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charset="UTF-8" Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 73CB84000E X-Stat-Signature: yn73zwsa3qeecfqtjow1oaobjrsjmuwy X-HE-Tag: 1677021228-992753 X-HE-Meta: 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 gBEghIzw lVgRPlFEaRMTmfu3YnS25IaLGVHvwDShtO9sUr9E6TkXLGRrSUeMcKcs6Z9/UP9mfG/7c41dHKLwxMxeWfClcffJIvMb6FXg8jL11s0ym+QaCEGOu8HT8vb8VuMjRnbgoSSgnB2jy1COhaRcy8zO8y3+3sDfANiYRG87y6xAs5eYzImzyCN+vdvrWsaJdJ50J+X72dCYjA+u7rzjXeYTokDohyL1KQ4RXwy2rma6JwpHsAUPnA/uQHzTIpvwasFRwLQ+ztnZtM1NQJJHUm254/4bFPrmw5kpnLj2gjE2gQshoeqmwySaogDM/F1kI74PC6CWBvE22Ez2xEHZAot+V0bYiXo8ohoqxxAa9oHhJUyqBd22GHZy22y5hJ40Fu52Q5Yp1OXxLyEgalMhu105e0Bgrm806ypTs1elIy/zak3OCeqq/bfiZNPehSkdLJUx9R2Xsnir9QG2N6ac= 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 2:38 PM Paul E. McKenney 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 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: > > > > > > > > > > > > > > > > =EF=BB=BFOn Mon, Feb 20, 2023 at 01:09:44PM -0800, Roman Gu= shchin wrote: > > > > > > > >>> On Mon, Feb 20, 2023 at 11:16:38PM +0800, Yue Zhao wrote: > > > > > > > >>> The knob for cgroup v2 memory controller: memory.oom.grou= p > > > > > > > >>> 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 patc= h is solving? > > > > > > > >> Can you, please, provide a bit more details. > > > > > > > >> > > > > > > > > > > > > > > > > IMHO such details are not needed. oom_group is being access= ed > > > > > > > > concurrently and one of them can be a write access. At leas= t > > > > > > > > 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 rea= d/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 b= yte also > > > > > being modified by another CPU, one of those CPUs can lose its sto= re? > > > > > And WRITE_ONCE would prevent the compiler from implementing the s= tore > > > > > in that way. > > > > > > > > Thanks Willy for pointing this out. If the compiler can really do t= his > > > > then [READ|WRITE]_ONCE are required here. I always have big bad > > > > compiler lwn article open in a tab. I couldn't map this transformat= ion > > > > 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. Thi= s is > > > after all a great way to keep the CPU's vector unit from getting bore= d. > > > 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 =3D new_a; > > > spin_unlock(&la); > > > } > > > > > > void change_b(char new_b) > > > { > > > spin_lock(&lb); > > > b =3D new_b; > > > spin_unlock(&lb); > > > } > > > > > > If the compiler "optimized" that "a =3D 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 compile= r > 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?