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
