Re: PMEM error-handling forces SIGKILL causes kernel panic

[Naoya Horiguchi]

Hi Dan, Jane,

Thanks for the report.

On Wed, Jan 09, 2019 at 03:49:32PM -0800, Dan Williams wrote:
> [ switch to text mail, add lkml and Naoya ]
> 
> On Wed, Jan 9, 2019 at 12:19 PM Jane Chu <jane.chu@xxxxxxxxxx> wrote:
...
> > 3. The hardware consists the latest revision CPU and Intel NVDIMM, we suspected
> >    the CPU faulty because it generated MCE over PMEM UE in a unlikely high
> >    rate for any reasonable NVDIMM (like a few per 24hours).
> >
> > After swapping the CPU, the problem stopped reproducing.
> >
> > But one could argue that perhaps the faulty CPU exposed a small race window
> > from collect_procs() to unmap_mapping_range() and to kill_procs(), hence
> > caught the kernel  PMEM error handler off guard.
> 
> There's definitely a race, and the implementation is buggy as can be
> seen in __exit_signal:
> 
>         sighand = rcu_dereference_check(tsk->sighand,
>                                         lockdep_tasklist_lock_is_held());
>         spin_lock(&sighand->siglock);
> 
> ...the memory-failure path needs to hold the proper locks before it
> can assume that de-referencing tsk->sighand is valid.
> 
> > Also note, the same workload on the same faulty CPU were run on Linux prior to
> > the 4.19 PMEM error handling and did not encounter kernel crash, probably because
> > the prior HWPOISON handler did not force SIGKILL?
> 
> Before 4.19 this test should result in a machine-check reboot, not
> much better than a kernel crash.
> 
> > Should we not to force the SIGKILL, or find a way to close the race window?
> 
> The race should be closed by holding the proper tasklist and rcu read lock(s).

This reasoning and proposal sound right to me. I'm trying to reproduce
this race (for non-pmem case,) but no luck for now. I'll investigate more.

Thanks,
Naoya Horiguchi