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

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On 08/09/2021 16.59, David Hildenbrand wrote:
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@xxxxxxx>
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.

Please don't draw this as a general conclusion.
As Linus describe in details, the IRQ disable/enable will be very micro-arch specific.

The preempt_disable/enable will likely be more stable/consistent across micro-archs. Keep an eye out for kernel config options when juding preempt_disable/enable performance [1]

[1] https://github.com/netoptimizer/prototype-kernel/blob/master/kernel/lib/time_bench_sample.c#L363-L367



Are the numbers trustworthy? :)


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


Of-cause not, this is a microbenchmark... they are per definition not trustworthy :-P

-Jesper






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