On 8/16/21 2:02 PM, Pavel Begunkov wrote: > 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. Apologies for this draft rumbling... 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, e.g. in 5.15 will be only requests operating on io_uring, and used during normal exit. Agree that the names may be not too descriptive. >> >> 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
