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

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Date:   Wed, 25 Oct 2023 19:54:36 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Chuck Lever <chuck.lever@...cle.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 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.

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);
 }
 
 /*