On 3/20/21 4:08 PM, Eric W. Biederman wrote: > > Added criu because I just realized that io_uring (which can open files > from an io worker thread) looks to require some special handling for > stopping and freezing processes. If not in the SIGSTOP case in the > related cgroup freezer case. > > Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> writes: > >> On Sat, Mar 20, 2021 at 10:51 AM Linus Torvalds >> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: >>> >>> Alternatively, make it not use >>> CLONE_SIGHAND|CLONE_THREAD at all, but that would make it >>> unnecessarily allocate its own signal state, so that's "cleaner" but >>> not great either. >> >> Thinking some more about that, it would be problematic for things like >> the resource counters too. They'd be much better shared. >> >> Not adding it to the thread list etc might be clever, but feels a bit too scary. >> >> So on the whole I think Jens' minor patches to just not have IO helper >> threads accept signals are probably the right thing to do. > > The way I see it we have two options: > > 1) Don't ask PF_IO_WORKERs to stop do_signal_stop and in > task_join_group_stop. > > The easiest comprehensive implementation looks like just > updating task_set_jobctl_pending to treat PF_IO_WORKER > as it treats PF_EXITING. > > 2) Have the main loop of the kernel thread test for JOBCTL_STOP_PENDING > and call into do_signal_stop. > > It is a wee bit trickier to modify the io_workers to stop, but it does > not look prohibitively difficult. > > All of the work performed by the io worker is work scheduled via > io_uring by the process being stopped. > > - Is the amount of work performed by the io worker thread sufficiently > negligible that we don't care? > > - Or is the amount of work performed by the io worker so great that it > becomes a way for an errant process to escape SIGSTOP? > > As the code is all intermingled with the cgroup_freezer. I am also > wondering creating checkpoints needs additional stopping guarantees. The work done is the same a syscall, basically. So it could be long running and essentially not doing anything (eg read from a socket, no data is there), or it's pretty short lived (eg read from a file, just waiting on DMA). This is outside of my domain of expertise, which is exactly why I added you and Linus to make some calls on what the best approach here would be. My two patches obviously go route #1 in terms of STOP. And fwiw, I tested this: > To solve the issue that SIGSTOP is simply broken right now I am totally > fine with something like: > > diff --git a/kernel/signal.c b/kernel/signal.c > index ba4d1ef39a9e..cb9acdfb32fa 100644 > --- a/kernel/signal.c > +++ b/kernel/signal.c > @@ -288,7 +288,8 @@ bool task_set_jobctl_pending(struct task_struct *task, unsigned long mask) > JOBCTL_STOP_SIGMASK | JOBCTL_TRAPPING)); > BUG_ON((mask & JOBCTL_TRAPPING) && !(mask & JOBCTL_PENDING_MASK)); > > - if (unlikely(fatal_signal_pending(task) || (task->flags & PF_EXITING))) > + if (unlikely(fatal_signal_pending(task) || > + (task->flags & (PF_EXITING | PF_IO_WORKER)))) > return false; > > if (mask & JOBCTL_STOP_SIGMASK) and can confirm it works fine for me with 2/2 reverted and this applied instead. > Which just keeps from creating unstoppable processes today. I am just > not convinced that is what we want as a long term solution. How about we go with either my 2/2 or yours above to at least ensure we don't leave workers looping as schedule() is a nop with sigpending? If there's a longer timeline concern that "evading" SIGSTOP is a concern, I have absolutely no qualms with making the IO threads participate. But since it seems conceptually simple but with potentially lurking minor issues, probably not the ideal approach for right now. -- Jens Axboe
