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McKenney" Cc: LKML , Frederic Weisbecker , quic_neeraju@quicinc.com, joel@joelfernandes.org, rcu@vger.kernel.org, Lai Jiangshan Subject: [PATCH 1/3] srcu: Warn when NMI-unsafe API is used in NMI Date: Thu, 13 Oct 2022 19:22:42 +0200 Message-Id: <20221013172244.1099010-2-frederic@kernel.org> X-Mailer: git-send-email 2.25.1 In-Reply-To: <20221013172244.1099010-1-frederic@kernel.org> References: <20221013172244.1099010-1-frederic@kernel.org> MIME-Version: 1.0 X-Spam-Status: No, score=-7.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_HI, SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org X-getmail-retrieved-from-mailbox: =?utf-8?q?INBOX?= X-GMAIL-THRID: =?utf-8?q?1746594543828276134?= X-GMAIL-MSGID: =?utf-8?q?1746594543828276134?= Using the NMI-unsafe reader API from within NMIs is very likely to be buggy for three reasons: 1) NMIs aren't strictly re-entrant (a pending nested NMI will execute at the end of the current one) so it should be fine to use a non-atomic increment here. However breakpoints can still interrupt NMIs and if a breakpoint callback has a reader on that same ssp, a racy increment can happen. 2) If the only reader site for a given ssp is in an NMI, RCU is definetly a better choice over SRCU. 3) Because of the previous reason (2), an ssp having an SRCU read side critical section in an NMI is likely to have another one from a task context. For all these reasons, warn if an nmi unsafe reader API is used from an NMI. Signed-off-by: Frederic Weisbecker Reviewed-by: Joel Fernandes (Google) --- kernel/rcu/srcutree.c | 2 ++ 1 file changed, 2 insertions(+) diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index c54142374793..8b7ef1031d89 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -642,6 +642,8 @@ static void srcu_check_nmi_safety(struct srcu_struct *ssp, bool nmi_safe) if (!IS_ENABLED(CONFIG_PROVE_RCU)) return; + /* NMI-unsafe use in NMI is a bad sign */ + WARN_ON_ONCE(!nmi_safe && in_nmi()); sdp = raw_cpu_ptr(ssp->sda); old_nmi_safe_mask = READ_ONCE(sdp->srcu_nmi_safety); if (!old_nmi_safe_mask) {