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Subject[PATCH] staging/lustre/llite: Close atomic_open race with several openers
Date
Right now, if it's an open of a negative dentry, a race is possible
with several openers who all try to instantiate/rehash the same
dentry and would hit a BUG_ON in d_add.
But in fact if we got a negative dentry in atomic_open, that means
we just revalidated it so no point in talking to MDS at all,
just return ENOENT and make the race go away completely.

Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
---
This problem was introduced during current development cycle, with
parallel openers now possible for non-creates due to vfs changes,
so it would be great if this could go into Linus' tree separatel
in time for 4.7

drivers/staging/lustre/lustre/llite/namei.c | 43 ++++++++++++++++-------------
1 file changed, 24 insertions(+), 19 deletions(-)

diff --git a/drivers/staging/lustre/lustre/llite/namei.c b/drivers/staging/lustre/lustre/llite/namei.c
index 3664bfd..2c4dc69 100644
--- a/drivers/staging/lustre/lustre/llite/namei.c
+++ b/drivers/staging/lustre/lustre/llite/namei.c
@@ -388,6 +388,7 @@ static int ll_lookup_it_finish(struct ptlrpc_request *request,
struct inode *inode = NULL;
__u64 bits = 0;
int rc = 0;
+ struct dentry *alias;

/* NB 1 request reference will be taken away by ll_intent_lock()
* when I return
@@ -412,26 +413,12 @@ static int ll_lookup_it_finish(struct ptlrpc_request *request,
*/
}

- /* Only hash *de if it is unhashed (new dentry).
- * Atoimc_open may passing hashed dentries for open.
- */
- if (d_unhashed(*de)) {
- struct dentry *alias;
-
- alias = ll_splice_alias(inode, *de);
- if (IS_ERR(alias)) {
- rc = PTR_ERR(alias);
- goto out;
- }
- *de = alias;
- } else if (!it_disposition(it, DISP_LOOKUP_NEG) &&
- !it_disposition(it, DISP_OPEN_CREATE)) {
- /* With DISP_OPEN_CREATE dentry will be
- * instantiated in ll_create_it.
- */
- LASSERT(!d_inode(*de));
- d_instantiate(*de, inode);
+ alias = ll_splice_alias(inode, *de);
+ if (IS_ERR(alias)) {
+ rc = PTR_ERR(alias);
+ goto out;
}
+ *de = alias;

if (!it_disposition(it, DISP_LOOKUP_NEG)) {
/* we have lookup look - unhide dentry */
@@ -587,6 +574,24 @@ static int ll_atomic_open(struct inode *dir, struct dentry *dentry,
dentry, PFID(ll_inode2fid(dir)), dir, file, open_flags, mode,
*opened);

+ /* Only negative dentries enter here */
+ LASSERT(!d_inode(dentry));
+
+ if (!d_in_lookup(dentry)) {
+ /* A valid negative dentry that just passed revalidation,
+ * there's little point to try and open it server-side,
+ * even though there's a minuscle chance it might succeed.
+ * Either way it's a valid race to just return -ENOENT here.
+ */
+ if (!(open_flags & O_CREAT))
+ return -ENOENT;
+
+ /* Otherwise we just unhash it to be rehashed afresh via
+ * lookup if necessary
+ */
+ d_drop(dentry);
+ }
+
it = kzalloc(sizeof(*it), GFP_NOFS);
if (!it)
return -ENOMEM;
--
2.7.4
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