linux-kernel - Re: [patch V2 18/29] lockdep: Move stack trace logic into check_prev

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Date:   Wed, 24 Apr 2019 21:45:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Alexey Dobriyan <adobriyan@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Pekka Enberg <penberg@...nel.org>, linux-mm@...ck.org,
        David Rientjes <rientjes@...gle.com>,
        Christoph Lameter <cl@...ux.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        kasan-dev@...glegroups.com,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Akinobu Mita <akinobu.mita@...il.com>,
        iommu@...ts.linux-foundation.org,
        Robin Murphy <robin.murphy@....com>,
        Christoph Hellwig <hch@....de>,
        Marek Szyprowski <m.szyprowski@...sung.com>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        David Sterba <dsterba@...e.com>, Chris Mason <clm@...com>,
        Josef Bacik <josef@...icpanda.com>,
        linux-btrfs@...r.kernel.org, dm-devel@...hat.com,
        Mike Snitzer <snitzer@...hat.com>,
        Alasdair Kergon <agk@...hat.com>,
        intel-gfx@...ts.freedesktop.org,
        Joonas Lahtinen <joonas.lahtinen@...ux.intel.com>,
        Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
        dri-devel@...ts.freedesktop.org, David Airlie <airlied@...ux.ie>,
        Jani Nikula <jani.nikula@...ux.intel.com>,
        Daniel Vetter <daniel@...ll.ch>,
        Rodrigo Vivi <rodrigo.vivi@...el.com>,
        linux-arch@...r.kernel.org
Subject: Re: [patch V2 18/29] lockdep: Move stack trace logic into
 check_prev_add()

On Thu, Apr 18, 2019 at 10:41:37AM +0200, Thomas Gleixner wrote:
> There is only one caller of check_prev_add() which hands in a zeroed struct
> stack trace and a function pointer to save_stack(). Inside check_prev_add()
> the stack_trace struct is checked for being empty, which is always
> true. Based on that one code path stores a stack trace which is unused. The
> comment there does not make sense either. It's all leftovers from
> historical lockdep code (cross release).

I was more or less expecting a revert of:

ce07a9415f26 ("locking/lockdep: Make check_prev_add() able to handle external stack_trace")

And then I read the comment that went with the "static struct
stack_trace trace" that got removed (in the above commit) and realized
that your patch will consume more stack entries.

The problem is when the held lock stack in check_prevs_add() has multple
trylock entries on top, in that case we call check_prev_add() multiple
times, and this patch will then save the exact same stack-trace multiple
times, consuming static resources.

Possibly we should copy what stackdepot does (but we cannot use it
directly because stackdepot uses locks; but possible we can share bits),
but that is a patch for another day I think.

So while convoluted, perhaps we should retain this code for now.