its own mutex protecting the * updating starting before the write_seqcountbeqin() and ending @@ -115,6 +131,7 @@ repeat: cpu_relax(); goto repeat; } + kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); return ret; } @@ -131,6 +148,7 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s) { unsigned ret = READ_ONCE(s->sequence); smp_rmb(); + kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); return ret; } @@ -183,6 +201,7 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s) { unsigned ret = READ_ONCE(s->sequence); smp_rmb(); + kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX); return ret & ~1; } @@ -202,7 +221,8 @@ static inline unsigned raw_seqcount_begin(const seqcount_t *s) */ static inline int __read_seqcount_retry(const seqcount_t *s, unsigned start) { - return unlikely(s->sequence != start); + kcsan_atomic_next(0); + return unlikely(READ_ONCE(s->sequence) != start); } /** @@ -225,6 +245,7 @@ static inline int read_seqcount_retry(const seqcount_t *s, unsigned start) static inline void raw_write_seqcount_begin(seqcount_t *s) { + kcsan_nestable_atomic_begin(); s->sequence++; smp_wmb(); } @@ -233,6 +254,7 @@ static inline void raw_write_seqcount_end(seqcount_t *s) { smp_wmb(); s->sequence++; + kcsan_nestable_atomic_end(); } /** @@ -271,9 +293,11 @@ static inline void raw_write_seqcount_end(seqcount_t *s) */ static inline void raw_write_seqcount_barrier(seqcount_t *s) { + kcsan_nestable_atomic_begin(); s->sequence++; smp_wmb(); s->sequence++; + kcsan_nestable_atomic_end(); } static inline int raw_read_seqcount_latch(seqcount_t *s) @@ -398,7 +422,9 @@ static inline void write_seqcount_end(seqcount_t *s) static inline void write_seqcount_invalidate(seqcount_t *s) { smp_wmb(); + kcsan_nestable_atomic_begin(); s->sequence+=2; + kcsan_nestable_atomic_end(); } typedef struct { @@ -430,11 +456,21 @@ typedef struct { */ static inline unsigned read_seqbegin(const seqlock_t *sl) { - return read_seqcount_begin(&sl->seqcount); + unsigned ret = read_seqcount_begin(&sl->seqcount); + + kcsan_atomic_next(0); /* non-raw usage, assume closing read_seqretry */ + kcsan_flat_atomic_begin(); + return ret; } static inline unsigned read_seqretry(const seqlock_t *sl, unsigned start) { + /* + * Assume not nested: read_seqretry may be called multiple times when + * completing read critical section. + */ + kcsan_flat_atomic_end(); + return read_seqcount_retry(&sl->seqcount, start); }