linux-kernel - Re: [BUG REPORT] ktime_get

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Date:	Tue, 19 Jan 2016 17:59:35 -0700
From:	Jeff Merkey <linux.mdb@...il.com>
To:	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [BUG REPORT] ktime_get_ts64 causes Hard Lockup

I have done some more research on this and due to the way vdso works
any fix must also be on inline macro are rely only on inline macros.

Jeff

On 1/19/16, Jeff Merkey <linux.mdb@...il.com> 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
> a bunch of macros to __iter_div_u64_rem().  It doesn't loop infinitely
> with a number that big, just for about 4 minutes but long enough for
> the hard lockup detector to fire off.  Setting RAX=0 when this bug
> occurs clears the problem from the debugger console when this function
> gets stuck in this loop and the system recovers fine.
>
> The problem code is in:
>
> include/linux/math64.h:117
> static __always_inline u32
> __iter_div_u64_rem(u64 dividend, u32 divisor, u64 *remainder)
> {
> 	u32 ret = 0;
>
> 	while (dividend >= divisor) {
> 		/* The following asm() prevents the compiler from
> 		   optimising this loop into a modulo operation.  */
> 		asm("" : "+rm"(dividend));
>
> 		dividend -= divisor;
> 		ret++;
> 	}
> 	*remainder = dividend;
>
> 	return ret;
> }
>
> which is called by
> include/linux/time.h:233
> static __always_inline void timespec_add_ns(struct timespec *a, u64 ns)
> {
> 	a->tv_sec += __iter_div_u64_rem(a->tv_nsec + ns, NSEC_PER_SEC, &ns);
> 	a->tv_nsec = ns;
> }
>
> it's defined so that timespec64_add_ns just points to this function
> which is called by ktime_get_ts64.  tv_nsec + ns gets passed as a huge
> number (RAX = 0x17AE7F57C671EA7D) with a very small divisor and just
> sits there and loops.
>
> 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;
> }
>
> Jeff
>