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[PATCH 4/5] xfs: avoid nesting transactions in xfs_qm_scall_setqlim()

To: xfs@xxxxxxxxxxx
Subject: [PATCH 4/5] xfs: avoid nesting transactions in xfs_qm_scall_setqlim()
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Fri, 17 May 2013 11:10:28 +1000
Delivered-to: xfs@xxxxxxxxxxx
In-reply-to: <1368753029-9977-1-git-send-email-david@xxxxxxxxxxxxx>
References: <1368753029-9977-1-git-send-email-david@xxxxxxxxxxxxx>
From: Dave Chinner <dchinner@xxxxxxxxxx>

Lockdep reports:

=============================================
[ INFO: possible recursive locking detected ]
3.9.0+ #3 Not tainted
---------------------------------------------
setquota/28368 is trying to acquire lock:
 (sb_internal){++++.?}, at: [<c11e8846>] xfs_trans_alloc+0x26/0x50

but task is already holding lock:
 (sb_internal){++++.?}, at: [<c11e8846>] xfs_trans_alloc+0x26/0x50

from xfs_qm_scall_setqlim()->xfs_dqread() when a dquot needs to be
allocated.

xfs_qm_scall_setqlim() is starting a transaction and then not
passing it into xfs_qm_dqet() and so it starts it's own transaction
when allocating the dquot.  Splat!

Fix this by not allocating the dquot in xfs_qm_scall_setqlim()
inside the setqlim transaction. This requires getting the dquot
first (and allocating it if necessary) then dropping and relocking
the dquot before joining it to the setqlim transaction.

Reported-by: Michael L. Semon <mlsemon35@xxxxxxxxx>
Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
---
 fs/xfs/xfs_qm_syscalls.c |   35 ++++++++++++++++++++---------------
 1 file changed, 20 insertions(+), 15 deletions(-)

diff --git a/fs/xfs/xfs_qm_syscalls.c b/fs/xfs/xfs_qm_syscalls.c
index c41190c..dfa5c05 100644
--- a/fs/xfs/xfs_qm_syscalls.c
+++ b/fs/xfs/xfs_qm_syscalls.c
@@ -489,31 +489,36 @@ xfs_qm_scall_setqlim(
        if ((newlim->d_fieldmask & XFS_DQ_MASK) == 0)
                return 0;
 
-       tp = xfs_trans_alloc(mp, XFS_TRANS_QM_SETQLIM);
-       error = xfs_trans_reserve(tp, 0, XFS_QM_SETQLIM_LOG_RES(mp),
-                                 0, 0, XFS_DEFAULT_LOG_COUNT);
-       if (error) {
-               xfs_trans_cancel(tp, 0);
-               return (error);
-       }
-
        /*
         * We don't want to race with a quotaoff so take the quotaoff lock.
-        * (We don't hold an inode lock, so there's nothing else to stop
-        * a quotaoff from happening). (XXXThis doesn't currently happen
-        * because we take the vfslock before calling xfs_qm_sysent).
+        * We don't hold an inode lock, so there's nothing else to stop
+        * a quotaoff from happening.
         */
        mutex_lock(&q->qi_quotaofflock);
 
        /*
-        * Get the dquot (locked), and join it to the transaction.
-        * Allocate the dquot if this doesn't exist.
+        * Get the dquot (locked) before we start, as we need to do a
+        * transaction to allocate it if it doesn't exist. Once we have the
+        * dquot, unlock it so we can start the next transaction safely. We hold
+        * a reference to the dquot, so it's safe to do this unlock/lock without
+        * it being reclaimed in the mean time.
         */
-       if ((error = xfs_qm_dqget(mp, NULL, id, type, XFS_QMOPT_DQALLOC, 
&dqp))) {
-               xfs_trans_cancel(tp, XFS_TRANS_ABORT);
+       error = xfs_qm_dqget(mp, NULL, id, type, XFS_QMOPT_DQALLOC, &dqp);
+       if (error) {
                ASSERT(error != ENOENT);
                goto out_unlock;
        }
+       xfs_dqunlock(dqp);
+
+       tp = xfs_trans_alloc(mp, XFS_TRANS_QM_SETQLIM);
+       error = xfs_trans_reserve(tp, 0, XFS_QM_SETQLIM_LOG_RES(mp),
+                                 0, 0, XFS_DEFAULT_LOG_COUNT);
+       if (error) {
+               xfs_trans_cancel(tp, 0);
+               goto out_unlock;
+       }
+
+       xfs_dqlock(dqp);
        xfs_trans_dqjoin(tp, dqp);
        ddq = &dqp->q_core;
 
-- 
1.7.10.4

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