nfsd: don't call locks_release_private() twice concurrently

It is possible for free_blocked_lock() to be called twice concurrently,
once from nfsd4_lock() and once from nfsd4_release_lockowner() calling
remove_blocked_locks().  This is why a kref was added.

It is perfectly safe for locks_delete_block() and kref_put() to be
called in parallel as they use locking or atomicity respectively as
protection.  However locks_release_private() has no locking.  It is
safe for it to be called twice sequentially, but not concurrently.

This patch moves that call from free_blocked_lock() where it could race
with itself, to free_nbl() where it cannot.  This will slightly delay
the freeing of private info or release of the owner - but not by much.
It is arguably more natural for this freeing to happen in free_nbl()
where the structure itself is freed.

This bug was found by code inspection - it has not been seen in practice.

Fixes: 47446d74f170 ("nfsd4: add refcount for nfsd4_blocked_lock")
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 4464e4d8aef520759ce3af8fd51a5fd844849b7b..7ac07a67740cf15bc5cc6b41755abd676a4e0327 100644 (file)
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -318,6 +318,7 @@ free_nbl(struct kref *kref)
        struct nfsd4_blocked_lock *nbl;
 
        nbl = container_of(kref, struct nfsd4_blocked_lock, nbl_kref);
+       locks_release_private(&nbl->nbl_lock);
        kfree(nbl);
 }
 
@@ -325,7 +326,6 @@ static void
 free_blocked_lock(struct nfsd4_blocked_lock *nbl)
 {
        locks_delete_block(&nbl->nbl_lock);
-       locks_release_private(&nbl->nbl_lock);
        kref_put(&nbl->nbl_kref, free_nbl);
 }