Re: [PATCH] mm: Prevent racy access to tlb_flush_pending

[Nadav Amit <nadav.amit@gmail.com>]

Andy Lutomirski <luto@kernel.org> wrote:

> On Mon, Jul 17, 2017 at 11:02 AM, Nadav Amit <namit@vmware.com> wrote:
>> Setting and clearing mm->tlb_flush_pending can be performed by multiple
>> threads, since mmap_sem may only be acquired for read in task_numa_work.
>> If this happens, tlb_flush_pending may be cleared while one of the
>> threads still changes PTEs and batches TLB flushes.
>> 
>> As a result, TLB flushes can be skipped because the indication of
>> pending TLB flushes is lost, for instance due to race between
>> migration and change_protection_range (just as in the scenario that
>> caused the introduction of tlb_flush_pending).
>> 
>> The feasibility of such a scenario was confirmed by adding assertion to
>> check tlb_flush_pending is not set by two threads, adding artificial
>> latency in change_protection_range() and using sysctl to reduce
>> kernel.numa_balancing_scan_delay_ms.
> 
> This thing is logically a refcount.  Should it be refcount_t?

I don’t think so. refcount_inc() would WARN_ONCE if the counter is zero
before the increase, although this is a valid scenario here.

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