Messages in this thread Patch in this message | | | Date | Tue, 1 Mar 2016 11:56:22 -0800 | Subject | Re: fs: uninterruptible hang in handle_userfault | From | Linus Torvalds <> |
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On Tue, Mar 1, 2016 at 3:29 AM, Dmitry Vyukov <dvyukov@google.com> wrote: > > The following program creates an unkillable process in D state:
It seems to be usefaultfd that *tries* to handle signals, but there's one special fault case where signals won't make it through: when we're exiting and doing the final child pid clearing access.
We could do this two ways:
(a) special-case the PF_EXITING case for usefaultfd, something like
diff --git a/fs/userfaultfd.c b/fs/userfaultfd.c index 50311703135b..66cdb44616d5 100644 --- a/fs/userfaultfd.c +++ b/fs/userfaultfd.c @@ -287,6 +287,12 @@ int handle_userfault(struct vm_area_struct *vma, unsigned long address, goto out;
/* + * We don't do userfault handling for the final child pid update. + */ + if (current->flags & PF_EXITING) + goto out; + + /* * Check that we can return VM_FAULT_RETRY. * * NOTE: it should become possible to return VM_FAULT_RETRY
or (b) always consider the exiting case be "fatal signal pending"
diff --git a/include/linux/sched.h b/include/linux/sched.h index a10494a94cc3..5adf9f001df3 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -2924,7 +2924,7 @@ static inline int __fatal_signal_pending(struct task_struct *p)
static inline int fatal_signal_pending(struct task_struct *p) { - return signal_pending(p) && __fatal_signal_pending(p); + return (p->flags & PF_EXITING) || (signal_pending(p) && __fatal_signal_pending(p)); } static inline int signal_pending_state(long state, struct task_struct *p)
either of which feels a bit hacky to me.
That general "consider the final exit always as if we have a fatal signal pending" feels like a more generic fix, but it makes me think that it will fail on NFS-backed mmap's too. That could be seen as a good thing (avoiding hangs when the NFS server dies), but it also means that the patch clearly changes *other* semantics too, not just the usefaultfd case.
So (a) is more targeted, and might be safer.
Does anybody have any other suggestions?
(The above patches are entirely untested, maybe I misread the reason it might be hanging and it's something else going on).
Linus
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