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xfs: use a lockref for the xfs_dquot reference count

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The xfs_dquot structure currently uses the anti-pattern of using the
in-object lock that protects the content to also serialize reference
count updates for the structure, leading to a cumbersome free path.
This is partially papered over by the fact that we never free the dquot
directly but always through the LRU.  Switch to use a lockref instead and
move the reference counter manipulations out of q_qlock.

To make this work, xfs_qm_flush_one and xfs_qm_flush_one are converted to
acquire a dquot reference while flushing to integrate with the lockref
"get if not dead" scheme.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>
This commit is contained in:
Christoph Hellwig 2025-11-10 14:22:57 +01:00 committed by Carlos Maiolino
parent 6129b088e1
commit 0c5e80bd57
5 changed files with 37 additions and 42 deletions
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4
fs/xfs/libxfs/xfs_quota_defs.h
View File

@ -29,11 +29,9 @@ typedef uint8_t xfs_dqtype_t;
* flags for q_flags field in the dquot.
*/
#define XFS_DQFLAG_DIRTY (1u << 0) /* dquot is dirty */
#define XFS_DQFLAG_FREEING (1u << 1) /* dquot is being torn down */
#define XFS_DQFLAG_STRINGS \
{ XFS_DQFLAG_DIRTY, "DIRTY" }, \
{ XFS_DQFLAG_FREEING, "FREEING" }
{ XFS_DQFLAG_DIRTY, "DIRTY" }
/*
* We have the possibility of all three quota types being active at once, and

17
fs/xfs/xfs_dquot.c
View File

@ -816,20 +816,17 @@ xfs_qm_dqget_cache_lookup(
return NULL;
}
mutex_lock(&dqp->q_qlock);
if (dqp->q_flags & XFS_DQFLAG_FREEING) {
mutex_unlock(&dqp->q_qlock);
if (!lockref_get_not_dead(&dqp->q_lockref)) {
mutex_unlock(&qi->qi_tree_lock);
trace_xfs_dqget_freeing(dqp);
delay(1);
goto restart;
}
dqp->q_nrefs++;
mutex_unlock(&qi->qi_tree_lock);
trace_xfs_dqget_hit(dqp);
XFS_STATS_INC(mp, xs_qm_dqcachehits);
mutex_lock(&dqp->q_qlock);
return dqp;
}
@ -866,7 +863,7 @@ xfs_qm_dqget_cache_insert(
/* Return a locked dquot to the caller, with a reference taken. */
mutex_lock(&dqp->q_qlock);
dqp->q_nrefs = 1;
lockref_init(&dqp->q_lockref);
qi->qi_dquots++;
out_unlock:
@ -1124,18 +1121,22 @@ void
xfs_qm_dqput(
struct xfs_dquot *dqp)
{
ASSERT(dqp->q_nrefs > 0);
ASSERT(XFS_DQ_IS_LOCKED(dqp));
trace_xfs_dqput(dqp);
if (--dqp->q_nrefs == 0) {
if (lockref_put_or_lock(&dqp->q_lockref))
goto out_unlock;
if (!--dqp->q_lockref.count) {
struct xfs_quotainfo *qi = dqp->q_mount->m_quotainfo;
trace_xfs_dqput_free(dqp);
if (list_lru_add_obj(&qi->qi_lru, &dqp->q_lru))
XFS_STATS_INC(dqp->q_mount, xs_qm_dquot_unused);
}
spin_unlock(&dqp->q_lockref.lock);
out_unlock:
mutex_unlock(&dqp->q_qlock);
}

6
fs/xfs/xfs_dquot.h
View File

@ -71,7 +71,7 @@ struct xfs_dquot {
xfs_dqtype_t q_type;
uint16_t q_flags;
xfs_dqid_t q_id;
uint q_nrefs;
struct lockref q_lockref;
int q_bufoffset;
xfs_daddr_t q_blkno;
xfs_fileoff_t q_fileoffset;
@ -231,9 +231,7 @@ void xfs_dquot_detach_buf(struct xfs_dquot *dqp);
static inline struct xfs_dquot *xfs_qm_dqhold(struct xfs_dquot *dqp)
{
mutex_lock(&dqp->q_qlock);
dqp->q_nrefs++;
mutex_unlock(&dqp->q_qlock);
lockref_get(&dqp->q_lockref);
return dqp;
}

50
fs/xfs/xfs_qm.c
View File

@ -126,14 +126,16 @@ xfs_qm_dqpurge(
void *data)
{
struct xfs_quotainfo *qi = dqp->q_mount->m_quotainfo;
int error = -EAGAIN;
spin_lock(&dqp->q_lockref.lock);
if (dqp->q_lockref.count > 0 || __lockref_is_dead(&dqp->q_lockref)) {
spin_unlock(&dqp->q_lockref.lock);
return -EAGAIN;
}
lockref_mark_dead(&dqp->q_lockref);
spin_unlock(&dqp->q_lockref.lock);
mutex_lock(&dqp->q_qlock);
if ((dqp->q_flags & XFS_DQFLAG_FREEING) || dqp->q_nrefs != 0)
goto out_unlock;
dqp->q_flags |= XFS_DQFLAG_FREEING;
xfs_qm_dqunpin_wait(dqp);
xfs_dqflock(dqp);
@ -144,6 +146,7 @@ xfs_qm_dqpurge(
*/
if (XFS_DQ_IS_DIRTY(dqp)) {
struct xfs_buf *bp = NULL;
int error;
/*
* We don't care about getting disk errors here. We need
@ -151,9 +154,9 @@ xfs_qm_dqpurge(
*/
error = xfs_dquot_use_attached_buf(dqp, &bp);
if (error == -EAGAIN) {
xfs_dqfunlock(dqp);
dqp->q_flags &= ~XFS_DQFLAG_FREEING;
goto out_unlock;
/* resurrect the refcount from the dead. */
dqp->q_lockref.count = 0;
goto out_funlock;
}
if (!bp)
goto out_funlock;
@ -192,10 +195,6 @@ xfs_qm_dqpurge(
xfs_qm_dqdestroy(dqp);
return 0;
out_unlock:
mutex_unlock(&dqp->q_qlock);
return error;
}
/*
@ -468,7 +467,7 @@ xfs_qm_dquot_isolate(
struct xfs_qm_isolate *isol = arg;
enum lru_status ret = LRU_SKIP;
if (!mutex_trylock(&dqp->q_qlock))
if (!spin_trylock(&dqp->q_lockref.lock))
goto out_miss_busy;
/*
@ -476,7 +475,7 @@ xfs_qm_dquot_isolate(
* from the LRU, leave it for the freeing task to complete the freeing
* process rather than risk it being free from under us here.
*/
if (dqp->q_flags & XFS_DQFLAG_FREEING)
if (__lockref_is_dead(&dqp->q_lockref))
goto out_miss_unlock;
/*
@ -485,16 +484,15 @@ xfs_qm_dquot_isolate(
* again.
*/
ret = LRU_ROTATE;
if (XFS_DQ_IS_DIRTY(dqp) || atomic_read(&dqp->q_pincount) > 0) {
if (XFS_DQ_IS_DIRTY(dqp) || atomic_read(&dqp->q_pincount) > 0)
goto out_miss_unlock;
}
/*
* This dquot has acquired a reference in the meantime remove it from
* the freelist and try again.
*/
if (dqp->q_nrefs) {
mutex_unlock(&dqp->q_qlock);
if (dqp->q_lockref.count) {
spin_unlock(&dqp->q_lockref.lock);
XFS_STATS_INC(dqp->q_mount, xs_qm_dqwants);
trace_xfs_dqreclaim_want(dqp);
@ -518,10 +516,9 @@ xfs_qm_dquot_isolate(
/*
* Prevent lookups now that we are past the point of no return.
*/
dqp->q_flags |= XFS_DQFLAG_FREEING;
mutex_unlock(&dqp->q_qlock);
lockref_mark_dead(&dqp->q_lockref);
spin_unlock(&dqp->q_lockref.lock);
ASSERT(dqp->q_nrefs == 0);
list_lru_isolate_move(lru, &dqp->q_lru, &isol->dispose);
XFS_STATS_DEC(dqp->q_mount, xs_qm_dquot_unused);
trace_xfs_dqreclaim_done(dqp);
@ -529,7 +526,7 @@ xfs_qm_dquot_isolate(
return LRU_REMOVED;
out_miss_unlock:
mutex_unlock(&dqp->q_qlock);
spin_unlock(&dqp->q_lockref.lock);
out_miss_busy:
trace_xfs_dqreclaim_busy(dqp);
XFS_STATS_INC(dqp->q_mount, xs_qm_dqreclaim_misses);
@ -1467,9 +1464,10 @@ xfs_qm_flush_one(
struct xfs_buf *bp = NULL;
int error = 0;
if (!lockref_get_not_dead(&dqp->q_lockref))
return 0;
mutex_lock(&dqp->q_qlock);
if (dqp->q_flags & XFS_DQFLAG_FREEING)
goto out_unlock;
if (!XFS_DQ_IS_DIRTY(dqp))
goto out_unlock;
@ -1489,7 +1487,7 @@ xfs_qm_flush_one(
xfs_buf_delwri_queue(bp, buffer_list);
xfs_buf_relse(bp);
out_unlock:
mutex_unlock(&dqp->q_qlock);
xfs_qm_dqput(dqp);
return error;
}

2
fs/xfs/xfs_trace.h
View File

@ -1350,7 +1350,7 @@ DECLARE_EVENT_CLASS(xfs_dquot_class,
__entry->id = dqp->q_id;
__entry->type = dqp->q_type;
__entry->flags = dqp->q_flags;
__entry->nrefs = dqp->q_nrefs;
__entry->nrefs = data_race(dqp->q_lockref.count);
__entry->res_bcount = dqp->q_blk.reserved;
__entry->res_rtbcount = dqp->q_rtb.reserved;