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McKenney" Cc: LKML , Frederic Weisbecker , rcu , Uladzislau Rezki , Neeraj Upadhyay , Boqun Feng , Joel Fernandes Subject: [PATCH 3/4] rcu/nocb: Recheck lazy callbacks under the ->nocb_lock from shrinker Date: Wed, 22 Mar 2023 20:44:55 +0100 Message-Id: <20230322194456.2331527-4-frederic@kernel.org> X-Mailer: git-send-email 2.34.1 In-Reply-To: <20230322194456.2331527-1-frederic@kernel.org> References: <20230322194456.2331527-1-frederic@kernel.org> MIME-Version: 1.0 X-Spam-Status: No, score=-2.5 required=5.0 tests=DKIMWL_WL_HIGH,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_MED,SPF_HELO_NONE, SPF_PASS autolearn=unavailable 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?1761098895817003241?= X-GMAIL-MSGID: =?utf-8?q?1761098895817003241?= The ->lazy_len is only checked locklessly. Recheck again under the ->nocb_lock to avoid spending more time on flushing/waking if not necessary. The ->lazy_len can still increment concurrently (from 1 to infinity) but under the ->nocb_lock we at least know for sure if there are lazy callbacks at all (->lazy_len > 0). Signed-off-by: Frederic Weisbecker --- kernel/rcu/tree_nocb.h | 16 ++++++++++++---- 1 file changed, 12 insertions(+), 4 deletions(-) diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h index cb57e8312231..a3dc7465b0b2 100644 --- a/kernel/rcu/tree_nocb.h +++ b/kernel/rcu/tree_nocb.h @@ -1350,12 +1350,20 @@ lazy_rcu_shrink_scan(struct shrinker *shrink, struct shrink_control *sc) if (!rcu_rdp_is_offloaded(rdp)) continue; + if (!READ_ONCE(rdp->lazy_len)) + continue; + + rcu_nocb_lock_irqsave(rdp, flags); + /* + * Recheck under the nocb lock. Since we are not holding the bypass + * lock we may still race with increments from the enqueuer but still + * we know for sure if there is at least one lazy callback. + */ _count = READ_ONCE(rdp->lazy_len); - - if (_count == 0) + if (!_count) { + rcu_nocb_unlock_irqrestore(rdp, flags); continue; - - rcu_nocb_lock_irqsave(rdp, flags); + } WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, false)); rcu_nocb_unlock_irqrestore(rdp, flags); wake_nocb_gp(rdp, false);