@@ -41,8 +41,16 @@ typedef struct poll_table_struct {
static inline void poll_wait(struct file * filp, wait_queue_head_t * wait_address, poll_table *p)
{
- if (p && p->_qproc && wait_address)
+ if (p && p->_qproc && wait_address) {
p->_qproc(filp, wait_address, p);
+ /*
+ * This memory barrier is paired in the wq_has_sleeper().
+ * See the comment above prepare_to_wait(), we need to
+ * ensure that subsequent tests in this thread can't be
+ * reordered with __add_wait_queue() in _qproc() paths.
+ */
+ smp_mb();
+ }
}
/*
As the comment above waitqueue_active() explains, it can only be used if both waker and waiter have mb()'s that pair with each other. However __pollwait() is broken in this respect. This is not pipe-specific, but let's look at pipe_poll() for example: poll_wait(...); // -> __pollwait() -> add_wait_queue() LOAD(pipe->head); LOAD(pipe->head); In theory these LOAD()'s can leak into the critical section inside add_wait_queue() and can happen before list_add(entry, wq_head), in this case pipe_poll() can race with wakeup_pipe_readers/writers which do smp_mb(); if (waitqueue_active(wq_head)) wake_up_interruptible(wq_head); There are more __pollwait()-like functions (grep init_poll_funcptr), and it seems that at least ep_ptable_queue_proc() has the same problem, so the patch adds smp_mb() into poll_wait(). Link: https://lore.kernel.org/all/20250102163320.GA17691@redhat.com/ Signed-off-by: Oleg Nesterov <oleg@redhat.com> --- include/linux/poll.h | 10 +++++++++- 1 file changed, 9 insertions(+), 1 deletion(-)