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Darwish" Reply-to: linux-kernel@vger.kernel.org To: linux-tip-commits@vger.kernel.org Subject: [tip: locking/core] seqlock: Implement raw_seqcount_begin() in terms of raw_read_seqcount() Cc: "Ahmed S. Darwish" , "Peter Zijlstra (Intel)" , x86 , LKML In-Reply-To: <20200720155530.1173732-7-a.darwish@linutronix.de> References: <20200720155530.1173732-7-a.darwish@linutronix.de> MIME-Version: 1.0 Message-ID: <159603321925.4006.790279186396450025.tip-bot2@tip-bot2> Robot-ID: Robot-Unsubscribe: Contact to get blacklisted from these emails Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit 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/core branch of tip: Commit-ID: 932e46365226324d2cf26d8bdec8b51ceb296948 Gitweb: https://git.kernel.org/tip/932e46365226324d2cf26d8bdec8b51ceb296948 Author: Ahmed S. Darwish AuthorDate: Mon, 20 Jul 2020 17:55:12 +02:00 Committer: Peter Zijlstra CommitterDate: Wed, 29 Jul 2020 16:14:24 +02:00 seqlock: Implement raw_seqcount_begin() in terms of raw_read_seqcount() raw_seqcount_begin() has the same code as raw_read_seqcount(), with the exception of masking the sequence counter's LSB before returning it to the caller. Note, raw_seqcount_begin() masks the counter's LSB before returning it to the caller so that read_seqcount_retry() can fail if the counter is odd -- without the overhead of an extra branching instruction. Signed-off-by: Ahmed S. Darwish Signed-off-by: Peter Zijlstra (Intel) Link: https://lkml.kernel.org/r/20200720155530.1173732-7-a.darwish@linutronix.de --- include/linux/seqlock.h | 9 +++++---- 1 file changed, 5 insertions(+), 4 deletions(-) diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h index 85fb3ac..e885702 100644 --- a/include/linux/seqlock.h +++ b/include/linux/seqlock.h @@ -199,10 +199,11 @@ static inline unsigned raw_read_seqcount(const seqcount_t *s) */ 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; + /* + * If the counter is odd, let read_seqcount_retry() fail + * by decrementing the counter. + */ + return raw_read_seqcount(s) & ~1; } /**