[PATCH] fuse: fix request_end() vs fuse_reset_request() race

The last fix for this function in fact opened up a much more often
triggering race.

It was uncommented tricky code, that was buggy.  Add comment, make it less
tricky and fix bug.

Signed-off-by: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/fs/fuse/dev.c b/fs/fuse/dev.c
index 4526da8..f556a0d 100644 (file)
--- a/fs/fuse/dev.c
+++ b/fs/fuse/dev.c
@@ -120,9 +120,9 @@ struct fuse_req *fuse_get_request(struct fuse_conn *fc)
        return do_get_request(fc);
 }
 
+/* Must be called with fuse_lock held */
 static void fuse_putback_request(struct fuse_conn *fc, struct fuse_req *req)
 {
-       spin_lock(&fuse_lock);
        if (req->preallocated) {
                atomic_dec(&fc->num_waiting);
                list_add(&req->list, &fc->unused_list);
@@ -134,10 +134,18 @@ static void fuse_putback_request(struct fuse_conn *fc, struct fuse_req *req)
                fc->outstanding_debt--;
        else
                up(&fc->outstanding_sem);
-       spin_unlock(&fuse_lock);
 }
 
 void fuse_put_request(struct fuse_conn *fc, struct fuse_req *req)
+{
+       if (atomic_dec_and_test(&req->count)) {
+               spin_lock(&fuse_lock);
+               fuse_putback_request(fc, req);
+               spin_unlock(&fuse_lock);
+       }
+}
+
+static void fuse_put_request_locked(struct fuse_conn *fc, struct fuse_req *req)
 {
        if (atomic_dec_and_test(&req->count))
                fuse_putback_request(fc, req);
@@ -163,26 +171,36 @@ void fuse_release_background(struct fuse_req *req)
  * still waiting), the 'end' callback is called if given, else the
  * reference to the request is released
  *
+ * Releasing extra reference for foreground requests must be done
+ * within the same locked region as setting state to finished.  This
+ * is because fuse_reset_request() may be called after request is
+ * finished and it must be the sole possessor.  If request is
+ * interrupted and put in the background, it will return with an error
+ * and hence never be reset and reused.
+ *
  * Called with fuse_lock, unlocks it
  */
 static void request_end(struct fuse_conn *fc, struct fuse_req *req)
 {
-       void (*end) (struct fuse_conn *, struct fuse_req *) = req->end;
-       req->end = NULL;
        list_del(&req->list);
        req->state = FUSE_REQ_FINISHED;
-       spin_unlock(&fuse_lock);
-       if (req->background) {
+       if (!req->background) {
+               wake_up(&req->waitq);
+               fuse_put_request_locked(fc, req);
+               spin_unlock(&fuse_lock);
+       } else {
+               void (*end) (struct fuse_conn *, struct fuse_req *) = req->end;
+               req->end = NULL;
+               spin_unlock(&fuse_lock);
                down_read(&fc->sbput_sem);
                if (fc->mounted)
                        fuse_release_background(req);
                up_read(&fc->sbput_sem);
+               if (end)
+                       end(fc, req);
+               else
+                       fuse_put_request(fc, req);
        }
-       wake_up(&req->waitq);
-       if (end)
-               end(fc, req);
-       else
-               fuse_put_request(fc, req);
 }
 
 /*