linux-kernel - Re: [PATCH 2/2] rcu: Dump vmalloc memory info safely

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Date:   Sat, 2 Sep 2023 22:03:28 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     linux-kernel@...r.kernel.org, Zqiang <qiang.zhang1211@...il.com>,
        Zhen Lei <thunder.leizhen@...weicloud.com>,
        "Paul E . McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH 2/2] rcu: Dump vmalloc memory info safely

On Sat, Sep 2, 2023 at 9:28 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Wed, 30 Aug 2023 11:04:00 +0000 "Joel Fernandes (Google)" <joel@...lfernandes.org> wrote:
>
> > Currently, for double invoke call_rcu(), will dump rcu_head objects
> > memory info, if the objects is not allocated from the slab allocator,
> > the vmalloc_dump_obj() will be invoke and the vmap_area_lock spinlock
> > need to be held, since the call_rcu() can be invoked in interrupt context,
> > therefore, there is a possibility of spinlock deadlock scenarios.
> >
> > And in Preempt-RT kernel, the rcutorture test also trigger the following
> > lockdep warning:
>
> "possibility of deadlock" sounds like something -stable kernels would
> like to have fixed.
>
> Did you consider the desirability of a -stable backport?
>
> If so, are we able to identify a suitable Fixes: target?

Good point, it should be:
Fixes: 98f180837a89 ("mm: Make mem_dump_obj() handle vmalloc() memory")

I am currently reworking the patch as Vlad was also concerned about
(the existing) issue of accessing vm_struct fields without holding the
lock [1].

I will add this fixes tag to both patches for the v3 on the respin.

Thanks!

 - Joel