[PATCH] shm_destroy lock hang

[Hugh Dickins]

Martin reported "Bug with shared memory" to LKML 14 May: hang due to
schedule in truncate_list_pages called from .... shm_destroy holding
the shm_lock spinlock.  shm_destroy needs that lock for shm_rmid, but
it can be safely unlocked once link from id to shp has been removed.
Patch against 2.4.19-rc1 or -pre10-ac2, applies at offset to 2.5.24.

--- 2.4.19-rc1/ipc/shm.c	Mon Jun 24 19:14:58 2002
+++ linux/ipc/shm.c	Thu Jun 27 19:34:51 2002
@@ -117,12 +117,14 @@
  *
  * @shp: struct to free
  *
- * It has to be called with shp and shm_ids.sem locked
+ * It has to be called with shp and shm_ids.sem locked,
+ * but returns with shp unlocked and freed.
  */
 static void shm_destroy (struct shmid_kernel *shp)
 {
 	shm_tot -= (shp->shm_segsz + PAGE_SIZE - 1) >> PAGE_SHIFT;
 	shm_rmid (shp->id);
+	shm_unlock(shp->id);
 	shmem_lock(shp->shm_file, 0);
 	fput (shp->shm_file);
 	kfree (shp);
@@ -150,8 +152,8 @@
 	if(shp->shm_nattch == 0 &&
 	   shp->shm_flags & SHM_DEST)
 		shm_destroy (shp);
-
-	shm_unlock(id);
+	else
+		shm_unlock(id);
 	up (&shm_ids.sem);
 }
 
@@ -511,11 +513,9 @@
 			shp->shm_flags |= SHM_DEST;
 			/* Do not find it any more */
 			shp->shm_perm.key = IPC_PRIVATE;
+			shm_unlock(shmid);
 		} else
 			shm_destroy (shp);
-
-		/* Unlock */
-		shm_unlock(shmid);
 		up(&shm_ids.sem);
 		return err;
 	}
@@ -653,7 +653,8 @@
 	if(shp->shm_nattch == 0 &&
 	   shp->shm_flags & SHM_DEST)
 		shm_destroy (shp);
-	shm_unlock(shmid);
+	else
+		shm_unlock(shmid);
 	up (&shm_ids.sem);
 
 	*raddr = (unsigned long) user_addr;