[PATCH] io_uring: fix IO hang in io_wq_put_and_exit from do_exit()

Ming Lei
2023-08-31T07:43:30Z

io_wq_put_and_exit() is called from do_exit(), but all requests in io_wq
aren't cancelled in io_uring_cancel_generic() called from do_exit().
Meantime io_wq IO code path may share resource with normal iopoll code
path.

So if any HIPRI request is pending in io_wq_submit_work(), this request
may not get resouce for moving on, given iopoll isn't possible in
io_wq_put_and_exit().

The issue can be triggered when terminating 't/io_uring -n4 /dev/nullb0'
with default null_blk parameters.

Fix it by always cancelling all requests in io_wq from io_uring_cancel_generic(),
and this way is reasonable because io_wq destroying follows cancelling
requests immediately. Based on one patch from Chengming.

Closes: https://lore.kernel.org/linux-block/3893581.1691785261@warthog.procyon.org.uk/
Reported-by: David Howells <dhowells@redhat.com>
Cc: Chengming Zhou <zhouchengming@bytedance.com>,
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
io_uring/io_uring.c | 40 ++++++++++++++++++++++++++++------------
1 file changed, 28 insertions(+), 12 deletions(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index e7675355048d..18d5ab969c29 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -144,7 +144,7 @@ struct io_defer_entry {

static bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
struct task_struct *task,
- bool cancel_all);
+ bool cancel_all, bool *wq_cancelled);

static void io_queue_sqe(struct io_kiocb *req);

@@ -3049,7 +3049,7 @@ static __cold void io_ring_exit_work(struct work_struct *work)
if (ctx->flags & IORING_SETUP_DEFER_TASKRUN)
io_move_task_work_from_local(ctx);

- while (io_uring_try_cancel_requests(ctx, NULL, true))
+ while (io_uring_try_cancel_requests(ctx, NULL, true, NULL))
cond_resched();

if (ctx->sq_data) {
@@ -3231,12 +3231,13 @@ static __cold bool io_uring_try_cancel_iowq(struct io_ring_ctx *ctx)

static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
struct task_struct *task,
- bool cancel_all)
+ bool cancel_all, bool *wq_cancelled)
{
- struct io_task_cancel cancel = { .task = task, .all = cancel_all, };
+ struct io_task_cancel cancel = { .task = task, .all = true, };
struct io_uring_task *tctx = task ? task->io_uring : NULL;
enum io_wq_cancel cret;
bool ret = false;
+ bool wq_active = false;

/* set it so io_req_local_work_add() would wake us up */
if (ctx->flags & IORING_SETUP_DEFER_TASKRUN) {
@@ -3249,7 +3250,7 @@ static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
return false;

if (!task) {
- ret |= io_uring_try_cancel_iowq(ctx);
+ wq_active = io_uring_try_cancel_iowq(ctx);
} else if (tctx && tctx->io_wq) {
/*
* Cancels requests of all rings, not only @ctx, but
@@ -3257,11 +3258,20 @@ static __cold bool io_uring_try_cancel_requests(struct io_ring_ctx *ctx,
*/
cret = io_wq_cancel_cb(tctx->io_wq, io_cancel_task_cb,
&cancel, true);
- ret |= (cret != IO_WQ_CANCEL_NOTFOUND);
+ wq_active = (cret != IO_WQ_CANCEL_NOTFOUND);
}
+ ret |= wq_active;
+ if (wq_cancelled)
+ *wq_cancelled = !wq_active;

- /* SQPOLL thread does its own polling */
- if ((!(ctx->flags & IORING_SETUP_SQPOLL) && cancel_all) ||
+ /*
+ * SQPOLL thread does its own polling
+ *
+ * io_wq may share IO resources(such as requests) with iopoll, so
+ * iopoll requests have to be reapped for providing forward
+ * progress to io_wq cancelling
+ */
+ if (!(ctx->flags & IORING_SETUP_SQPOLL) ||
(ctx->sq_data && ctx->sq_data->thread == current)) {
while (!wq_list_empty(&ctx->iopoll_list)) {
io_iopoll_try_reap_events(ctx);
@@ -3313,11 +3323,12 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
atomic_inc(&tctx->in_cancel);
do {
bool loop = false;
+ bool wq_cancelled;

io_uring_drop_tctx_refs(current);
/* read completions before cancelations */
inflight = tctx_inflight(tctx, !cancel_all);
- if (!inflight)
+ if (!inflight && !tctx->io_wq)
break;

if (!sqd) {
@@ -3326,20 +3337,25 @@ __cold void io_uring_cancel_generic(bool cancel_all, struct io_sq_data *sqd)
if (node->ctx->sq_data)
continue;
loop |= io_uring_try_cancel_requests(node->ctx,
- current, cancel_all);
+ current, cancel_all,
+ &wq_cancelled);
}
} else {
list_for_each_entry(ctx, &sqd->ctx_list, sqd_list)
loop |= io_uring_try_cancel_requests(ctx,
current,
- cancel_all);
+ cancel_all,
+ &wq_cancelled);
}

- if (loop) {
+ if (!wq_cancelled || (inflight && loop)) {
cond_resched();
continue;
}

+ if (!inflight)
+ break;
+
prepare_to_wait(&tctx->wait, &wait, TASK_INTERRUPTIBLE);
io_run_task_work();
io_uring_drop_tctx_refs(current);
--
2.41.0