On 8/15/21 9:42 PM, Olivier Langlois wrote: [...] > When I have first encountered the issue, the very first thing that I > did try was to create a simple test program that would synthetize the > problem. > > After few time consumming failed attempts, I just gave up the idea and > simply settle to my prod program that showcase systematically the > problem every time that I kill the process with a SEGV signal. > > In a nutshell, all the program does is to issue read operations with > io_uring on a TCP socket on which there is a constant data stream. > > Now that I have a better understanding of what is going on, I think > that one way that could reproduce the problem consistently could be > along those lines: > > 1. Create a pipe > 2. fork a child > 3. Initiate a read operation on the pipe with io_uring from the child > 4. Let the parent kill its child with a core dump generating signal. > 5. Write something in the pipe from the parent so that the io_uring > read operation completes while the core dump is generated. > > I guess that I'll end up doing that if I cannot fix the issue with my > current setup but here is what I have attempted so far: > > 1. Call io_uring_files_cancel from do_coredump > 2. Same as #1 but also make sure that TIF_NOTIFY_SIGNAL is cleared on > returning from io_uring_files_cancel > > Those attempts didn't work but lurking in the io_uring dev mailing list > is starting to pay off. I thought that I did reach the bottom of the > rabbit hole in my journey of understanding io_uring but the recent > patch set sent by Hao Xu > > https://lore.kernel.org/io-uring/90fce498-968e-6812-7b6a-fdf8520ea8d9@xxxxxxxxx/T/#t > > made me realize that I still haven't assimilated all the small io_uring > nuances... > > Here is my feedback. From my casual io_uring code reader point of view, > it is not 100% obvious what the difference is between > io_uring_files_cancel and io_uring_task_cancel As you mentioned, io_uring_task_cancel() cancels and waits for all requests submitted by current task, used in exec() and SQPOLL because of potential races. io_uring_task_cancel() cancels only selected ones and io_uring_files_cancel() cancels and waits only some specific requests that we absolutely have to, e.g. in 5.15 it'll be only requests referencing the ring itself. It's used on normal task exit. io_uring_task_cancel() cancels and waits all requests submitted by current task, used on exec() because of races. As you mentioned > > It seems like io_uring_files_cancel is cancelling polls only if they > have the REQ_F_INFLIGHT flag set. > > I have no idea what an inflight request means and why someone would > want to call io_uring_files_cancel over io_uring_task_cancel. > > I guess that if I was to meditate on the question for few hours, I > would at some point get some illumination strike me but I believe that > it could be a good idea to document in the code those concepts for > helping casual readers... > > Bottomline, I now understand that io_uring_files_cancel does not cancel > all the requests. Therefore, without fully understanding what I am > doing, I am going to replace my call to io_uring_files_cancel from > do_coredump with io_uring_task_cancel and see if this finally fix the > issue for good. > > What I am trying to do is to cancel pending io_uring requests to make > sure that TIF_NOTIFY_SIGNAL isn't set while core dump is generated. > > Maybe another solution would simply be to modify __dump_emit to make it > resilient to TIF_NOTIFY_SIGNAL as Eric W. Biederman originally > suggested. > > or maybe do both... > > Not sure which approach is best. If someone has an opinion, I would be > curious to hear it. > > Greetings, > > -- Pavel Begunkov
