linux-kernel - Re: [PATCH] stackdepot: fix stack_depot_save

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Message-ID: <CANpmjNPDVORFLCqnm3n1RHnJBKdim_x=kurbOJ0QYxaztT4q=g@mail.gmail.com>
Date: Thu, 28 Nov 2024 10:13:59 +0100
From: Marco Elver <elver@...gle.com>
To: Andrew Morton <akpm@...ux-foundation.org>
Cc: Andrey Konovalov <andreyknvl@...il.com>, Alexander Potapenko <glider@...gle.com>, 
	Dmitry Vyukov <dvyukov@...gle.com>, Vlastimil Babka <vbabka@...e.cz>, Oscar Salvador <osalvador@...e.de>, 
	linux-kernel@...r.kernel.org, linux-mm@...ck.org, kasan-dev@...glegroups.com, 
	Sebastian Andrzej Siewior <bigeasy@...utronix.de>
Subject: Re: [PATCH] stackdepot: fix stack_depot_save_flags() in NMI context

On Thu, 28 Nov 2024 at 02:25, Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Fri, 22 Nov 2024 16:39:47 +0100 Marco Elver <elver@...gle.com> wrote:
>
> > Per documentation, stack_depot_save_flags() was meant to be usable from
> > NMI context if STACK_DEPOT_FLAG_CAN_ALLOC is unset. However, it still
> > would try to take the pool_lock in an attempt to save a stack trace in
> > the current pool (if space is available).
> >
> > This could result in deadlock if an NMI is handled while pool_lock is
> > already held. To avoid deadlock, only try to take the lock in NMI
> > context and give up if unsuccessful.
>
> Is it possible to trigger this deadlock in current kernels, or is this
> a might-happen-in-the-future thing?

I can't find evidence this happens right now (at least with the quick
test I just ran), so it's more of a might happen if use of this API
broadens.

Thanks,
-- Marco