diff options
| -rw-r--r-- | fs/pipe.c | 18 |
1 files changed, 15 insertions, 3 deletions
diff --git a/fs/pipe.c b/fs/pipe.c index b901c8eefafd..3e8b11e3b764 100644 --- a/fs/pipe.c +++ b/fs/pipe.c @@ -574,12 +574,24 @@ pipe_poll(struct file *filp, poll_table *wait) { __poll_t mask; struct pipe_inode_info *pipe = filp->private_data; - unsigned int head = READ_ONCE(pipe->head); - unsigned int tail = READ_ONCE(pipe->tail); + unsigned int head, tail; + /* + * Reading only -- no need for acquiring the semaphore. + * + * But because this is racy, the code has to add the + * entry to the poll table _first_ .. + */ poll_wait(filp, &pipe->wait, wait); - /* Reading only -- no need for acquiring the semaphore. */ + /* + * .. and only then can you do the racy tests. That way, + * if something changes and you got it wrong, the poll + * table entry will wake you up and fix it. + */ + head = READ_ONCE(pipe->head); + tail = READ_ONCE(pipe->tail); + mask = 0; if (filp->f_mode & FMODE_READ) { if (!pipe_empty(head, tail)) |