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    SubjectRe: ARC show_regs() triggers preempt debug splat, lockdep
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    On 08/01/2018 12:53 AM, Peter Zijlstra wrote:
    > On Tue, Jul 31, 2018 at 02:26:32PM -0700, Vineet Gupta wrote:
    >> Hi Peter, Al,
    >>
    >> Reaching out about a problem I understand, but not quite sure how to fix it.
    >> Its the weird feeling of how was this working all along, if at all.
    >>
    >> With print-fatal-signals enabled, there's CONFIG_DEBUG_PREEMPT splat all over,
    >> even with a simple single threaded segv inducing program (console log below). This
    >> originally came to light with a glibc test suite tst-tls3-malloc which is a
    >> multi-threaded monster.
    >>
    >> ARC show_regs() is a bit more fancy as it tries to print the executable path,
    >> faulting vma name (in case it was a shared lib etc). This involves taking a bunch
    >> of customary locks which seems to be tripping the debug infra.
    > Right, so I think that that is a fairly dodgy thing to do. As shown in
    > your subsequent email, if a pagefault generates a signal we might
    > already be holding the mmap_sem.
    >
    > The thing you could do is maybe use down_read_trylock() there.
    >
    > diff --git a/arch/arc/kernel/troubleshoot.c b/arch/arc/kernel/troubleshoot.c
    > index 783b20354f8b..bb7bde11d2c8 100644
    > --- a/arch/arc/kernel/troubleshoot.c
    > +++ b/arch/arc/kernel/troubleshoot.c
    > @@ -92,7 +92,10 @@ static void show_faulting_vma(unsigned long address, char *buf)
    > /* can't use print_vma_addr() yet as it doesn't check for
    > * non-inclusive vma
    > */
    > - down_read(&active_mm->mmap_sem);
    > + if (!down_read_trylock(&active_mm->mmap_sem)) {
    > + pr_info(" @Trylock failed\n");
    > + return;
    > + }
    > vma = find_vma(active_mm, address);
    >
    > /* check against the find_vma( ) behaviour which returns the next VMA

    That's not the only issue here. We also call page allocator in show_regs code path
    and that barfs as well with __might_sleep.

    I think for us, it would make sense to re-enable preemption inside ARC show_regs()
    - undoing the generic disable from get_signal()
    The rationale for that in first place, per commit 3a9f84d354ce1 was some arch
    (x86?) show_regs() calling smp_processor_id(). Arguable that could call the
    raw_smp variant, but we don't want to needlessly bother rest of the world.

    Do you see any pitfall with my proposal ?

    -Vineet

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