linux-kernel - nfsd_copy_write_verifier: wrong usage of read_seqbegin_or

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Date:   Wed, 25 Oct 2023 18:30:06 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Chuck Lever <chuck.lever@...cle.com>,
        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>
Cc:     Ingo Molnar <mingo@...hat.com>, linux-nfs@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: nfsd_copy_write_verifier: wrong usage of read_seqbegin_or_lock()

Hello,

The usage of writeverf_lock is wrong and misleading no matter what and
I can not understand the intent.

nfsd_copy_write_verifier() uses read_seqbegin_or_lock() incorrectly.
"seq" is always even, so read_seqbegin_or_lock() can never take the
lock for writing. We need to make the counter odd for the 2nd round:

	--- a/fs/nfsd/nfssvc.c
	+++ b/fs/nfsd/nfssvc.c
	@@ -359,11 +359,14 @@ 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, nextseq = 0;

		do {
	+		seq = nextseq;
			read_seqbegin_or_lock(&nn->writeverf_lock, &seq);
			memcpy(verf, nn->writeverf, sizeof(nn->writeverf));
	+		/* If lockless access failed, take the lock. */
	+		nextseq = 1;
		} while (need_seqretry(&nn->writeverf_lock, seq));
		done_seqretry(&nn->writeverf_lock, seq);
	 }

OTOH. This function just copies 8 bytes, this makes me think that it doesn't
need the conditional locking and read_seqbegin_or_lock() at all. So perhaps
the (untested) patch below makes more sense? Please note that it should not
change the current behaviour, it just makes the code look correct (and more
optimal but this is minor).

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?

Oleg.
---

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)