io_uring: fix no lock protection for ctx->cq_extra

[ Upstream commit e302f1046f ]

ctx->cq_extra should be protected by completion lock so that the
req_need_defer() does the right check.

Cc: stable@vger.kernel.org
Signed-off-by: Hao Xu <haoxu@linux.alibaba.com>
Link: https://lore.kernel.org/r/20211125092103.224502-2-haoxu@linux.alibaba.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/io_uring.c

@@ -6573,11 +6573,14 @@ static bool io_drain_req(struct io_kiocb *req)
 	}
 
 	/* Still need defer if there is pending req in defer list. */
+	spin_lock(&ctx->completion_lock);
 	if (likely(list_empty_careful(&ctx->defer_list) &&
 		!(req->flags & REQ_F_IO_DRAIN))) {
+		spin_unlock(&ctx->completion_lock);
 		ctx->drain_active = false;
 		return false;
 	}
+	spin_unlock(&ctx->completion_lock);
 
 	seq = io_get_sequence(req);
 	/* Still a chance to pass the sequence check */