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From:   Thomas Gleixner <tglx@linutronix.de>
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To:     Peter Zijlstra <peterz@infradead.org>, bigeasy@linutronix.de
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Subject: Re: [RFC][PATCH 1/9] seqlock/latch: Provide
 raw_read_seqcount_latch_retry()
In-Reply-To: <20230508213147.448097252@infradead.org>
References: <20230508211951.901961964@infradead.org>
 <20230508213147.448097252@infradead.org>
Date:   Tue, 09 May 2023 10:13:20 +0200
Message-ID: <871qjp3otb.ffs@tglx>
MIME-Version: 1.0
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List-ID: <kvm.vger.kernel.org>
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On Mon, May 08 2023 at 23:19, Peter Zijlstra wrote:
> The read side of seqcount_latch consists of:
>
>   do {
>     seq = raw_read_seqcount_latch(&latch->seq);
>     ...
>   } while (read_seqcount_latch_retry(&latch->seq, seq));
>
> which is asymmetric in the raw_ department, and sure enough,
> read_seqcount_latch_retry() includes (explicit) instrumentation where
> raw_read_seqcount_latch() does not.
>
> This inconsistency becomes a problem when trying to use it from
> noinstr code. As such, fix it by renaming and re-implementing
> raw_read_seqcount_latch_retry() without the instrumentation.
>
> Specifically the instrumentation in question is kcsan_atomic_next(0)
> in do___read_seqcount_retry(). Loosing this annotation is not a
> problem because raw_read_seqcount_latch() does not pass through
> kcsan_atomic_next(KCSAN_SEQLOCK_REGION_MAX).
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Reviewed-by: Thomas Gleixner <tglx@linutronix.de>