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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , Simon Horman , Kaiyuan Zhang , Stanislav Fomichev , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Bobby Eshleman Subject: Re: [PATCH net] net: devmem: use READ_ONCE/WRITE_ONCE on binding->dev Message-ID: References: <20260223-devmem-membar-fix-v1-1-37dcae1e49f8@meta.com> Precedence: bulk X-Mailing-List: netdev@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: On Wed, Feb 25, 2026 at 09:31:48AM -0800, Mina Almasry wrote: > On Wed, Feb 25, 2026 at 7:14 AM Bobby Eshleman wrote: > > > > On Tue, Feb 24, 2026 at 05:49:42PM -0800, Stanislav Fomichev wrote: > > > On 02/23, Bobby Eshleman wrote: > > > > From: Bobby Eshleman > > > > > > > > binding->dev is protected on the write-side in > > > > mp_dmabuf_devmem_uninstall() against concurrent writes, but due to the > > > > concurrent bare read in net_devmem_get_binding() it should be wrapped in > > > > a READ_ONCE/WRITE_ONCE pair to make sure no compiler optimizations play > > > > with the underlying register in unforeseen ways. > > > > > > > > Fixes: bd61848900bf ("net: devmem: Implement TX path") > > > > Signed-off-by: Bobby Eshleman > > Looks correct to me, and AFAIU Stan is right, we might as well > annotate all the reads of ->dev as technically there could be a dmabuf > uninstall happing concurrently on another CPU. I also think it's > probably good to annotate potential races. > > The ->dev write in dmabuf binding doesn't need WRITE_ONCE annotation I > guess because it's initialization, it can't race with any reads. > > This makes me wonder what other fields in dmabuf need annotations. I > hope I didn't miss many more. > > I would add this is really not a critical bug because > net_devmem_get_binding() is in TX path, and it is more than fine here > if we fail this check if there is an unbind happening in paraller with > sendmsg(), but it's probably good to annotate potential races anyway. > > -- > Thanks, > Mina I also noticed in tools/memory-model/Documentation/access-marking.txt section "Lock-Protected Writes With Lockless Reads" it is recommended to also use ASSERT_EXCLUSIVE_WRITER(), with explanation: "The purpose of the ASSERT_EXCLUSIVE_WRITER() is to allow KCSAN to check for a buggy concurrent write, whether marked or not." Do we think it is worth adding here? I'd think so but didn't want to throw it in without checking. Best, Bobby