linux-kernel - Re: [RFC][PATCH 1/2] locking: Introduce __cleanup_

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Message-ID: <cf20bbbc-c435-326d-f31a-86b1f4ce927a@redhat.com>
Date:   Fri, 26 May 2023 14:49:33 -0400
From:   Waiman Long <longman@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        torvalds@...ux-foundation.org, keescook@...omium.org,
        gregkh@...uxfoundation.org, pbonzini@...hat.com
Cc:     linux-kernel@...r.kernel.org, ojeda@...nel.org,
        ndesaulniers@...gle.com, mingo@...hat.com, will@...nel.org,
        boqun.feng@...il.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com, paulmck@...nel.org,
        frederic@...nel.org, quic_neeraju@...cinc.com,
        joel@...lfernandes.org, josh@...htriplett.org,
        mathieu.desnoyers@...icios.com, jiangshanlai@...il.com,
        qiang1.zhang@...el.com, rcu@...r.kernel.org, tj@...nel.org,
        tglx@...utronix.de
Subject: Re: [RFC][PATCH 1/2] locking: Introduce __cleanup__ based guards

On 5/26/23 11:05, Peter Zijlstra wrote:
> Use __attribute__((__cleanup__(func))) to buid various guards:
>
>   - ptr_guard()
>   - void_guard() / void_scope()
>   - lock_guard() / lock_scope()
>   - double_lock_guard() / double_lock_scope()
>
> Where the _guard thingies are variables with scope-based cleanup and
> the _scope thingies are basically do-once for-loops with the same.
>
> The CPP is rather impenetrable -- but I'll attempt to write proper
> comments if/when people think this is worth pursuing.
>
> Actual usage in the next patch
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>   include/linux/compiler_attributes.h |    2
>   include/linux/irqflags.h            |    7 ++
>   include/linux/guards.h          |  118 ++++++++++++++++++++++++++++++++++++
>   include/linux/mutex.h               |    5 +
>   include/linux/preempt.h             |    4 +
>   include/linux/rcupdate.h            |    3
>   include/linux/sched/task.h          |    2
>   include/linux/spinlock.h            |   23 +++++++
>   8 files changed, 164 insertions(+)

That is an interesting idea and may help to simplify some of the common 
code patterns that we have in the kernel. The macros are a bit hard to 
read and understand though I thought I got a rough idea of what they are 
trying to do.

BTW, do we have a use case for double_lock_guard/double_lock_scope? I 
can envision a nested lock_scope inside a lock_scope, but taking 2 auto 
locks of the same type at init time and then unlock them at exit just 
doesn't make sense to me.

Cheers,
Longman