[fuse] fix deadlock between fuse_put_super() and request_end()

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 chosen soltuion is to get rid of sbput_sem, and instead use the
spinlock to ensure the referenced inodes/file are released only once.
Since the actual release may sleep, defer these outside the locked
region, but using local variables instead of the structure members.

This is a much more rubust solution.

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

1 files changed, 16 insertions, 12 deletions

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 6c740f860665..d4efb6223e2c 100644
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -120,20 +120,14 @@ void fuse_put_request(struct fuse_conn *fc, struct fuse_req *req)
         }
 }
 
-void fuse_release_background(struct fuse_conn *fc, struct fuse_req *req)
+void fuse_remove_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);
+        list_del_init(&req->bg_entry);
         if (fc->num_background == FUSE_MAX_BACKGROUND) {
                 fc->blocked = 0;
                 wake_up_all(&fc->blocked_waitq);
         }
         fc->num_background--;
-        spin_unlock(&fc->lock);
 }
 
 /*
@@ -163,17 +157,27 @@ static void request_end(struct fuse_conn *fc, struct fuse_req *req)
                 wake_up(&req->waitq);
                 fuse_put_request(fc, req);
         } else {
+                struct inode *inode = req->inode;
+                struct inode *inode2 = req->inode2;
+                struct file *file = req->file;
                 void (*end) (struct fuse_conn *, struct fuse_req *) = req->end;
                 req->end = NULL;
+                req->inode = NULL;
+                req->inode2 = NULL;
+                req->file = NULL;
+                if (!list_empty(&req->bg_entry))
+                        fuse_remove_background(fc, req);
                 spin_unlock(&fc->lock);
-                down_read(&fc->sbput_sem);
-                if (fc->mounted)
-                        fuse_release_background(fc, req);
-                up_read(&fc->sbput_sem);
+
                 if (end)
                         end(fc, req);
                 else
                         fuse_put_request(fc, req);
+
+                if (file)
+                        fput(file);
+                iput(inode);
+                iput(inode2);
         }
 }