From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id DF1D41ED38 for ; Tue, 25 Jul 2023 11:26:09 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 60F39C433CB; Tue, 25 Jul 2023 11:26:09 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1690284369; bh=F7PoIq55H4DQZ0kzjQ/vJZDTCsckSefymbwhRlMQaOY=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=FPIb/2u93Hud2wwuwFQGcYQ8zln5V8eOWWaAZO4vzh6981W09B2/EhBoPVRXTf4L/ yMoAz8cYcDC3eD0qYgUUOO77pUUYgAYZBxQ0nXyrDuWu5FAUzhQHHHUyzzlXUKpa+s LLoDjdX/bYMsMHR8OdcMvuQS5w3Ho/FTN5HuFMJw= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, "Paul E. McKenney" , "Joel Fernandes (Google)" Subject: [PATCH 5.10 331/509] rcu-tasks: Simplify trc_read_check_handler() atomic operations Date: Tue, 25 Jul 2023 12:44:30 +0200 Message-ID: <20230725104608.860158097@linuxfoundation.org> X-Mailer: git-send-email 2.41.0 In-Reply-To: <20230725104553.588743331@linuxfoundation.org> References: <20230725104553.588743331@linuxfoundation.org> User-Agent: quilt/0.67 Precedence: bulk X-Mailing-List: patches@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit From: "Paul E. McKenney" [ Upstream commit 96017bf9039763a2e02dcc6adaa18592cd73a39d ] Currently, trc_wait_for_one_reader() atomically increments the trc_n_readers_need_end counter before sending the IPI invoking trc_read_check_handler(). All failure paths out of trc_read_check_handler() and also from the smp_call_function_single() within trc_wait_for_one_reader() must carefully atomically decrement this counter. This is more complex than it needs to be. This commit therefore simplifies things and saves a few lines of code by dispensing with the atomic decrements in favor of having trc_read_check_handler() do the atomic increment only in the success case. In theory, this represents no change in functionality. Cc: # 5.10.x Signed-off-by: Paul E. McKenney Signed-off-by: Joel Fernandes (Google) Signed-off-by: Greg Kroah-Hartman --- kernel/rcu/tasks.h | 20 +++----------------- 1 file changed, 3 insertions(+), 17 deletions(-) --- a/kernel/rcu/tasks.h +++ b/kernel/rcu/tasks.h @@ -841,32 +841,24 @@ static void trc_read_check_handler(void // If the task is no longer running on this CPU, leave. if (unlikely(texp != t)) { - if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end))) - wake_up(&trc_wait); goto reset_ipi; // Already on holdout list, so will check later. } // If the task is not in a read-side critical section, and // if this is the last reader, awaken the grace-period kthread. if (likely(!READ_ONCE(t->trc_reader_nesting))) { - if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end))) - wake_up(&trc_wait); - // Mark as checked after decrement to avoid false - // positives on the above WARN_ON_ONCE(). WRITE_ONCE(t->trc_reader_checked, true); goto reset_ipi; } // If we are racing with an rcu_read_unlock_trace(), try again later. - if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0)) { - if (WARN_ON_ONCE(atomic_dec_and_test(&trc_n_readers_need_end))) - wake_up(&trc_wait); + if (unlikely(READ_ONCE(t->trc_reader_nesting) < 0)) goto reset_ipi; - } WRITE_ONCE(t->trc_reader_checked, true); // Get here if the task is in a read-side critical section. Set // its state so that it will awaken the grace-period kthread upon // exit from that critical section. + atomic_inc(&trc_n_readers_need_end); // One more to wait on. WARN_ON_ONCE(READ_ONCE(t->trc_reader_special.b.need_qs)); WRITE_ONCE(t->trc_reader_special.b.need_qs, true); @@ -960,21 +952,15 @@ static void trc_wait_for_one_reader(stru if (per_cpu(trc_ipi_to_cpu, cpu) || t->trc_ipi_to_cpu >= 0) return; - atomic_inc(&trc_n_readers_need_end); per_cpu(trc_ipi_to_cpu, cpu) = true; t->trc_ipi_to_cpu = cpu; rcu_tasks_trace.n_ipis++; - if (smp_call_function_single(cpu, - trc_read_check_handler, t, 0)) { + if (smp_call_function_single(cpu, trc_read_check_handler, t, 0)) { // Just in case there is some other reason for // failure than the target CPU being offline. rcu_tasks_trace.n_ipis_fails++; per_cpu(trc_ipi_to_cpu, cpu) = false; t->trc_ipi_to_cpu = cpu; - if (atomic_dec_and_test(&trc_n_readers_need_end)) { - WARN_ON_ONCE(1); - wake_up(&trc_wait); - } } } }