linux-kernel - Re: [BUG REPORT] ktime_get

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Date:	Wed, 20 Jan 2016 10:21:53 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jeff Merkey <linux.mdb@...il.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	John Stultz <john.stultz@...aro.org>
Subject: Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup

On Tue, 19 Jan 2016, Jeff Merkey wrote:
> Nasty bug but trivial fix for this.  What happens here is RAX (nsecs)
> gets set to a huge value (RAX = 0x17AE7F57C671EA7D) and passed through

And how exactly does that happen?

0x17AE7F57C671EA7D = 1.70644e+18  nsec
		   = 1.70644e+09  sec
		   = 2.84407e+07  min
		   = 474011	  hrs
		   = 19750.5	  days
		   = 54.1109	  years

That's the real issue, not what you are trying to 'fix' in timespec_add_ns()

> Submitting a patch to fix this after I regress and test it.   Since it
> makes no sense to loop on a simple calculation, fix should be:
> 
> static __always_inline void timespec_add_ns(struct timespec *a, u64 ns)
> {
> 	a->tv_sec += div64_u64_rem(a->tv_nsec + ns, NSEC_PER_SEC, &ns);
> 	a->tv_nsec = ns;
> }

No. It's not that simple, because div64_u64_rem() is expensive on 32bit
architectures which have no hardware 64/32 division. And that's going to hurt
for the normal tick case where we have at max one iteration.

Thanks,

	tglx