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Date: Fri, 21 Oct 2016 18:47:27 +0200
From: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To: Trond Myklebust <trond.myklebust@...marydata.com>
Cc: Anna Schumaker <anna.schumaker@...app.com>,
linux-nfs@...r.kernel.org, linux-kernel@...r.kernel.org,
tglx@...utronix.de,
Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: [RFC PATCH] 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").
I don't understand how it is possible that we don't end up with two
writers for the same resource because the `sp->so_lock' lock is dropped
is soon in the list_for_each_entry() loop. It might be the
test_and_clear_bit() check in nfs4_do_reclaim() but it might clear one
bit on each iteration so I *think* we could have two invocations of the
same struct nfs4_state_owner in nfs4_reclaim_open_state().
So there is that.
But back to the list_for_each_entry() macro.
It seems that this `so_lock' lock protects the ->so_states list among
other atomic_t & flags members. So at the begin of the loop we inc ->count
ensuring that this field is not removed while we use it. So we drop the
->so_lock lock during the loop. And after nfs4_reclaim_locks() invocation we
nfs4_put_open_state() and then grab the ->so_lock again. So if we were the last
user of this struct and we remove it, then the following list_next_entry()
invocation is a use-after-free. Even if we use list_for_each_entry_safe() there
is no guarantee that the following member is still valid because it might have
been removed by something that invoked nfs4_put_open_state(), right?
So there is this.
However to address my initial problem I have here a patch :) So it uses
a seqlock_t which ensures that there is only one writer at a time. So it
should be basically what is happening now plus a tiny tiny tiny lock
plus lockdep coverage. I tried to test this myself but I don't manage to get
into this code path at all so I might be doing something wrong.
Could you please check if this patch is working for you and whether my
list_for_each_entry() observation is correct or not?
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
---
fs/nfs/delegation.c | 4 ++--
fs/nfs/nfs4_fs.h | 2 +-
fs/nfs/nfs4proc.c | 4 ++--
fs/nfs/nfs4state.c | 10 ++++------
4 files changed, 9 insertions(+), 11 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 ad917bd72b38..3d6be8405c08 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..74be6378ca08 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.
*/
+ write_seqlock(&sp->so_reclaim_seqlock);
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);
+ write_sequnlock(&sp->so_reclaim_seqlock);
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);
+ write_sequnlock(&sp->so_reclaim_seqlock);
return status;
}
--
2.9.3
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