Re: [patch 031/147] mm, slub: protect put_cpu_partial() with disabled irqs instead of cmpxchg

[David Hildenbrand <david@redhat.com>]

On 08.09.21 16:55, David Hildenbrand wrote:
> On 08.09.21 15:58, Vlastimil Babka wrote:
>> On 9/8/21 15:05, Jesper Dangaard Brouer wrote:
>>>
>>>
>>> On 08/09/2021 04.54, Andrew Morton wrote:
>>>> From: Vlastimil Babka <vbabka@suse.cz>
>>>> Subject: mm, slub: protect put_cpu_partial() with disabled irqs instead of cmpxchg
>>>>
>>>> Jann Horn reported [1] the following theoretically possible race:
>>>>
>>>>      task A: put_cpu_partial() calls preempt_disable()
>>>>      task A: oldpage = this_cpu_read(s->cpu_slab->partial)
>>>>      interrupt: kfree() reaches unfreeze_partials() and discards the page
>>>>      task B (on another CPU): reallocates page as page cache
>>>>      task A: reads page->pages and page->pobjects, which are actually
>>>>      halves of the pointer page->lru.prev
>>>>      task B (on another CPU): frees page
>>>>      interrupt: allocates page as SLUB page and places it on the percpu partial list
>>>>      task A: this_cpu_cmpxchg() succeeds
>>>>
>>>>      which would cause page->pages and page->pobjects to end up containing
>>>>      halves of pointers that would then influence when put_cpu_partial()
>>>>      happens and show up in root-only sysfs files. Maybe that's acceptable,
>>>>      I don't know. But there should probably at least be a comment for now
>>>>      to point out that we're reading union fields of a page that might be
>>>>      in a completely different state.
>>>>
>>>> Additionally, the this_cpu_cmpxchg() approach in put_cpu_partial() is only
>>>> safe against s->cpu_slab->partial manipulation in ___slab_alloc() if the
>>>> latter disables irqs, otherwise a __slab_free() in an irq handler could
>>>> call put_cpu_partial() in the middle of ___slab_alloc() manipulating
>>>> ->partial and corrupt it.  This becomes an issue on RT after a local_lock
>>>> is introduced in later patch.  The fix means taking the local_lock also in
>>>> put_cpu_partial() on RT.
>>>>
>>>> After debugging this issue, Mike Galbraith suggested [2] that to avoid
>>>> different locking schemes on RT and !RT, we can just protect
>>>> put_cpu_partial() with disabled irqs (to be converted to
>>>> local_lock_irqsave() later) everywhere.  This should be acceptable as it's
>>>> not a fast path, and moving the actual partial unfreezing outside of the
>>>> irq disabled section makes it short, and with the retry loop gone the code
>>>> can be also simplified.  In addition, the race reported by Jann should no
>>>> longer be possible.
>>>
>>> Based on my microbench[0] measurement changing preempt_disable to
>>> local_irq_save will cost us 11 cycles (TSC).  I'm not against the
>>> change, I just want people to keep this in mind.
>>
>> OK, but this is not a fast path for every allocation/free, so it gets
>> amortized. Also it eliminates a this_cpu_cmpxchg loop, and I'd expect
>> cmpxchg to be expensive too?
>>
>>> On my E5-1650 v4 @ 3.60GHz:
>>>     - preempt_disable(+enable)  cost: 11 cycles(tsc) 3.161 ns
>>>     - local_irq_save (+restore) cost: 22 cycles(tsc) 6.331 ns
>>>
>>> Notice the non-save/restore variant is superfast:
>>>     - local_irq_disable(+enable) cost: 6 cycles(tsc) 1.844 ns
>>
>> It actually surprises me that it's that cheap, and would have expected
>> changing the irq state would be the costly part, not the saving/restoring.
>> Incidentally, would you know what's the cost of save+restore when the
>> irqs are already disabled, so it's effectively a no-op?
> 
> It surprises me as well. That would imply that protecting short RCU
> sections using
> 
> local_irq_disable
> local_irq_enable
> 
> instead of via
> 
> preempt_disable
> preempt_enable
> 
> would actually be very beneficial.
> 
> Are the numbers trustworthy? :)
> 

.. and especially did the benchmark consider side effects of 
enabling/disabling interrupts (pipeline flushes etc ..)?

-- 
Thanks,

David / dhildenb