linux-kernel - Re: [PATCH RT 3/3] fix get_monotonic

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Message-ID: <Pine.LNX.4.58.0708241454250.4331@gandalf.stny.rr.com>
Date:	Fri, 24 Aug 2007 14:56:13 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	john stultz <johnstul@...ibm.com>
cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	RT <linux-rt-users@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH RT 3/3] fix get_monotonic_cycles for latency tracer



--
On Fri, 24 Aug 2007, john stultz wrote:

> >
> > -	/* calculate the delta since the last update_wall_time: */
> > -	cycle_delta = (cycle_now - clock->cycle_last) & clock->mask;
> > +	} while (cycle_raw != clock->cycle_raw ||
> > +		 cycle_last != clock->cycle_last);
>
> So if I'm understanding this right, not taking a lock isn't an
> optimization (as the seq read lock code is almost the same), but a
> requirement (as this might be called while xtime_lock is held),
> correct?

Exactly. The latency tracer uses this so no locks can be grabbed
(seq_locks included).

>
> Might want to clarify that a bit in the comment.
>

OK, will do and send a updated patch.

I wasn't in the best state writing this patch in the wee hours of the
night ;-)

-- Steve

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