linux-kernel - [PATCH 0/2] completion: Reduce stack usage caused by COMPLETION_INITIALIZER

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Message-Id: <20170823152542.5150-1-boqun.feng@gmail.com>
Date:   Wed, 23 Aug 2017 23:25:36 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     linux-kernel@...r.kernel.org, linux-mm@...ck.org
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>, walken@...gle.com,
        Byungchul Park <byungchul.park@....com>,
        Arnd Bergmann <arnd@...db.de>,
        Andrew Morton <akpm@...ux-foundation.org>, willy@...radead.org,
        Nicholas Piggin <npiggin@...il.com>, kernel-team@....com
Subject: [PATCH 0/2] completion: Reduce stack usage caused by COMPLETION_INITIALIZER_ONSTACK()

With LOCKDEP_CROSSRELEASE and LOCKDEP_COMPLETIONS introduced, the growth
in kernel stack usage of several functions were reported:

	https://marc.info/?l=linux-kernel&m=150270063231284&w=2

The root cause of this is in COMPLETION_INITIALIZER_ONSTACK(), we use

	({init_completion(&work); work})

, which will create a temporary object when returned. However this
temporary object is unnecessary. And this patch fixes it by making the
statement expression in COMPLETION_INITIALIZER_ONSTACK() return a
pointer rather than a whole structure. This will reduce the stack usage
even if !LOCKDEP.

However, such a change does make one COMPLETION_INITIALIZER_ONSTACK()
callsite invalid, so we fix this first via converting to
init_completion().

Regards,
Boqun