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SubjectRe: [PATCH] mm: change memcg->oom_group access with atomic operations
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> On Feb 21, 2023, at 4:38 PM, Paul E. McKenney <paulmck@kernel.org> wrote:
>
> On Tue, Feb 21, 2023 at 03:57:58PM -0800, Roman Gushchin wrote:
>>> On Tue, Feb 21, 2023 at 03:38:24PM -0800, Paul E. McKenney wrote:
>>> On Tue, Feb 21, 2023 at 03:13:36PM -0800, Shakeel Butt wrote:
>>>> On Tue, Feb 21, 2023 at 2:38 PM Paul E. McKenney <paulmck@kernel.org> wrote:
>>>>>
>>>>> On Tue, Feb 21, 2023 at 02:23:31PM -0800, Roman Gushchin wrote:
>>>>>> On Tue, Feb 21, 2023 at 10:23:59AM -0800, Paul E. McKenney wrote:
>>>>>>> On Tue, Feb 21, 2023 at 08:56:59AM -0800, Shakeel Butt wrote:
>>>>>>>> +Paul & Marco
>>>>>>>>
>>>>>>>> On Tue, Feb 21, 2023 at 5:51 AM Matthew Wilcox <willy@infradead.org> wrote:
>>>>>>>>>
>>>>>>>>> On Mon, Feb 20, 2023 at 10:52:10PM -0800, Shakeel Butt wrote:
>>>>>>>>>> On Mon, Feb 20, 2023 at 9:17 PM Roman Gushchin <roman.gushchin@linux.dev> wrote:
>>>>>>>>>>>> On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@google.com> wrote:
>>>>>>>>>>>>
>>>>>>>>>>>> On Mon, Feb 20, 2023 at 01:09:44PM -0800, Roman Gushchin wrote:
>>>>>>>>>>>>>> On Mon, Feb 20, 2023 at 11:16:38PM +0800, Yue Zhao wrote:
>>>>>>>>>>>>>> The knob for cgroup v2 memory controller: memory.oom.group
>>>>>>>>>>>>>> will be read and written simultaneously by user space
>>>>>>>>>>>>>> programs, thus we'd better change memcg->oom_group access
>>>>>>>>>>>>>> with atomic operations to avoid concurrency problems.
>>>>>>>>>>>>>>
>>>>>>>>>>>>>> Signed-off-by: Yue Zhao <findns94@gmail.com>
>>>>>>>>>>>>>
>>>>>>>>>>>>> Hi Yue!
>>>>>>>>>>>>>
>>>>>>>>>>>>> I'm curious, have any seen any real issues which your patch is solving?
>>>>>>>>>>>>> Can you, please, provide a bit more details.
>>>>>>>>>>>>>
>>>>>>>>>>>>
>>>>>>>>>>>> IMHO such details are not needed. oom_group is being accessed
>>>>>>>>>>>> concurrently and one of them can be a write access. At least
>>>>>>>>>>>> READ_ONCE/WRITE_ONCE is needed here.
>>>>>>>>>>>
>>>>>>>>>>> Needed for what?
>>>>>>>>>>
>>>>>>>>>> For this particular case, documenting such an access. Though I don't
>>>>>>>>>> think there are any architectures which may tear a one byte read/write
>>>>>>>>>> and merging/refetching is not an issue for this.
>>>>>>>>>
>>>>>>>>> Wouldn't a compiler be within its rights to implement a one byte store as:
>>>>>>>>>
>>>>>>>>> load-word
>>>>>>>>> modify-byte-in-word
>>>>>>>>> store-word
>>>>>>>>>
>>>>>>>>> and if this is a lockless store to a word which has an adjacent byte also
>>>>>>>>> being modified by another CPU, one of those CPUs can lose its store?
>>>>>>>>> And WRITE_ONCE would prevent the compiler from implementing the store
>>>>>>>>> in that way.
>>>>>>>>
>>>>>>>> Thanks Willy for pointing this out. If the compiler can really do this
>>>>>>>> then [READ|WRITE]_ONCE are required here. I always have big bad
>>>>>>>> compiler lwn article open in a tab. I couldn't map this transformation
>>>>>>>> to ones mentioned in that article. Do we have name of this one?
>>>>>>>
>>>>>>> No, recent compilers are absolutely forbidden from doing this sort of
>>>>>>> thing except under very special circumstances.
>>>>>>>
>>>>>>> Before C11, compilers could and in fact did do things like this. This is
>>>>>>> after all a great way to keep the CPU's vector unit from getting bored.
>>>>>>> Unfortunately for those who prize optimization above all else, doing
>>>>>>> this can introduce data races, for example:
>>>>>>>
>>>>>>> char a;
>>>>>>> char b;
>>>>>>> spin_lock la;
>>>>>>> spin_lock lb;
>>>>>>>
>>>>>>> void change_a(char new_a)
>>>>>>> {
>>>>>>> spin_lock(&la);
>>>>>>> a = new_a;
>>>>>>> spin_unlock(&la);
>>>>>>> }
>>>>>>>
>>>>>>> void change_b(char new_b)
>>>>>>> {
>>>>>>> spin_lock(&lb);
>>>>>>> b = new_b;
>>>>>>> spin_unlock(&lb);
>>>>>>> }
>>>>>>>
>>>>>>> If the compiler "optimized" that "a = new_a" so as to produce a non-atomic
>>>>>>> read-modify-write sequence, it would be introducing a data race.
>>>>>>> And since C11, the compiler is absolutely forbidden from introducing
>>>>>>> data races. So, again, no, the compiler cannot invent writes to
>>>>>>> variables.
>>>>>>>
>>>>>>> What are those very special circumstances?
>>>>>>>
>>>>>>> 1. The other variables were going to be written to anyway, and
>>>>>>> none of the writes was non-volatile and there was no ordering
>>>>>>> directive between any of those writes.
>>>>>>>
>>>>>>> 2. The other variables are dead, as in there are no subsequent
>>>>>>> reads from them anywhere in the program. Of course in that case,
>>>>>>> there is no need to read the prior values of those variables.
>>>>>>>
>>>>>>> 3. All accesses to all of the variables are visible to the compiler,
>>>>>>> and the compiler can prove that there are no concurrent accesses
>>>>>>> to any of them. For example, all of the variables are on-stack
>>>>>>> variables whose addresses are never taken.
>>>>>>>
>>>>>>> Does that help, or am I misunderstanding the question?
>>>>>>
>>>>>> Thank you, Paul!
>>>>>>
>>>>>> So it seems like READ_ONCE()/WRITE_ONCE() are totally useless here.
>>>>>> Or I still miss something?
>>>>>
>>>>> Yes, given that the compiler will already avoid inventing data-race-prone
>>>>> C-language accesses to shared variables, so if that was the only reason
>>>>> that you were using READ_ONCE() or WRITE_ONCE(), then READ_ONCE() and
>>>>> WRITE_ONCE() won't be helping you.
>>>>>
>>>>> Or perhaps better to put it a different way... The fact that the compiler
>>>>> is not permitted to invent data-racy reads and writes is exactly why
>>>>> you do not normally need READ_ONCE() and WRITE_ONCE() for accesses in
>>>>> lock-based critical sections. Instead, you only need READ_ONCE() and
>>>>> WRITE_ONCE() when you have lockless accesses to the same shared variables.
>>>>
>>>> This is lockless access to memcg->oom_group potentially from multiple
>>>> CPUs, so, READ_ONCE() and WRITE_ONCE() are needed, right?
>>>
>>> Agreed, lockless concurrent accesses should use READ_ONCE() and WRITE_ONCE().
>>> And if either conflicting access is lockless, it is lockless. ;-)
>>
>> Now I'm confused, why we should use it here?
>> Writing is happening from a separate syscall (a single write from a syscall),
>> reading is happening from a oom context. The variable is boolean, it's either
>> 0 or 1. What difference READ_ONCE()/WRITE_ONCE() will make here?
>> Thanks!
>
> In practice, not much difference other than documenting shared accesses.
> Which can be valuable.
>
> In theory, when you do a normal C-language store, the compiler is within
> its rights to use the variable for temporary storage between the time
> of the last read from that variable and the next write to that variable.
> Back to practice, I have not heard of this happening for shared variables.
> On the other hand, compilers really do this for on-stack variables whose
> addresses are not taken, which is one of the reasons that gdb might say
> that the variable is optimized out when you try to look at its value.
>
> So the potential is there, and if it was my code, I would therefore use
> READ_ONCE() and WRITE_ONCE().

Got it, Paul, thank you for the explanation!

It seems like the resolution is that putting READ_ONCE()/WRITE_ONCE() across knobs in mm/memcontrol.c is generally a good idea, but mostly for cosmetic reasons.

Yue, can you, please, update the patch?

Btw, what a thread! Apparently writing & reading a single boolean is not that simple… :)
Thanks for all participants!

Roman
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