Re: [PATCH] mm: change memcg->oom_group access with atomic operations

[Paul E. McKenney]

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@xxxxxxxxxx> 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@xxxxxxxxxxxxx> 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@xxxxxxxxx> wrote:
> > > > > > > > > On Feb 20, 2023, at 3:06 PM, Shakeel Butt <shakeelb@xxxxxxxxxx> 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@xxxxxxxxx>
> > > > > > > > >>
> > > > > > > > >> 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.  ;-)

							Thanx, Paul