linux-kernel - Re: [PATCH] rcu: use try_cmpxchg in check_cpu

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Message-Id: <00F4E7D7-6E54-4BC0-9735-4038F8DBAE20@joelfernandes.org>
Date:   Wed, 1 Mar 2023 14:46:25 -0500
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Uros Bizjak <ubizjak@...il.com>,
        "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] rcu: use try_cmpxchg in check_cpu_stall



> On Mar 1, 2023, at 2:43 PM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> On Wed, 1 Mar 2023 20:18:11 +0100
> Uros Bizjak <ubizjak@...il.com> wrote:
> 
>> If we want to put the definition in generic headers, then we also need
>> to define acquire/release/relaxed and 64bit variants. ATM, we have two
>> sites that require this definition and I think that for now we could
>> live with two instances of the same definition in two separate
>> subsystems. But this would definitely be a good future addition. There
>> is some code in the form of
>> 
>> if (cmpxchg (ptr, 0, 1) == 0)
>> 
>> that can not be converted to try_cmpxchg, but can use cmpxchg_success.
> 
> And even modify code that uses temp variables. For example, where you
> modified the ring buffer code to use try_cmpxchg(), I could convert your:
> 
> static int rb_head_page_replace(struct buffer_page *old,
>                struct buffer_page *new)
> {
>    unsigned long *ptr = (unsigned long *)&old->list.prev->next;
>    unsigned long val;
> 
>    val = *ptr & ~RB_FLAG_MASK;
>    val |= RB_PAGE_HEAD;
> 
>    return try_cmpxchg(ptr, &val, (unsigned long)&new->list);
> }
> 
> Into just:
> 
> static int rb_head_page_replace(struct buffer_page *old,
>                struct buffer_page *new)
> {
>    unsigned long *ptr = (unsigned long *)&old->list.prev->next;
>    unsigned long val;
> 
>    val = *ptr & ~RB_FLAG_MASK;
> 
>    return cmpxchg_success(ptr, val | RB_PAGE_HEAD, (unsigned long)&new->list);
> }

Nice! Looks much better.

Thanks,

 - Joel

> 
> -- Steve