Re: [PATCH] nfsd_copy_write_verifier: use read_seqbegin() rather than read_seqbegin_or_lock()

From: Oleg Nesterov <oleg@redhat.com>
To: Chuck Lever <chuck.lever@oracle.com>
Cc: Jeff Layton <jlayton@kernel.org>, Neil Brown <neilb@suse.de>,
	Olga Kornievskaia <kolga@netapp.com>,
	Dai Ngo <Dai.Ngo@oracle.com>, Tom Talpey <tom@talpey.com>,
	Ingo Molnar <mingo@redhat.com>,
	linux-nfs@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] nfsd_copy_write_verifier: use read_seqbegin() rather than read_seqbegin_or_lock()
Date: Fri, 27 Oct 2023 21:34:00 +0200	[thread overview]
Message-ID: <20231027193359.GB24128@redhat.com> (raw)
In-Reply-To: <ZTvc0Z6DJEYXI/TL@tissot.1015granger.net>

On 10/27, Chuck Lever wrote:
>
> 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.

Yes, yes, sorry for confusion. This code is not buggy. Just a) it looks
confusing because read_seqbegin_or_lock() doesn't do what it is supposed
to do, and b) I am going to change the semantics of done_seqretry() to
enforce the locking on the 2nd pass.

Chuck, I can reword the changelog to make it more clear and send V2 if
you think this makes sense.

Thanks,

Oleg.