io_uring/timeout: don't flush with the sequence behind the last flush

io_flush_timeouts() measures the events since the last flush as the
distance of the current sequence from ->cq_last_tm_flush. A fired
timeout bumps ->cq_timeouts before its CQE is posted, so until it is
the sequence can be behind the last flush, and the distance wraps:

  last flush: seq 100, cq_last_tm_flush = 100
  Y: IORING_OP_TIMEOUT, off 50, target_seq 150
  pure timeout A fires, cq_timeouts = 1
  task_work posts A's CQE, cached_cq_tail = 101
  pure timeout B fires before the flush, cq_timeouts = 2
  io_flush_timeouts(): seq = 99, events_got = 0xffffffff

Y, and every sequence timeout behind it, is ended with -ETIME although
no event happened. Two timeouts expiring close together is all it takes,
a completion with IOSQE_CQE_SKIP_SUCCESS after a single one does it too.

Nothing but those in flight CQEs separates the two, so count from the
last flush when the sequence is behind it, as io_timeout() does.

Fixes: f010505b78 ("io_uring: flush timeouts that should already have expired")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/timeout.c

@@ -171,6 +171,14 @@ __cold void io_flush_timeouts(struct io_ring_ctx *ctx)
 	raw_spin_lock_irq(&ctx->timeout_lock);
 	seq = READ_ONCE(ctx->cached_cq_tail) - atomic_read(&ctx->cq_timeouts);
 
+	/*
+	 * A fired timeout bumps ->cq_timeouts before its CQE is posted, which
+	 * leaves the sequence behind the last flush until it is. No event has
+	 * happened then, see io_timeout().
+	 */
+	if ((s32)(seq - ctx->cq_last_tm_flush) < 0)
+		seq = ctx->cq_last_tm_flush;
+
 	list_for_each_entry_safe(timeout, tmp, &ctx->timeout_list, list) {
 		struct io_kiocb *req = cmd_to_io_kiocb(timeout);
 		u32 events_needed, events_got;