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. Thanx, Paul