On 8/23/22 12:22 PM, Eric W. Biederman wrote: > Olivier Langlois <olivier@xxxxxxxxxxxxxx> writes: > >> On Mon, 2022-08-22 at 17:16 -0400, Olivier Langlois wrote: >>> >>> What is stopping the task calling do_coredump() to be interrupted and >>> call task_work_add() from the interrupt context? >>> >>> This is precisely what I was experiencing last summer when I did work >>> on this issue. >>> >>> My understanding of how async I/O works with io_uring is that the >>> task >>> is added to a wait queue without being put to sleep and when the >>> io_uring callback is called from the interrupt context, >>> task_work_add() >>> is called so that the next time io_uring syscall is invoked, pending >>> work is processed to complete the I/O. >>> >>> So if: >>> >>> 1. io_uring request is initiated AND the task is in a wait queue >>> 2. do_coredump() is called before the I/O is completed >>> >>> IMHO, this is how you end up having task_work_add() called while the >>> coredump is generated. >>> >> I forgot to add that I have experienced the issue with TCP/IP I/O. >> >> I suspect that with a TCP socket, the race condition window is much >> larger than if it was disk I/O and this might make it easier to >> reproduce the issue this way... > > I was under the apparently mistaken impression that the io_uring > task_work_add only comes from the io_uring userspace helper threads. > Those are definitely suppressed by my change. > > Do you have any idea in the code where io_uring code is being called in > an interrupt context? I would really like to trace that code path so I > have a better grasp on what is happening. > > If task_work_add is being called from interrupt context then something > additional from what I have proposed certainly needs to be done. task_work may come from the helper threads, but generally it does not. One example would be doing a read from a socket. There's no data there, poll is armed to trigger a retry. When we get the poll notification that there's now data to be read, then we kick that off with task_work. Since it's from the poll handler, it can trigger from interrupt context. See the path from io_uring/poll.c:io_poll_wake() -> __io_poll_execute() -> io_req_task_work_add() -> task_work_add(). It can also happen for regular IRQ based reads from regular files, where the completion is actually done via task_work added from the potentially IRQ based completion path. -- Jens Axboe
