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Message-ID: <ZTvc0Z6DJEYXI/TL@tissot.1015granger.net>
Date: Fri, 27 Oct 2023 11:52:49 -0400
From: Chuck Lever <chuck.lever@...cle.com>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Jeff Layton <jlayton@...nel.org>, Neil Brown <neilb@...e.de>,
Olga Kornievskaia <kolga@...app.com>,
Dai Ngo <Dai.Ngo@...cle.com>, Tom Talpey <tom@...pey.com>,
Ingo Molnar <mingo@...hat.com>, linux-nfs@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] nfsd_copy_write_verifier: use read_seqbegin() rather
than read_seqbegin_or_lock()
On Thu, Oct 26, 2023 at 04:50:18PM +0200, Oleg Nesterov wrote:
> The usage of read_seqbegin_or_lock() in nfsd_copy_write_verifier()
> is wrong. "seq" is always even and thus "or_lock" has no effect,
> this code can never take ->writeverf_lock for writing.
>
> I guess this is fine, nfsd_copy_write_verifier() just copies 8 bytes
> and nfsd_reset_write_verifier() is supposed to be very rare operation
> so we do not need the adaptive locking in this case.
>
> Yet the code looks wrong and sub-optimal, it can use read_seqbegin()
> without changing the behaviour.
I was debating whether to add Fixes/Cc-stable, but if the behavior
doesn't change, this doesn't need a backport.
Just waiting now on R-b.
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
> fs/nfsd/nfssvc.c | 7 +++----
> 1 file changed, 3 insertions(+), 4 deletions(-)
>
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index c7af1095f6b5..094b765c5397 100644
> --- a/fs/nfsd/nfssvc.c
> +++ b/fs/nfsd/nfssvc.c
> @@ -359,13 +359,12 @@ static bool nfsd_needs_lockd(struct nfsd_net *nn)
> */
> void nfsd_copy_write_verifier(__be32 verf[2], struct nfsd_net *nn)
> {
> - int seq = 0;
> + unsigned seq;
>
> do {
> - read_seqbegin_or_lock(&nn->writeverf_lock, &seq);
> + seq = read_seqbegin(&nn->writeverf_lock);
> memcpy(verf, nn->writeverf, sizeof(nn->writeverf));
> - } while (need_seqretry(&nn->writeverf_lock, seq));
> - done_seqretry(&nn->writeverf_lock, seq);
> + } while (read_seqretry(&nn->writeverf_lock, seq));
> }
>
> static void nfsd_reset_write_verifier_locked(struct nfsd_net *nn)
> --
> 2.25.1.362.g51ebf55
>
>
--
Chuck Lever
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