linux-kernel - Re: [PATCH v2] context_tracking: remove local_irq_save from __acct_update

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Message-ID: <20150425094346.GA5897@osiris>
Date:	Sat, 25 Apr 2015 11:43:46 +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>, williams@...hat.com
Subject: Re: [PATCH v2] context_tracking: remove local_irq_save from
 __acct_update_integrals

On Fri, Apr 24, 2015 at 11:16:53AM -0400, Rik van Riel wrote:
> V2: introduce signed_cputime_t to deal with 64 bit cputime_t on
>     32 bit architectures, and use READ_ONCE to ensure the value
>     is always read atomically (Heiko Karstens)

Erm, that's not what I said ;)
READ_ONCE() only fixes the isssue that with your previous code the
compiler was free to generate code that accesses the memory value
several times.

But..

> -		local_irq_save(flags);
>  		time = stime + utime;
> -		dtime = time - tsk->acct_timexpd;
> +		dtime = time - READ_ONCE(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_cputime_t)dtime <= 0))
> +			return;
> +

...the READ_ONCE() doesn't give you any guarantees about reading
tsk->acct_timexpd in an atomic way.
Well, actually you don't need atomic semantics, but only to make sure that
the read access happens with a single instruction, since you want to protect
against interrupts.
But still: if the size of acct_timexpd is 64 bit READ_ONCE() may still result
in two instructions on 32 bit architectures.
(or isn't there currently no 32 bit architecture with 64 bit cputime_t left?)

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