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From: Roman Gushchin To: "Paul E. McKenney" 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: 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: <20230221182359.GJ2948950@paulmck-ThinkPad-P17-Gen-1> X-Migadu-Flow: FLOW_OUT X-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: 98E6480004 X-Stat-Signature: b9mqddhzwnr8nufpmiyt6ctygtkmgtmj X-HE-Tag: 1677018241-62451 X-HE-Meta: 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 N6zFp42S af/teeGgol+EYqVTnYPxmX+f3b1gmX/cQLC21FvF5RXfLks7k2RC2EPkNxTDB0zxhToqzh3YB1r2nl2IZKZFAPyooAPOj1mReBQHwVwr1zIYBilcJIQXx7w5eCaagHrskzlNipaz6CuZx+8GBJehS+aXLwJ5nyQ9pZi3QVGOUAF+9LQ6ZOFA/to4qfXG+9Cs32qR5bbil2wy65XM1QpliKpIqbpqSCg65eLfSR2Jk5XC3hoqmicg9DFK+6cChO9RxtiRF0S/NOo0A8LaA45ZCgBE2/F0LEbILAwZEB9dAk0F/TnP28a4uYPu5Cw== 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 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? Thanks!