Re: [syzbot] INFO: task hung in io_wqe_worker

[Pavel Begunkov <asml.silence@gmail.com>]

On 10/22/21 14:49, Pavel Begunkov wrote:
> On 10/22/21 05:38, syzbot wrote:
>> Hello,
>>
>> syzbot has tested the proposed patch but the reproducer is still triggering an issue:
>> INFO: task hung in io_wqe_worker
>>
>> INFO: task iou-wrk-9392:9401 blocked for more than 143 seconds.
>>        Not tainted 5.15.0-rc2-syzkaller #0
>> "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> task:iou-wrk-9392    state:D stack:27952 pid: 9401 ppid:  7038 flags:0x00004004
>> Call Trace:
>>   context_switch kernel/sched/core.c:4940 [inline]
>>   __schedule+0xb44/0x5960 kernel/sched/core.c:6287
>>   schedule+0xd3/0x270 kernel/sched/core.c:6366
>>   schedule_timeout+0x1db/0x2a0 kernel/time/timer.c:1857
>>   do_wait_for_common kernel/sched/completion.c:85 [inline]
>>   __wait_for_common kernel/sched/completion.c:106 [inline]
>>   wait_for_common kernel/sched/completion.c:117 [inline]
>>   wait_for_completion+0x176/0x280 kernel/sched/completion.c:138
>>   io_worker_exit fs/io-wq.c:183 [inline]
>>   io_wqe_worker+0x66d/0xc40 fs/io-wq.c:597
>>   ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:295
> 
> Easily reproducible, it's stuck in
> 
> static void io_worker_exit(struct io_worker *worker)
> {
>      ...
>      wait_for_completion(&worker->ref_done);
>      ...
> }
> 
> The reference belongs to a create_worker_cb() task_work item. It's expected
> to either be executed or cancelled by io_wq_exit_workers(), but the owner
> task never goes __io_uring_cancel (called in do_exit()) and so never
> reaches io_wq_exit_workers().
> 
> Following the owner task, cat /proc/<pid>/stack:
> 
> [<0>] do_coredump+0x1d0/0x10e0
> [<0>] get_signal+0x4a3/0x960
> [<0>] arch_do_signal_or_restart+0xc3/0x6d0
> [<0>] exit_to_user_mode_prepare+0x10e/0x190
> [<0>] irqentry_exit_to_user_mode+0x9/0x20
> [<0>] irqentry_exit+0x36/0x40
> [<0>] exc_page_fault+0x95/0x190
> [<0>] asm_exc_page_fault+0x1e/0x30
> 
> (gdb) l *(do_coredump+0x1d0-5)
> 0xffffffff81343ccb is in do_coredump (fs/coredump.c:469).
> 464
> 465             if (core_waiters > 0) {
> 466                     struct core_thread *ptr;
> 467
> 468                     freezer_do_not_count();
> 469                     wait_for_completion(&core_state->startup);
> 470                     freezer_count();
> 
> Can't say anything more at the moment as not familiar with coredump

A simple hack allowing task works to be executed from there
workarounds the problem


diff --git a/fs/coredump.c b/fs/coredump.c
index 3224dee44d30..f6f9dfb02296 100644
--- a/fs/coredump.c
+++ b/fs/coredump.c
@@ -466,7 +466,8 @@ static int coredump_wait(int exit_code, struct core_state *core_state)
  		struct core_thread *ptr;
  
  		freezer_do_not_count();
-		wait_for_completion(&core_state->startup);
+		while (wait_for_completion_interruptible(&core_state->startup))
+			tracehook_notify_signal();
  		freezer_count();
  		/*
  		 * Wait for all the threads to become inactive, so that



-- 
Pavel Begunkov