Re: [PATCH 4/7] xfs: lockdep annotations for xfs_dqlock2

Christoph Hellwig
2009-01-12T15:16:18Z

On Mon, Jan 12, 2009 at 10:06:37AM +1100, Dave Chinner wrote:
> This looks a bit wierd.
>
> Yes, xfs_dqlock() is just a wrapper around mutex_lock, but we should
> be consistent here. Can you add a xfs_dqlock_nested() wrapper to do
> this?

I don't think we should add more of the silly wrappers. What about
the version below that always uses plain mutex_lock* in xfs_dqlock2?

---

Subject: xfs: lockdep annotations for xfs_dqlock2
From: Christoph Hellwig <hch@lst.de>

xfs_dqlock2 locks two xfs_dquots, which is fine as it always locks the
dquot with the lower id first. Use mutex_lock_nested to tell lockdep
about this fact. Also clean up xfs_dqlock2 a bit by rationalizing
the conditionals and always using the mutex_lock family of functions
directly.

Signed-off-by: Christoph Hellwig <hch@lst.de>

Index: xfs/fs/xfs/quota/xfs_dquot.c
===================================================================
--- xfs.orig/fs/xfs/quota/xfs_dquot.c 2009-01-12 14:37:49.445796033 +0100
+++ xfs/fs/xfs/quota/xfs_dquot.c 2009-01-12 14:46:59.913795998 +0100
@@ -1383,6 +1383,12 @@ xfs_dqunlock_nonotify(
mutex_unlock(&(dqp->q_qlock));
}

+/*
+ * Lock two xfs_dquot structures.
+ *
+ * To avoid deadlocks we always lock the quota structure with
+ * the lowerd id first.
+ */
void
xfs_dqlock2(
xfs_dquot_t *d1,
@@ -1392,18 +1398,16 @@ xfs_dqlock2(
ASSERT(d1 != d2);
if (be32_to_cpu(d1->q_core.d_id) >
be32_to_cpu(d2->q_core.d_id)) {
- xfs_dqlock(d2);
- xfs_dqlock(d1);
+ mutex_lock(&d2->q_qlock);
+ mutex_lock_nested(&d1->q_qlock, XFS_QLOCK_NESTED);
} else {
- xfs_dqlock(d1);
- xfs_dqlock(d2);
- }
- } else {
- if (d1) {
- xfs_dqlock(d1);
- } else if (d2) {
- xfs_dqlock(d2);
+ mutex_lock(&d1->q_qlock);
+ mutex_lock_nested(&d2->q_qlock, XFS_QLOCK_NESTED);
}
+ } else if (d1) {
+ mutex_lock(&d1->q_qlock);
+ } else if (d2) {
+ mutex_lock(&d2->q_qlock);
}
}

Index: xfs/fs/xfs/quota/xfs_dquot.h
===================================================================
--- xfs.orig/fs/xfs/quota/xfs_dquot.h 2009-01-12 14:37:49.459795861 +0100
+++ xfs/fs/xfs/quota/xfs_dquot.h 2009-01-12 14:45:30.902817471 +0100
@@ -97,6 +97,16 @@ typedef struct xfs_dquot {
#define dq_hashlist q_lists.dqm_hashlist
#define dq_flags q_lists.dqm_flags

+/*
+ * Lock hierachy for q_qlock:
+ * XFS_QLOCK_NORMAL is the implicit default,
+ * XFS_QLOCK_NESTED is the dquot with the higher id in xfs_dqlock2
+ */
+enum {
+ XFS_QLOCK_NORMAL = 0,
+ XFS_QLOCK_NESTED,
+};
+
#define XFS_DQHOLD(dqp) ((dqp)->q_nrefs++)

#ifdef DEBUG