linux-kernel - Re: nfsd_copy_write_verifier: wrong usage of read_seqbegin_or

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Date:   Wed, 25 Oct 2023 14:07:38 -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: nfsd_copy_write_verifier: wrong usage of read_seqbegin_or_lock()

On Wed, Oct 25, 2023 at 07:54:36PM +0200, Oleg Nesterov wrote:
> On 10/25, Chuck Lever wrote:
> >
> > > Another question is why we can't simply turn nn->writeverf into seqcount_t.
> > > I guess we can't because nfsd_reset_write_verifier() needs spin_lock() to
> > > serialise with itself, right?
> >
> > "reset" is supposed to be very rare operation. Using a lock in that
> > case is probably quite acceptable, as long as reading the verifier
> > is wait-free and guaranteed to be untorn.
> >
> > But a seqcount_t is only 32 bits.
> 
> Again, I don't understand you.
> 
> Once again, we can turn writeverf into seqcount_t, see the patch below.

The patch below does not turn "writeverf" into a seqcount_t, it
turns "writeverf_lock" into a seqcount_t. "writeverf" is an 8-byte
field, that's why I said "seqcount_t is only 32 bits" -- that is
not an adequate replacement for the 8-byte "writeverf" field.

Your original proposal made no sense. But I see now what you
would like to change.

I'm not familiar enough with these primitives to have a strong
opinion. What do you think would be the benefit?


> But this way nfsd_reset_write_verifier() can race with itself, no?

> Oleg
> ---
> 
> diff --git a/fs/nfsd/netns.h b/fs/nfsd/netns.h
> index ec49b200b797..3e2adf3eb15f 100644
> --- a/fs/nfsd/netns.h
> +++ b/fs/nfsd/netns.h
> @@ -110,7 +110,7 @@ struct nfsd_net {
>  	bool nfsd_net_up;
>  	bool lockd_up;
>  
> -	seqlock_t writeverf_lock;
> +	seqcount_t writeverf_lock;
>  	unsigned char writeverf[8];
>  
>  	/*
> diff --git a/fs/nfsd/nfsctl.c b/fs/nfsd/nfsctl.c
> index 7ed02fb88a36..6320491018f8 100644
> --- a/fs/nfsd/nfsctl.c
> +++ b/fs/nfsd/nfsctl.c
> @@ -1523,7 +1523,7 @@ static __net_init int nfsd_net_init(struct net *net)
>  	nn->nfsd4_minorversions = NULL;
>  	nfsd4_init_leases_net(nn);
>  	get_random_bytes(&nn->siphash_key, sizeof(nn->siphash_key));
> -	seqlock_init(&nn->writeverf_lock);
> +	seqcount_init(&nn->writeverf_lock);
>  
>  	return 0;
>  
> diff --git a/fs/nfsd/nfssvc.c b/fs/nfsd/nfssvc.c
> index c7af1095f6b5..fc4e31411508 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;
> +	int seq;
>  
>  	do {
> -		read_seqbegin_or_lock(&nn->writeverf_lock, &seq);
> +		seq = read_seqcount_begin(&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_seqcount_retry(&nn->writeverf_lock, seq));
>  }
>  
>  static void nfsd_reset_write_verifier_locked(struct nfsd_net *nn)
> @@ -397,9 +396,9 @@ static void nfsd_reset_write_verifier_locked(struct nfsd_net *nn)
>   */
>  void nfsd_reset_write_verifier(struct nfsd_net *nn)
>  {
> -	write_seqlock(&nn->writeverf_lock);
> +	write_seqcount_begin(&nn->writeverf_lock);
>  	nfsd_reset_write_verifier_locked(nn);
> -	write_sequnlock(&nn->writeverf_lock);
> +	write_seqcount_end(&nn->writeverf_lock);
>  }
>  
>  /*
> 

-- 
Chuck Lever