Re: ARC show_regs() triggers preempt debug splat, lockdep

[Peter Zijlstra <peterz@infradead.org>]

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