Re: [PATCH 04/14 V2] xfs: avoid nesting transactions in xfs_qm_scall_setqlim()

Brian Foster
2013-05-21T10:54:03Z

On 05/20/2013 08:36 PM, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> 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@gmail.com>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> V2: fix error handling path that failed to release the dquot.
>

Looks good.

Reviewed-by: Brian Foster <bfoster@redhat.com>

> fs/xfs/xfs_qm_syscalls.c | 40 +++++++++++++++++++++++-----------------
> 1 file changed, 23 insertions(+), 17 deletions(-)
>
> diff --git a/fs/xfs/xfs_qm_syscalls.c b/fs/xfs/xfs_qm_syscalls.c
> index c41190c..6cdf6ff 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_rele;
> + }
> +
> + xfs_dqlock(dqp);
> xfs_trans_dqjoin(tp, dqp);
> ddq = &dqp->q_core;
>
> @@ -621,9 +626,10 @@ xfs_qm_scall_setqlim(
> xfs_trans_log_dquot(tp, dqp);
>
> error = xfs_trans_commit(tp, 0);
> - xfs_qm_dqrele(dqp);
>
> - out_unlock:
> +out_rele:
> + xfs_qm_dqrele(dqp);
> +out_unlock:
> mutex_unlock(&q->qi_quotaofflock);
> return error;
> }
>
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