Re: [patch 13/19] xfs: nest qm_dqfrlist_lock inside the dquot qlock

Ben Myers
2011-12-15T03:02:15Z

On Tue, Dec 06, 2011 at 04:58:19PM -0500, Christoph Hellwig wrote:
> Allow xfs_qm_dqput to work without trylock loops by nesting the freelist lock
> inside the dquot qlock. In turn that requires trylocks in the reclaim path
> instead, but given it's a classic tradeoff between fast and slow path, and
> we follow the model of the inode and dentry caches.
>
> Document our new lock order now that it has settled.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> Reviewed-by: Dave Chinner <dchinner@redhat.com>

Looks good.
Reviewed-by: Ben Myers <bpm@sgi.com>

> ---
> fs/xfs/xfs_dquot.c | 99 ++++++++++++++++++++---------------------------------
> fs/xfs/xfs_qm.c | 4 +-
> 2 files changed, 42 insertions(+), 61 deletions(-)
>
> Index: xfs/fs/xfs/xfs_dquot.c
> ===================================================================
> --- xfs.orig/fs/xfs/xfs_dquot.c 2011-12-06 15:46:19.730356139 +0100
> +++ xfs/fs/xfs/xfs_dquot.c 2011-12-06 15:51:23.630361773 +0100
> @@ -39,20 +39,19 @@
> #include "xfs_qm.h"
> #include "xfs_trace.h"
>
> -
> /*
> - LOCK ORDER
> -
> - inode lock (ilock)
> - dquot hash-chain lock (hashlock)
> - xqm dquot freelist lock (freelistlock
> - mount's dquot list lock (mplistlock)
> - user dquot lock - lock ordering among dquots is based on the uid or gid
> - group dquot lock - similar to udquots. Between the two dquots, the udquot
> - has to be locked first.
> - pin lock - the dquot lock must be held to take this lock.
> - flush lock - ditto.
> -*/
> + * Lock order:
> + *
> + * ip->i_lock
> + * qh->qh_lock
> + * qi->qi_dqlist_lock
> + * dquot->q_qlock (xfs_dqlock() and friends)
> + * dquot->q_flush (xfs_dqflock() and friends)
> + * xfs_Gqm->qm_dqfrlist_lock
> + *
> + * If two dquots need to be locked the order is user before group/project,
> + * otherwise by the lowest id first, see xfs_dqlock2.
> + */
>
> #ifdef DEBUG
> xfs_buftarg_t *xfs_dqerror_target;
> @@ -984,69 +983,49 @@ restart:
> */
> void
> xfs_qm_dqput(
> - xfs_dquot_t *dqp)
> + struct xfs_dquot *dqp)
> {
> - xfs_dquot_t *gdqp;
> + struct xfs_dquot *gdqp;
>
> ASSERT(dqp->q_nrefs > 0);
> ASSERT(XFS_DQ_IS_LOCKED(dqp));
>
> trace_xfs_dqput(dqp);
>
> - if (dqp->q_nrefs != 1) {
> - dqp->q_nrefs--;
> +recurse:
> + if (--dqp->q_nrefs > 0) {
> xfs_dqunlock(dqp);
> return;
> }
>
> - /*
> - * drop the dqlock and acquire the freelist and dqlock
> - * in the right order; but try to get it out-of-order first
> - */
> - if (!mutex_trylock(&xfs_Gqm->qm_dqfrlist_lock)) {
> - trace_xfs_dqput_wait(dqp);
> - xfs_dqunlock(dqp);
> - mutex_lock(&xfs_Gqm->qm_dqfrlist_lock);
> - xfs_dqlock(dqp);
> - }
> -
> - while (1) {
> - gdqp = NULL;
> + trace_xfs_dqput_free(dqp);
>
> - /* We can't depend on nrefs being == 1 here */
> - if (--dqp->q_nrefs == 0) {
> - trace_xfs_dqput_free(dqp);
> -
> - if (list_empty(&dqp->q_freelist)) {
> - list_add_tail(&dqp->q_freelist, &xfs_Gqm->qm_dqfrlist);
> - xfs_Gqm->qm_dqfrlist_cnt++;
> - }
> + mutex_lock(&xfs_Gqm->qm_dqfrlist_lock);
> + if (list_empty(&dqp->q_freelist)) {
> + list_add_tail(&dqp->q_freelist, &xfs_Gqm->qm_dqfrlist);
> + xfs_Gqm->qm_dqfrlist_cnt++;
> + }
> + mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
>
> - /*
> - * If we just added a udquot to the freelist, then
> - * we want to release the gdquot reference that
> - * it (probably) has. Otherwise it'll keep the
> - * gdquot from getting reclaimed.
> - */
> - if ((gdqp = dqp->q_gdquot)) {
> - /*
> - * Avoid a recursive dqput call
> - */
> - xfs_dqlock(gdqp);
> - dqp->q_gdquot = NULL;
> - }
> - }
> - xfs_dqunlock(dqp);
> + /*
> + * If we just added a udquot to the freelist, then we want to release
> + * the gdquot reference that it (probably) has. Otherwise it'll keep
> + * the gdquot from getting reclaimed.
> + */
> + gdqp = dqp->q_gdquot;
> + if (gdqp) {
> + xfs_dqlock(gdqp);
> + dqp->q_gdquot = NULL;
> + }
> + xfs_dqunlock(dqp);
>
> - /*
> - * If we had a group quota inside the user quota as a hint,
> - * release it now.
> - */
> - if (! gdqp)
> - break;
> + /*
> + * If we had a group quota hint, release it now.
> + */
> + if (gdqp) {
> dqp = gdqp;
> + goto recurse;
> }
> - mutex_unlock(&xfs_Gqm->qm_dqfrlist_lock);
> }
>
> /*
> Index: xfs/fs/xfs/xfs_qm.c
> ===================================================================
> --- xfs.orig/fs/xfs/xfs_qm.c 2011-12-06 15:48:38.753692050 +0100
> +++ xfs/fs/xfs/xfs_qm.c 2011-12-06 15:49:31.303693024 +0100
> @@ -1668,7 +1668,9 @@ xfs_qm_dqreclaim_one(void)
> restart:
> list_for_each_entry(dqp, &xfs_Gqm->qm_dqfrlist, q_freelist) {
> struct xfs_mount *mp = dqp->q_mount;
> - xfs_dqlock(dqp);
> +
> + if (!xfs_dqlock_nowait(dqp))
> + continue;
>
> /*
> * This dquot has already been grabbed by dqlookup.
>
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