io_uring: fix lack of protection for compl_nr

coml_nr in ctx_flush_and_put() is not protected by uring_lock, this
may cause problems when accessing in parallel:

say coml_nr > 0

  ctx_flush_and put                  other context
   if (compl_nr)                      get mutex
                                      coml_nr > 0
                                      do flush
                                          coml_nr = 0
                                      release mutex
        get mutex
           do flush (*)
        release mutex

in (*) place, we call io_cqring_ev_posted() and users likely get
no events there. To avoid spurious events, re-check the value when
under the lock.

Fixes: 2c32395d ("io_uring: fix __tctx_task_work() ctx race")
Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20210820221954.61815-1-haoxu@linux.alibaba.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2007,7 +2007,8 @@ static void ctx_flush_and_put(struct io_ring_ctx *ctx)
 		return;
 	if (ctx->submit_state.compl_nr) {
 		mutex_lock(&ctx->uring_lock);
-		io_submit_flush_completions(ctx);
+		if (ctx->submit_state.compl_nr)
+			io_submit_flush_completions(ctx);
 		mutex_unlock(&ctx->uring_lock);
 	}
 	percpu_ref_put(&ctx->refs);