Two (or more) concurrent calls of shrink_dcache_parent() on the same
dentry with the right timing may cause shrink_dcache_parent() to loop
forever.
Here's what appears to happen:
1 - CPU0: select_parent(P) finds C and puts it on sb->s_dentry_lru, returns 1
2 - CPU0: __shrink_dcache_sb() moves C from sb->s_dentry_lru to tmp
3 - CPU1: select_parent(P) locks P->d_lock
4 - CPU0: shrink_dentry_list() locks C->d_lock
dentry_kill(C) tries to lock P->d_lock but fails, unlocks C->d_lock
5 - CPU1: select_parent(P) locks C->d_lock,
moves C from tmp to sb->s_dentry_lru, returns 1
6 - CPU0: shrink_dentry_list() finds tmp empty, returns
7 - Goto 2 with CPU0 and CPU1 switched
So basically select_parent() steals the dentry from shrink_dentry_list()
and thinks it found a new one, causing shrink_dentry_list() to think
it's making progress and loop over and over.
This was actually triggered by a combination of the bonding driver
removing a directory in sysfs and a udev rule causing udev to do stat
multiple times on the just removed directory.
One fix would be to check all callers and prevent concurrent calls to
shrink_dcache_parent(). But I think a better solution is to stop the
stealing behavior.
This patch adds a new dentry flag that is set when the dentry is removed
from the lru and put on the list being processed by
shrink_dentry_list(). The flag is cleared in dentry_lru_del() which is
called if the dentry gets a new reference just before it's pruned.
Thoughts?
diff --git a/fs/dcache.c b/fs/dcache.c
index 89509b5..09805eb 100644
--- a/fs/dcache.c
+++ b/fs/dcache.c
@@ -242,6 +242,7 @@ static void dentry_lru_add(struct dentry *dentry)
static void __dentry_lru_del(struct dentry *dentry)
{
list_del_init(&dentry->d_lru);
+ dentry->d_flags &= ~DCACHE_SHRINK_LIST;
dentry->d_sb->s_nr_dentry_unused--;
dentry_stat.nr_unused--;
}
@@ -807,6 +808,7 @@ relock:
spin_unlock(&dentry->d_lock);
} else {
list_move_tail(&dentry->d_lru, &tmp);
+ dentry->d_flags |= DCACHE_SHRINK_LIST;
spin_unlock(&dentry->d_lock);
if (!--count)
break;
@@ -1117,11 +1119,11 @@ resume:
* move only zero ref count dentries to the end
* of the unused list for prune_dcache
*/
- if (!dentry->d_count) {
+ if (dentry->d_count) {
+ dentry_lru_del(dentry);
+ } else if(!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
dentry_lru_move_tail(dentry);
found++;
- } else {
- dentry_lru_del(dentry);
}
/*
diff --git a/include/linux/dcache.h b/include/linux/dcache.h
index ed9f74f..4eb8c80 100644
--- a/include/linux/dcache.h
+++ b/include/linux/dcache.h
@@ -203,6 +203,7 @@ struct dentry_operations {
#define DCACHE_CANT_MOUNT 0x0100
#define DCACHE_GENOCIDE 0x0200
+#define DCACHE_SHRINK_LIST 0x0400
#define DCACHE_NFSFS_RENAMED 0x1000
/* this dentry has been "silly renamed" and has to be deleted on the last
--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
