linux-kernel - Re: [PATCH] time: Add locking to xtime access in get

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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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