[PATCH 1/2] io_uring: add a schedule point in io_free_batch_list()

[Yuanhe Shu]

Completed requests accumulate in submit_state.compl_reqs - deferred
completions queued by task_work handlers via io_req_complete_defer(),
plus everything io_do_iopoll() reaps - and are then all freed in one
io_free_batch_list() pass under ctx->uring_lock.  The loop has no
scheduling point.

Each iteration does real work: io_clean_op() freeing request
payloads, file, task and rsrc puts.  With debug features like KASAN
or kmemleak enabled, that work gets expensive enough that a large
batch can hold the CPU well beyond the soft lockup threshold.

Caught by io stress testing (stress-ng -a 1 --class io -t 7h) on a
384 CPU machine running a v6.6-based debug kernel (KASAN, kmemleak,
lockdep; CONFIG_PREEMPT_NONE=y): a stress-ng worker was looping
here past the watchdog threshold, entered from exit-time task_work,
until the machine panicked on "softlockup: hung tasks".  Frames
from the dump:

    io_clean_op
    io_free_batch_list
    __io_submit_flush_completions
    ctx_flush_and_put
    tctx_task_work
    task_work_run
    get_signal

The same test later caught a completion fallback work item holding
uring_lock for 671 seconds in this same loop ("BUG: soft lockup -
CPU#61 stuck for 671s").

Also measured on mainline v7.3-rc7 (192-CPU debug kernel with the
same debug features): a single pass freeing a 32K completion batch
took 443ms with no scheduling point in between.

Insert a cond_resched() in the loop, like other potentially long
io_uring loops already have.  The loop runs in process context with
ctx->uring_lock held, which is a mutex, so scheduling is allowed.

Assisted-by: LLM
Signed-off-by: Yuanhe Shu <xiangzao@xxxxxxxxxxxxxxxxx>
---
 io_uring/io_uring.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index cf5218d100d9..26cd91a91aa2 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -1123,6 +1123,7 @@ static void io_free_batch_list(struct io_ring_ctx *ctx,
 
 		node = req->comp_list.next;
 		io_req_add_to_cache(req, ctx);
+		cond_resched();
 	} while (node);
 }
 
-- 
2.43.7