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fix shrink_dcache_parent() livelock

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Two (or more) concurrent calls of shrink_dcache_parent() on the same dentry 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 dispose list, returns 1

2 - CPU1: select_parent(P) locks P->d_lock

3 - CPU0: shrink_dentry_list() locks C->d_lock
   dentry_kill(C) tries to lock P->d_lock but fails, unlocks C->d_lock

4 - CPU1: select_parent(P) locks C->d_lock,
         moves C from dispose list being processed on CPU0 to the new
dispose list, returns 1

5 - CPU0: shrink_dentry_list() finds dispose list empty, returns

6 - Goto 2 with CPU0 and CPU1 switched

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.

One way to trigger this is to make udev calls stat() on the sysfs file while it
is going away.

Having a file in /lib/udev/rules.d/ with only this one rule seems to the trick:

ATTR{vendor}=="0x8086", ATTR{device}=="0x10ca", ENV{PCI_SLOT_NAME}="%k", ENV{MATCHADDR}="$attr{address}", RUN+="/bin/true"

Then execute the following loop:

while true; do
        echo -bond0 > /sys/class/net/bonding_masters
        echo +bond0 > /sys/class/net/bonding_masters
        echo -bond1 > /sys/class/net/bonding_masters
        echo +bond1 > /sys/class/net/bonding_masters
done

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 added to the
dispose list.  The flag is cleared in dentry_lru_del() in case the dentry gets a
new reference just before being pruned.

If the dentry has this flag, select_parent() will skip it and let
shrink_dentry_list() retry pruning it.  With select_parent() skipping those
dentries there will not be the appearance of progress (new dentries found) when
there is none, hence shrink_dcache_parent() will not loop forever.

Set the flag is also set in prune_dcache_sb() for consistency as suggested by
Linus.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: stable@vger.kernel.org
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Miklos Szeredi 2012-01-10 18:22:25 +01:00 committed by Al Viro
parent f4947fbce2
commit eaf5f90735
2 changed files with 13 additions and 5 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

17
fs/dcache.c
View file

@ -243,6 +243,7 @@ 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--;
}
@ -806,6 +807,7 @@ void prune_dcache_sb(struct super_block *sb, int count)
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;
@ -1097,14 +1099,19 @@ static int select_parent(struct dentry *parent, struct list_head *dispose)
/*
* move only zero ref count dentries to the dispose list.
*
* Those which are presently on the shrink list, being processed
* by shrink_dentry_list(), shouldn't be moved. Otherwise the
* loop in shrink_dcache_parent() might not make any progress
* and loop forever.
*/
if (!dentry->d_count) {
dentry_lru_move_list(dentry, dispose);
found++;
} else {
if (dentry->d_count) {
dentry_lru_del(dentry);
} else if (!(dentry->d_flags & DCACHE_SHRINK_LIST)) {
dentry_lru_move_list(dentry, dispose);
dentry->d_flags |= DCACHE_SHRINK_LIST;
found++;
}
/*
* We can return to the caller if we have found some (this
* ensures forward progress). We'll be coming back to find

1
include/linux/dcache.h
View file

@ -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