[fuse] fix deadlock between fuse_put_super() and request_end(), try #2

A deadlock was possible, when the last reference to the superblock was
held due to a background request containing a file reference.

Releasing the file would release the vfsmount which in turn would
release the superblock.  Since sbput_sem is held during the fput() and
fuse_put_super() tries to acquire this same semaphore, a deadlock
results.

The solution is to move the fput() outside the region protected by
sbput_sem.

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>

1 files changed, 11 insertions, 2 deletions

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 4967bd40b953..104a62dadb94 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -128,12 +128,16 @@ void fuse_put_request(struct fuse_conn *fc, struct fuse_req *req)
         }
 }
 
+/*
+ * Called with sbput_sem held for read (request_end) or write
+ * (fuse_put_super).  By the time fuse_put_super() is finished, all
+ * inodes belonging to background requests must be released, so the
+ * iputs have to be done within the locked region.
+ */
 void fuse_release_background(struct fuse_conn *fc, struct fuse_req *req)
 {
         iput(req->inode);
         iput(req->inode2);
-        if (req->file)
-                fput(req->file);
         spin_lock(&fc->lock);
         list_del(&req->bg_entry);
         if (fc->num_background == FUSE_MAX_BACKGROUND) {
@@ -178,6 +182,11 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
                 if (fc->mounted)
                         fuse_release_background(fc, req);
                 up_read(&fc->sbput_sem);
+
+                /* fput must go outside sbput_sem, otherwise it can deadlock */
+                if (req->file)
+                        fput(req->file);
+
                 if (end)
                         end(fc, req);
                 else