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McKenney" Subject: Re: [PATCH v3 1/7] kernel/fork: convert vma assignment to a memcpy Message-ID: References: <20230125233554.153109-1-surenb@google.com> <20230125233554.153109-2-surenb@google.com> <20230125162159.a66e5ef05fecb405e85ffec9@linux-foundation.org> <20230125173449.5472cffc989dfab4b83c491d@linux-foundation.org> <20230126172726.GA682281@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230126172726.GA682281@paulmck-ThinkPad-P17-Gen-1> User-Agent: Mutt/2.2.9 (2022-11-12) X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: michel@lespinasse.org, joelaf@google.com, songliubraving@fb.com, mhocko@suse.com, leewalsh@google.com, david@redhat.com, peterz@infradead.org, bigeasy@linutronix.de, peterx@redhat.com, dhowells@redhat.com, linux-mm@kvack.org, edumazet@google.com, jglisse@google.com, punit.agrawal@bytedance.com, will@kernel.org, arjunroy@google.com, dave@stgolabs.net, x86@kernel.org, hughd@google.com, willy@infradead.org, gurua@google.com, mingo@redhat.com, linux-arm-kernel@lists.infradead.org, rientjes@google.com, axelrasmussen@google.com, kernel-team@android.com, soheil@google.com, minchan@google.com, jannh@google.com, liam.howlett@oracle.com, shakeelb@google.com, luto@kernel.org, gthelen@google.com, ldufour@linux.ibm.com, Suren Baghdasaryan , vbabka@suse.cz, posk@google.com, lstoakes@gmail.com, peterjung1337@gmail.com, linuxppc-dev@lists.ozlabs.org, kent.overstreet@linux.dev, hughlynch@google.com, linux-kernel@vger.kernel.org, hannes@cmpxchg.org, Andrew Morton , tatashin@google.com, mgorman@techsingularity.net Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" 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, -- Marco