On Tue, Jun 30, 2020 at 7:26 PM Yonghong Song [off-list ref] wrote:

On 6/30/20 5:10 PM, Hao Luo wrote:

quoted

Ok, with the help of my colleague Ian Rogers, I think we solved the
mystery. Clang actually inlined hrtimer_nanosleep() inside
SyS_nanosleep(), so there is no call to that function throughout the
path of the nanosleep syscall. I've been looking at the function body
of hrtimer_nanosleep for quite some time, but clearly overlooked the
caller of hrtimer_nanosleep. hrtimer_nanosleep is pretty short and
there are many constants, inlining would not be too surprising.

Oh thanks for explanation. inlining makes sense. We have many other
instances like this in the past where kprobe won't work properly.

Could you reword your commit message then?

 > causing fentry and kprobe to not hook on this function properly on a
 > Clang build kernel.

The above is a little vague on what happens. What really happens is
fentry/kprobe does hook on this function but has no effect since
its caller has inlined the function.

Sure, sending a v2 with a more accurate description of the issue.

Hao