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McKenney" , Andrew Morton , michel@lespinasse.org, jglisse@google.com, mhocko@suse.com, vbabka@suse.cz, hannes@cmpxchg.org, mgorman@techsingularity.net, dave@stgolabs.net, willy@infradead.org, liam.howlett@oracle.com, peterz@infradead.org, ldufour@linux.ibm.com, mingo@redhat.com, will@kernel.org, luto@kernel.org, songliubraving@fb.com, peterx@redhat.com, david@redhat.com, dhowells@redhat.com, hughd@google.com, bigeasy@linutronix.de, kent.overstreet@linux.dev, punit.agrawal@bytedance.com, lstoakes@gmail.com, peterjung1337@gmail.com, rientjes@google.com, axelrasmussen@google.com, joelaf@google.com, minchan@google.com, jannh@google.com, shakeelb@google.com, tatashin@google.com, edumazet@google.com, gthelen@google.com, gurua@google.com, arjunroy@google.com, soheil@google.com, hughlynch@google.com, leewalsh@google.com, posk@google.com, linux-mm@kvack.org, linux-arm-kernel@lists.infradead.org, linuxppc-dev@lists.ozlabs.org, x86@kernel.org, linux-kernel@vger.kernel.org, kernel-team@android.com Content-Type: text/plain; charset="UTF-8" X-Rspam-User: X-Rspamd-Server: rspam02 X-Rspamd-Queue-Id: 9B1FF40023 X-Stat-Signature: tzjchij3piww918c4j1tdp3okr5zzk9b X-HE-Tag: 1675790644-643285 X-HE-Meta: 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 J0PoVpk1 gzBMsMtycHs8LlvI7d0VLbuhIXfLdVOYXz3BnNF17/mleUrfiu/H7jlTxV/et1hHSKcAjulnzNSJavtVMaI7X9EN94xXuL4Ffw6XnW+/tgFZ28bKXl2TDNTcv5245ewtufMzcHmhbMw2JAB3cfCKoV9yk5Qs+AQ4T70NjM+zYGzxo7smB4A/eKRdFGjzxjmCI6khjfgaJf5aJEzsl7gAwShMabJpeJ3xTYise/i/ae2+6i0UKnmgIN6BIJjzoTVFNqRELW3PlE0bbmV6SKuXqUSCo/cIS4C6hxPhoJSGnkOXpO1P+doHOuMTrkI8t5kTmeAHdb0a/EH3knNau6TeWFSabMO2lskG6hlOkvVjr8d3Mdt9Gc3nIo0TccsWpCtPDX0qJ 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 7, 2023 at 9:16 AM Marco Elver wrote: > > On Thu, Jan 26, 2023 at 09:27AM -0800, Paul E. McKenney wrote: > > On Wed, Jan 25, 2023 at 05:34:49PM -0800, Andrew Morton wrote: > > > On Wed, 25 Jan 2023 16:50:01 -0800 Suren Baghdasaryan wrote: > > > > > > > On Wed, Jan 25, 2023 at 4:22 PM Andrew Morton wrote: > > > > > > > > > > On Wed, 25 Jan 2023 15:35:48 -0800 Suren Baghdasaryan wrote: > > > > > > > > > > > Convert vma assignment in vm_area_dup() to a memcpy() to prevent compiler > > > > > > errors when we add a const modifier to vma->vm_flags. > > > > > > > > > > > > ... > > > > > > > > > > > > --- a/kernel/fork.c > > > > > > +++ b/kernel/fork.c > > > > > > @@ -482,7 +482,7 @@ struct vm_area_struct *vm_area_dup(struct vm_area_struct *orig) > > > > > > * orig->shared.rb may be modified concurrently, but the clone > > > > > > * will be reinitialized. > > > > > > */ > > > > > > - *new = data_race(*orig); > > > > > > + memcpy(new, orig, sizeof(*new)); > > > > > > > > > > The data_race() removal is unchangelogged? > > > > > > > > True. I'll add a note in the changelog about that. Ideally I would > > > > like to preserve it but I could not find a way to do that. > > > > > > Perhaps Paul can comment? > > > > > > I wonder if KCSAN knows how to detect this race, given that it's now in > > > a memcpy. I assume so. > > > > I ran an experiment memcpy()ing between a static array and an onstack > > array, and KCSAN did not complain. But maybe I was setting it up wrong. > > > > This is what I did: > > > > long myid = (long)arg; /* different value for each task */ > > static unsigned long z1[10] = { 0 }; > > unsigned long z2[10]; > > > > ... > > > > memcpy(z1, z2, ARRAY_SIZE(z1) * sizeof(z1[0])); > > for (zi = 0; zi < ARRAY_SIZE(z1); zi++) > > z2[zi] += myid; > > memcpy(z2, z1, ARRAY_SIZE(z1) * sizeof(z1[0])); > > > > Adding Marco on CC for his thoughts. > > ( Sorry for not seeing it earlier - just saw this by chance. ) > > memcpy() data races will be detected as of (given a relatively recent > Clang compiler): > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=7c201739beef > > Also beware that the compiler is free to "optimize" things by either > inlining memcpy() (turning an explicit memcpy() into just a bunch of > loads/stores), or outline plain assignments into memcpy() calls. So the > only way to be sure what ends up there is to look at the disassembled > code. > > The data_race() was introduced by: > > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=cda099b37d716 > > It says: > "vm_area_dup() blindly copies all fields of original VMA to the new one. > This includes coping vm_area_struct::shared.rb which is normally > protected by i_mmap_lock. But this is fine because the read value will > be overwritten on the following __vma_link_file() under proper > protection. Thus, mark it as an intentional data race and insert a few > assertions for the fields that should not be modified concurrently." > > And as far as I can tell this hasn't changed. Thanks for the feedback, Marco! So, IIUC Mel's proposal to use data_race(memcpy(new, orig, sizeof(*new))); is fine in this case, right? Thanks, Suren. > > Thanks, > -- Marco