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McKenney" To: Roman Gushchin Cc: Shakeel Butt , Matthew Wilcox , Marco Elver , Yue Zhao , linux-mm@kvack.org, akpm@linux-foundation.org, hannes@cmpxchg.org, mhocko@kernel.org, muchun.song@linux.dev, cgroups@vger.kernel.org, linux-kernel@vger.kernel.org Subject: Re: [PATCH] mm: change memcg->oom_group access with atomic operations Message-ID: <20230221223811.GK2948950@paulmck-ThinkPad-P17-Gen-1> Reply-To: paulmck@kernel.org References: <20230220230624.lkobqeagycx7bi7p@google.com> <6563189C-7765-4FFA-A8F2-A5CC4860A1EF@linux.dev> <20230221182359.GJ2948950@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Stat-Signature: mrbzcp3uwe5t5rrt8hw7ejui3k3brx66 X-Rspam-User: X-Rspamd-Queue-Id: 8B37F120012 X-Rspamd-Server: rspam06 X-HE-Tag: 1677019093-898947 X-HE-Meta: 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 DemmjUYI 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 X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: 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 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 wrote: > > > > > > > On Feb 20, 2023, at 3:06 PM, Shakeel Butt 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 > > > > > > >> > > > > > > >> 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