linux-kernel - [PATCH v4] NFSv4: replace seqcount_t with a seqlock

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Date:   Wed, 2 Nov 2016 18:15:25 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     Trond Myklebust <trondmy@...marydata.com>
Cc:     Schumaker Anna <anna.schumaker@...app.com>,
        List Linux NFS Mailing <linux-nfs@...r.kernel.org>,
        List Linux Kernel Mailing <linux-kernel@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>
Subject: [PATCH v4] NFSv4: replace seqcount_t with a seqlock_t

The raw_write_seqcount_begin() in nfs4_reclaim_open_state() bugs me
because it maps to preempt_disable() in -RT which I can't have at this
point. So I took a look at the code.
It the lockdep part was removed in commit abbec2da13f0 ("NFS: Use
raw_write_seqcount_begin/end int nfs4_reclaim_open_state") because
lockdep complained. The whole seqcount thing was introduced in commit
c137afabe330 ("NFSv4: Allow the state manager to mark an open_owner as
being recovered").
The recovery threads runs only once.
write_seqlock() does not work on !RT because it disables preemption and it the
writer side is preemptible (has to remain so despite the fact that it will
block readers).

Reported-by: kernel test robot <xiaolong.ye@...el.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---

Is the lockdep okay in the read path. I would like to see the lockdep
warning on the warning side but for now it is disabled.

 fs/nfs/delegation.c |  4 ++--
 fs/nfs/nfs4_fs.h    |  2 +-
 fs/nfs/nfs4proc.c   |  4 ++--
 fs/nfs/nfs4state.c  | 10 ++++------
 5 files changed, 10 insertions(+), 12 deletions(-)

diff --git a/fs/nfs/delegation.c b/fs/nfs/delegation.c
index dff600ae0d74..d726d2e09353 100644
--- a/fs/nfs/delegation.c
+++ b/fs/nfs/delegation.c
@@ -150,11 +150,11 @@ static int nfs_delegation_claim_opens(struct inode *inode,
 		sp = state->owner;
 		/* Block nfs4_proc_unlck */
 		mutex_lock(&sp->so_delegreturn_mutex);
-		seq = raw_seqcount_begin(&sp->so_reclaim_seqcount);
+		seq = read_seqbegin(&sp->so_reclaim_seqlock);
 		err = nfs4_open_delegation_recall(ctx, state, stateid, type);
 		if (!err)
 			err = nfs_delegation_claim_locks(ctx, state, stateid);
-		if (!err && read_seqcount_retry(&sp->so_reclaim_seqcount, seq))
+		if (!err && read_seqretry(&sp->so_reclaim_seqlock, seq))
 			err = -EAGAIN;
 		mutex_unlock(&sp->so_delegreturn_mutex);
 		put_nfs_open_context(ctx);
diff --git a/fs/nfs/nfs4_fs.h b/fs/nfs/nfs4_fs.h
index 9b3a82abab07..9b5089bf0f2e 100644
--- a/fs/nfs/nfs4_fs.h
+++ b/fs/nfs/nfs4_fs.h
@@ -111,7 +111,7 @@ struct nfs4_state_owner {
 	unsigned long	     so_flags;
 	struct list_head     so_states;
 	struct nfs_seqid_counter so_seqid;
-	seqcount_t	     so_reclaim_seqcount;
+	seqlock_t	     so_reclaim_seqlock;
 	struct mutex	     so_delegreturn_mutex;
 };
 
diff --git a/fs/nfs/nfs4proc.c b/fs/nfs/nfs4proc.c
index 7897826d7c51..9b9d53cd85f9 100644
--- a/fs/nfs/nfs4proc.c
+++ b/fs/nfs/nfs4proc.c
@@ -2685,7 +2685,7 @@ static int _nfs4_open_and_get_state(struct nfs4_opendata *opendata,
 	unsigned int seq;
 	int ret;
 
-	seq = raw_seqcount_begin(&sp->so_reclaim_seqcount);
+	seq = raw_seqcount_begin(&sp->so_reclaim_seqlock.seqcount);
 
 	ret = _nfs4_proc_open(opendata);
 	if (ret != 0)
@@ -2723,7 +2723,7 @@ static int _nfs4_open_and_get_state(struct nfs4_opendata *opendata,
 	ctx->state = state;
 	if (d_inode(dentry) == state->inode) {
 		nfs_inode_attach_open_context(ctx);
-		if (read_seqcount_retry(&sp->so_reclaim_seqcount, seq))
+		if (read_seqretry(&sp->so_reclaim_seqlock, seq))
 			nfs4_schedule_stateid_recovery(server, state);
 	}
 out:
diff --git a/fs/nfs/nfs4state.c b/fs/nfs/nfs4state.c
index 5f4281ec5f72..400a9d4186de 100644
--- a/fs/nfs/nfs4state.c
+++ b/fs/nfs/nfs4state.c
@@ -488,7 +488,7 @@ nfs4_alloc_state_owner(struct nfs_server *server,
 	nfs4_init_seqid_counter(&sp->so_seqid);
 	atomic_set(&sp->so_count, 1);
 	INIT_LIST_HEAD(&sp->so_lru);
-	seqcount_init(&sp->so_reclaim_seqcount);
+	seqlock_init(&sp->so_reclaim_seqlock);
 	mutex_init(&sp->so_delegreturn_mutex);
 	return sp;
 }
@@ -1497,8 +1497,8 @@ static int nfs4_reclaim_open_state(struct nfs4_state_owner *sp, const struct nfs
 	 * recovering after a network partition or a reboot from a
 	 * server that doesn't support a grace period.
 	 */
+	raw_write_seqcount_begin(&sp->so_reclaim_seqlock.seqcount);
 	spin_lock(&sp->so_lock);
-	raw_write_seqcount_begin(&sp->so_reclaim_seqcount);
 restart:
 	list_for_each_entry(state, &sp->so_states, open_states) {
 		if (!test_and_clear_bit(ops->state_flag_bit, &state->flags))
@@ -1566,14 +1566,12 @@ static int nfs4_reclaim_open_state(struct nfs4_state_owner *sp, const struct nfs
 		spin_lock(&sp->so_lock);
 		goto restart;
 	}
-	raw_write_seqcount_end(&sp->so_reclaim_seqcount);
 	spin_unlock(&sp->so_lock);
+	raw_write_seqcount_end(&sp->so_reclaim_seqlock.seqcount);
 	return 0;
 out_err:
 	nfs4_put_open_state(state);
-	spin_lock(&sp->so_lock);
-	raw_write_seqcount_end(&sp->so_reclaim_seqcount);
-	spin_unlock(&sp->so_lock);
+	raw_write_seqcount_end(&sp->so_reclaim_seqlock.seqcount);
 	return status;
 }
 
-- 
2.10.2