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Message-ID: <52cc5910-af30-42d3-85a3-5eab06fea658@quicinc.com>
Date: Thu, 24 Aug 2023 11:43:02 +0530
From: Pavan Kondeti <quic_pkondeti@...cinc.com>
To: Kassey Li <quic_yingangl@...cinc.com>
CC: Peter Zijlstra <peterz@...radead.org>, <mingo@...hat.com>,
<will@...nel.org>, <boqun.feng@...il.com>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: seqlock:do_raw_write_seqcount_end smp_wmb
On Thu, Aug 24, 2023 at 09:07:57AM +0800, Kassey Li wrote:
> hi, Peter:
> I not quit sure on this, but is that a type error in
> do_raw_write_seqcount_end here ?
> is this change necessary to match with do_raw_write_seqcount_begin ?
>
>
> diff --git a/include/linux/seqlock.h b/include/linux/seqlock.h
> index 987a59d977c5..eb8807ed3a00 100644
> --- a/include/linux/seqlock.h
> +++ b/include/linux/seqlock.h
> @@ -486,8 +486,8 @@ do { \
>
> static inline void do_raw_write_seqcount_end(seqcount_t *s)
> {
> - smp_wmb();
> s->sequence++;
> + smp_wmb();
> kcsan_nestable_atomic_end();
> }
>
>
> to match with
>
> static inline void do_raw_write_seqcount_begin(seqcount_t *s)
> {
> kcsan_nestable_atomic_begin();
> s->sequence++;
> smp_wmb();
> }
>
>
>
I don't see anything wrong here. As per my understanding,
do_raw_write_seqcount_end() marks the end of the writer side critical
section. The data modified in the critical section should be visisble to
the *reader* before seqcount::sequence increment is visible. The
smp_wmb() here pairs with the smp_rmb() in the read_seqcount_begin().
Pls see kernel/sched/cputime.c for an example.
Thanks,
Pavan
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