linux-kernel - Re: [PATCH 3/3] early_printk: Add simple serialization to early

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20161018135329.3e8eef38@gandalf.local.home>
Date:   Tue, 18 Oct 2016 13:53:29 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>, Tejun Heo <tj@...nel.org>,
        Calvin Owens <calvinowens@...com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] early_printk: Add simple serialization to
 early_vprintk()

On Tue, 18 Oct 2016 19:30:46 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Tue, Oct 18, 2016 at 01:19:51PM -0400, Steven Rostedt wrote:

> Looking at this again, we really should not spin using cmpxchg(), that
> thrashes the cacheline.
> 
> The below is slightly better spinning... then again, this isn't
> performance code.

Yeah, this is similar to the hack I had.

May want to fold this in your previous version.

-- Steve

> 
> --- a/kernel/printk/printk.c
> +++ b/kernel/printk/printk.c
> @@ -376,10 +376,16 @@ static int early_vprintk(const char *fmt
>  
>  	cpu = get_cpu();
>  	for (;;) {
> -		old = cmpxchg(&early_printk_cpu, -1, cpu);
> -		if (old == -1 || old == cpu)
> +		old = READ_ONCE(early_printk_cpu);
> +		if (old == cpu)
>  			break;
>  
> +		if (old == -1) {
> +			old = cmpxchg(&early_printk_cpu, -1, cpu);
> +			if (old == -1 || old == cpu)
> +				break;
> +		}
> +
>  		cpu_relax();
>  	}
>