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Date: Thu, 05 May 2011 22:24:58 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: john stultz <johnstul@...ibm.com>
Cc: Andi Kleen <andi@...stfloor.org>,
lkml <linux-kernel@...r.kernel.org>,
Paul Mackerras <paulus@...ba.org>,
"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
Anton Blanchard <anton@...ba.org>,
Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] time: Add locking to xtime access in get_seconds()
Le jeudi 05 mai 2011 à 13:17 -0700, john stultz a écrit :
> On Thu, 2011-05-05 at 19:57 +0200, Andi Kleen wrote:
> > > I suspect the reason this hasn't been triggered on x86 or power6 is due
> > > to compiler or processor optimizations reordering the assignment to in
> > > effect make it atomic. Or maybe the timing window to see the issue is
> > > harder to observe?
> >
> > On x86 all aligned stores are atomic. So I don't see how this
> > could be a problem ever.
>
> No no. The issue was with the fact that in update_xtime_cache we modify
> xtime_cache twice (once setting it possibly backwards to xtime, then
> adding in the nsec offset).
>
> Since get_seconds does no locking, this issue should be visible
> anywhere, as long as you manage to hit the race window between the first
> assignment and the second.
>
> However, in the testing, the issue only showed up on P7, but not P6 or
> x86.
>
> My guess was that the code:
>
> xtime_cache.sec = xtime.sec
> xtime_cache.nsec = xtime.nsec
> xtime_cache.sec = xtime_cache.sec
> + div(xtime_cache.nsec + nsec, NSEC_PER_SEC, &rem);
> xtime_cache.nsec = rem
>
> Was getting rearranged to:
>
> xtime_cache.sec = xtime.sec
> + div(xtime.nsec + nsec, NSEC_PER_SEC, &rem);
> xtime_cache.nsec = rem
>
>
> Which makes the xtime_cache.sec update atomic.
>
> But its just a guess.
Sure (disassembly could help to check this), but get_seconds() reads
xtime.tv_sec ;)
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