linux-kernel - Re: [PATCH] nfsd_copy_write_verifier: use read_seqbegin() rather than read_seqbegin_or

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Date:   Fri, 27 Oct 2023 19:40:35 +0000
From:   Chuck Lever III <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 Mailing List <linux-nfs@...r.kernel.org>,
        "linux-kernel@...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 Oct 27, 2023, at 12:34 PM, Oleg Nesterov <oleg@...hat.com> wrote:
> 
> 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.

No confusion, the changelog is clear to me. I'm simply stating
my intention for other reviewers and the lore archive that I
will leave off Fixes/Cc-stable when I commit your patch.

So far there has been no review comment that suggests we need a v2.


--
Chuck Lever