Re: [PATCH] io_uring: prune request from overflow list on flush

[Jens Axboe <axboe@kernel.dk>]

On 2/13/20 5:17 PM, Jens Axboe wrote:
> Carter reported an issue where he could produce a stall on ring exit,
> when we're cleaning up requests that match the given file table. For
> this particular test case, a combination of a few things caused the
> issue:
> 
> - The cq ring was overflown
> - The request being canceled was in the overflow list
> 
> The combination of the above means that the cq overflow list holds a
> reference to the request. The request is canceled correctly, but since
> the overflow list holds a reference to it, the final put won't happen.
> Since the final put doesn't happen, the request remains in the inflight.
> Hence we never finish the cancelation flush.
> 
> Fix this by removing requests from the overflow list if we're canceling
> them.

What I queued up was a v2, only difference being that we increment the
overflow counter if we prune it. Below for reference:


commit 2ca10259b4189a433c309054496dd6af1415f992
Author: Jens Axboe <axboe@kernel.dk>
Date:   Thu Feb 13 17:17:35 2020 -0700

    io_uring: prune request from overflow list on flush
    
    Carter reported an issue where he could produce a stall on ring exit,
    when we're cleaning up requests that match the given file table. For
    this particular test case, a combination of a few things caused the
    issue:
    
    - The cq ring was overflown
    - The request being canceled was in the overflow list
    
    The combination of the above means that the cq overflow list holds a
    reference to the request. The request is canceled correctly, but since
    the overflow list holds a reference to it, the final put won't happen.
    Since the final put doesn't happen, the request remains in the inflight.
    Hence we never finish the cancelation flush.
    
    Fix this by removing requests from the overflow list if we're canceling
    them.
    
    Cc: stable@vger.kernel.org # 5.5
    Reported-by: Carter Li 李通洲 <carter.li@eoitek.com>
    Signed-off-by: Jens Axboe <axboe@kernel.dk>

diff --git a/fs/io_uring.c b/fs/io_uring.c
index 6d4e20d59729..5a826017ebb8 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -481,6 +481,7 @@ enum {
 	REQ_F_TIMEOUT_NOSEQ_BIT,
 	REQ_F_COMP_LOCKED_BIT,
 	REQ_F_NEED_CLEANUP_BIT,
+	REQ_F_OVERFLOW_BIT,
 };
 
 enum {
@@ -521,6 +522,8 @@ enum {
 	REQ_F_COMP_LOCKED	= BIT(REQ_F_COMP_LOCKED_BIT),
 	/* needs cleanup */
 	REQ_F_NEED_CLEANUP	= BIT(REQ_F_NEED_CLEANUP_BIT),
+	/* in overflow list */
+	REQ_F_OVERFLOW		= BIT(REQ_F_OVERFLOW_BIT),
 };
 
 /*
@@ -1103,6 +1106,7 @@ static bool io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force)
 		req = list_first_entry(&ctx->cq_overflow_list, struct io_kiocb,
 						list);
 		list_move(&req->list, &list);
+		req->flags &= ~REQ_F_OVERFLOW;
 		if (cqe) {
 			WRITE_ONCE(cqe->user_data, req->user_data);
 			WRITE_ONCE(cqe->res, req->result);
@@ -1155,6 +1159,7 @@ static void io_cqring_fill_event(struct io_kiocb *req, long res)
 			set_bit(0, &ctx->sq_check_overflow);
 			set_bit(0, &ctx->cq_check_overflow);
 		}
+		req->flags |= REQ_F_OVERFLOW;
 		refcount_inc(&req->refs);
 		req->result = res;
 		list_add_tail(&req->list, &ctx->cq_overflow_list);
@@ -6463,6 +6468,29 @@ static void io_uring_cancel_files(struct io_ring_ctx *ctx,
 		if (!cancel_req)
 			break;
 
+		if (cancel_req->flags & REQ_F_OVERFLOW) {
+			spin_lock_irq(&ctx->completion_lock);
+			list_del(&cancel_req->list);
+			cancel_req->flags &= ~REQ_F_OVERFLOW;
+			if (list_empty(&ctx->cq_overflow_list)) {
+				clear_bit(0, &ctx->sq_check_overflow);
+				clear_bit(0, &ctx->cq_check_overflow);
+			}
+			spin_unlock_irq(&ctx->completion_lock);
+
+			WRITE_ONCE(ctx->rings->cq_overflow,
+				atomic_inc_return(&ctx->cached_cq_overflow));
+
+			/*
+			 * Put inflight ref and overflow ref. If that's
+			 * all we had, then we're done with this request.
+			 */
+			if (refcount_sub_and_test(2, &cancel_req->refs)) {
+				io_put_req(cancel_req);
+				continue;
+			}
+		}
+
 		io_wq_cancel_work(ctx->io_wq, &cancel_req->work);
 		io_put_req(cancel_req);
 		schedule();

-- 
Jens Axboe