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

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Date:   Sat, 28 Oct 2023 09:52:05 +1100
From:   "NeilBrown" <neilb@...e.de>
To:     "Oleg Nesterov" <oleg@...hat.com>
Cc:     "Chuck Lever" <chuck.lever@...cle.com>,
        "Jeff Layton" <jlayton@...nel.org>,
        "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 Fri, 27 Oct 2023, 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.

Wow! read_seqbegin_or_lock() has never locked since

Commit: 88a411c07b6f ("seqlock: livelock fix") in Linux v2.6.26 (2008).

That's rather embarrassing.

I agree we don't need the lock on the read-side for nfsd.

Reviewed-by: NeilBrown <neilb@...e.de>

NeilBrown


> 
> 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
> 
> 
>