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vfs: check submounts and drop atomically

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We check submounts before doing d_drop() on a non-empty directory dentry in
NFS (have_submounts()), but we do not exclude a racing mount.

 Process A: have_submounts() -> returns false
 Process B: mount() -> success
 Process A: d_drop()

This patch prepares the ground for the fix by doing the following
operations all under the same rename lock:

  have_submounts()
  shrink_dcache_parent()
  d_drop()

This is actually an optimization since have_submounts() and
shrink_dcache_parent() both traverse the same dentry tree separately.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
CC: David Howells <dhowells@redhat.com>
CC: Steven Whitehouse <swhiteho@redhat.com>
CC: Trond Myklebust <Trond.Myklebust@netapp.com>
CC: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
This commit is contained in:
Miklos Szeredi 2013-09-05 11:44:36 +02:00 committed by Al Viro
parent db14fc3abc
commit 848ac114e8
2 changed files with 66 additions and 0 deletions
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65
fs/dcache.c
View File

@ -1264,6 +1264,71 @@ void shrink_dcache_parent(struct dentry *parent)
}
EXPORT_SYMBOL(shrink_dcache_parent);
static enum d_walk_ret check_and_collect(void *_data, struct dentry *dentry)
{
struct select_data *data = _data;
if (d_mountpoint(dentry)) {
data->found = -EBUSY;
return D_WALK_QUIT;
}
return select_collect(_data, dentry);
}
static void check_and_drop(void *_data)
{
struct select_data *data = _data;
if (d_mountpoint(data->start))
data->found = -EBUSY;
if (!data->found)
__d_drop(data->start);
}
/**
* check_submounts_and_drop - prune dcache, check for submounts and drop
*
* All done as a single atomic operation relative to has_unlinked_ancestor().
* Returns 0 if successfully unhashed @parent. If there were submounts then
* return -EBUSY.
*
* @dentry: dentry to prune and drop
*/
int check_submounts_and_drop(struct dentry *dentry)
{
int ret = 0;
/* Negative dentries can be dropped without further checks */
if (!dentry->d_inode) {
d_drop(dentry);
goto out;
}
for (;;) {
struct select_data data;
INIT_LIST_HEAD(&data.dispose);
data.start = dentry;
data.found = 0;
d_walk(dentry, &data, check_and_collect, check_and_drop);
ret = data.found;
if (!list_empty(&data.dispose))
shrink_dentry_list(&data.dispose);
if (ret <= 0)
break;
cond_resched();
}
out:
return ret;
}
EXPORT_SYMBOL(check_submounts_and_drop);
/**
* __d_alloc - allocate a dcache entry
* @sb: filesystem it will belong to

1
include/linux/dcache.h
View File

@ -253,6 +253,7 @@ extern void d_prune_aliases(struct inode *);
/* test whether we have any submounts in a subdir tree */
extern int have_submounts(struct dentry *);
extern int check_submounts_and_drop(struct dentry *);
/*
* This adds the entry to the hash queues.