linux-kernel - [tip: locking/core] seqlock: seqcount_t latch: End read sections with read_seqcount

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Message-ID: <159603322133.4006.2775605538857308036.tip-bot2@tip-bot2>
Date:   Wed, 29 Jul 2020 14:33:41 -0000
From:   "tip-bot2 for Ahmed S. Darwish" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     "Ahmed S. Darwish" <a.darwish@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: [tip: locking/core] seqlock: seqcount_t latch: End read sections with
 read_seqcount_retry()

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     d3b35b87f436c1b226a8061bee9c8875ba6658bd
Gitweb:        https://git.kernel.org/tip/d3b35b87f436c1b226a8061bee9c8875ba6658bd
Author:        Ahmed S. Darwish <a.darwish@...utronix.de>
AuthorDate:    Mon, 20 Jul 2020 17:55:09 +02:00
Committer:     Peter Zijlstra <peterz@...radead.org>
CommitterDate: Wed, 29 Jul 2020 16:14:23 +02:00

seqlock: seqcount_t latch: End read sections with read_seqcount_retry()

The seqcount_t latch reader example at the raw_write_seqcount_latch()
kernel-doc comment ends the latch read section with a manual smp memory
barrier and sequence counter comparison.

This is technically correct, but it is suboptimal: read_seqcount_retry()
already contains the same logic of an smp memory barrier and sequence
counter comparison.

End the latch read critical section example with read_seqcount_retry().

Signed-off-by: Ahmed S. Darwish <a.darwish@...utronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20200720155530.1173732-4-a.darwish@linutronix.de
---
 include/linux/seqlock.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
index 6c4f68e..d724b5e 100644
--- a/include/linux/seqlock.h
+++ b/include/linux/seqlock.h
@@ -363,8 +363,8 @@ static inline int raw_read_seqcount_latch(seqcount_t *s)
  *			idx = seq & 0x01;
  *			entry = data_query(latch->data[idx], ...);
  *
- *			smp_rmb();
- *		} while (seq != latch->seq);
+ *		// read_seqcount_retry() includes needed smp_rmb()
+ *		} while (read_seqcount_retry(&latch->seq, seq));
  *
  *		return entry;
  *	}