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-rw-r--r--fs/pipe.c18
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))