lkml.org 
[lkml]   [2016]   [Jan]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
SubjectRe: [BUG REPORT] ktime_get_ts64 causes Hard Lockup
From
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@gmail.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
>

\
 
 \ /
  Last update: 2016-01-20 03:01    [W:0.125 / U:0.156 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site