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Miller" , Eric Dumazet , Jakub Kicinski , Paolo Abeni , netdev@vger.kernel.org, linux-kernel@vger.kernel.org, llvm@lists.linux.dev, stable@vger.kernel.org Subject: Re: [PATCH net] net: microchip: encx24j600: Avoid unsequenced accesses to jiffies in encx24j600_tx() Message-ID: <58014e4c-fd79-44d7-af47-35da26616570@lunn.ch> References: <20261114-encx24j600-wunsequenced-v1-1-c978fc36cacd@kernel.org> X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261114-encx24j600-wunsequenced-v1-1-c978fc36cacd@kernel.org> On Sat, Nov 14, 2026 at 09:38:26PM -0700, Nathan Chancellor wrote: > After a recent improvement in clang to warn on unsequenced accesses to > volatile variables [1], an instance appears in encx24j600_tx(), which > can break the build with CONFIG_WERROR=y / W=e: > > drivers/net/ethernet/microchip/encx24j600.c:898:14: error: unsequenced accesses to volatile qualified 'jiffies' [-Werror,-Wunsequenced] > 898 | jiffies, jiffies - dev_trans_start(dev)); > | ~~~~~~~ ^ > > Per C11 5.1.2.3.2, accessing a volatile object is a side effect. Per C11 > 6.5.2, unsequenced side effects on the same scalar object are undefined > behavior. Per C11 6.5.2.2.10/12, the evaluation of arguments in a > function call is indeterminately sequenced, meaning any side effects in > the arguments are unsequenced. > > Add a local variable to hold the value of jiffies before the function > call so that there are no unsequenced side effects, clearing up the > warning. > > Cc: stable@vger.kernel.org > Fixes: 04fbfce7a222 ("net: Microchip encx24j600 driver") > Closes: https://github.com/ClangBuiltLinux/linux/issues/2196 > Link: https://github.com/llvm/llvm-project/commit/955715460eaf9188c241c74750637fc82dfcc423 [1] > Signed-off-by: Nathan Chancellor Reviewed-by: Andrew Lunn Andrew