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....@oracle.com> 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
