linux-kernel - Re: [PATCH] context_tracking: remove local_irq_save from __acct_update

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Message-ID: <20150424091156.GB4089@osiris>
Date:	Fri, 24 Apr 2015 11:11:56 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Rik van Riel <riel@...riel.com>
Cc:	linux-kernel@...r.kernel.org,
	Andy Lutomirsky <amluto@...capital.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Clark Williams <williams@...hat.com>
Subject: Re: [PATCH] context_tracking: remove local_irq_save from
 __acct_update_integrals

On Thu, Apr 23, 2015 at 09:57:13PM -0400, Rik van Riel wrote:
> diff --git a/kernel/tsacct.c b/kernel/tsacct.c
> index 975cb49e32bf..0b967f116a6b 100644
> --- a/kernel/tsacct.c
> +++ b/kernel/tsacct.c
> @@ -126,23 +126,29 @@ static void __acct_update_integrals(struct task_struct *tsk,
>  	if (likely(tsk->mm)) {
>  		cputime_t time, dtime;
>  		struct timeval value;
> -		unsigned long flags;
>  		u64 delta;
> 
> -		local_irq_save(flags);
>  		time = stime + utime;
>  		dtime = time - tsk->acct_timexpd;
> +		/*
> +		 * This code is called both from irq context and from
> +		 * task context. There is a race where irq context advances
> +		 * tsk->acct_timexpd to a value larger than time, creating
> +		 * a negative value. In that case, the irq has already
> +		 * updated the statistics.
> +		 */
> +		if (unlikely((signed long)dtime <= 0))
> +			return;

FWIW, I think you either need a barrier() before the if-statement or use
READ_ONCE() when reading tsk->acct_timexpd above.

Otherwise the compiler could (in theory at least) generate code which
would translate to 
		if (unlikely(time <= tsk->acct_timexpd))
in order to achieve the same result, no?

Besides that cputime_t might be 64 bit in size, therefore you don't have
much of a guarentee that reading tsk->acct_timexpd happens atomically on
32 bit architectures, so you _may_ end up with garbage, no?

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