linux-kernel - Re: [PATCH v8 09/14] lockdep: Apply crossrelease to completions

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Date:   Mon, 14 Aug 2017 10:50:24 +0200
From:   Arnd Bergmann <arnd@...db.de>
To:     Byungchul Park <byungchul.park@....com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, walken@...gle.com,
        Boqun Feng <boqun.feng@...il.com>, kirill@...temov.name,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Andrew Morton <akpm@...ux-foundation.org>, willy@...radead.org,
        Nicholas Piggin <npiggin@...il.com>, kernel-team@....com
Subject: Re: [PATCH v8 09/14] lockdep: Apply crossrelease to completions

On Mon, Aug 7, 2017 at 9:12 AM, Byungchul Park <byungchul.park@....com> wrote:
> Although wait_for_completion() and its family can cause deadlock, the
> lock correctness validator could not be applied to them until now,
> because things like complete() are usually called in a different context
> from the waiting context, which violates lockdep's assumption.
>
> Thanks to CONFIG_LOCKDEP_CROSSRELEASE, we can now apply the lockdep
> detector to those completion operations. Applied it.
>
> Signed-off-by: Byungchul Park <byungchul.park@....com>

This patch introduced a significant growth in kernel stack usage for a small
set of functions. I see two new warnings for functions that get tipped over the
1024 or 2048 byte frame size limit in linux-next (with a few other patches
applied):

Before:

drivers/md/dm-integrity.c: In function 'write_journal':
drivers/md/dm-integrity.c:827:1: error: the frame size of 504 bytes is
larger than xxx bytes [-Werror=frame-larger-than=]
drivers/mmc/core/mmc_test.c: In function 'mmc_test_area_io_seq':
drivers/mmc/core/mmc_test.c:1491:1: error: the frame size of 680 bytes
is larger than 104 bytes [-Werror=frame-larger-than=]

After:

drivers/md/dm-integrity.c: In function 'write_journal':
drivers/md/dm-integrity.c:827:1: error: the frame size of 1280 bytes
is larger than 1024 bytes [-Werror=frame-larger-than=]
drivers/mmc/core/mmc_test.c: In function 'mmc_test_area_io_seq':
drivers/mmc/core/mmc_test.c:1491:1: error: the frame size of 1072
bytes is larger than 1024 bytes [-Werror=frame-larger-than=]

I have not checked in detail why this happens, but I'm guessing that
there is an overall increase in stack usage with
CONFIG_LOCKDEP_COMPLETE in functions using completions,
and I think it would be good to try to come up with a version that doesn't
add as much.

        Arnd