From patchwork Fri May 24 14:05:24 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Frederic Weisbecker X-Patchwork-Id: 13673209 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 3050D12BF38; Fri, 24 May 2024 14:05:27 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716559528; cv=none; b=pgT5aO6K8/7i5DW/5CHibBwj2iiZBNdPog67fGmRtQZQzD948IuTAETb0OkfZjOGC1XjsRwaUzWZ7ImWseUJ2wZsfMmmbAISJUJaKNwljnst8Qt9Qw8Sz5JeplT5wclE9hFcPcGTuqTKV2A9nLOuwwNKSls+gC9wO/+9tGKRoPc= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1716559528; c=relaxed/simple; bh=lEpDUMndEWxswvmAvM++8Gc2QtrqlIBZpwvAiQ7Z65E=; h=Date:From:To:Cc:Subject:Message-ID:MIME-Version:Content-Type: Content-Disposition; b=m5IBdjOHq2auq0wWCwEsTrHpYb7TGWMZnlUWbZ6EvLRT/S3pYbF3A8nz4XOce4arZLLBoT/PH3ZoNVhHlGSS4pWn/39cILuTYv0APPs1b5YUf1miAw4OUUda//mfFIrYTRVZOuutnDBnLmjgmRX1Kprs1qjD8IDKyFheIolRtXE= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=JdW6F8IB; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="JdW6F8IB" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 29CB7C2BBFC; Fri, 24 May 2024 14:05:26 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1716559527; bh=lEpDUMndEWxswvmAvM++8Gc2QtrqlIBZpwvAiQ7Z65E=; h=Date:From:To:Cc:Subject:From; b=JdW6F8IB+hgDYDynBhunl/81pomaGV4o1g5U1Nyx0UXvTk897yltvSF0JZK+5j9h3 GyOE0gTKFJ/YeffUGVHujVSngeoLHAvuDIv7LMiqfnGRliBrv67lIif0Jji45wady0 EG5LWbvzYN12+7v/xnI+6OWMQPqe/vOfytq/hnIJLMUkopOpN17X5PhThfn1c3zdx9 ke95NeTj2HqrYxYaGCpMykccpcV7j6uss03m5uyHAGMIPsttqemy/GB50nFfh75Rwd 3UI7e2bqYB5LiBWW5VN+Ou8AppaoK1Mj3ifhHjFvRtLk+NRSv08glVmH853ReS1C0M jG8n3PEgeMsTg== Date: Fri, 24 May 2024 16:05:24 +0200 From: Frederic Weisbecker To: linux-kernel@vger.kernel.org Cc: Frederic Weisbecker , "Paul E . McKenney" , Boqun Feng , Joel Fernandes , Neeraj Upadhyay , Uladzislau Rezki , Zqiang , rcu Subject: [PATCH] rcu: Fix rcu_barrier() VS post CPUHP_TEARDOWN_CPU invocation Message-ID: Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Disposition: inline Hi, I have found a potential race, but I might have missed something on the way that makes it actually impossible... Please double check. Thanks. --- Subject: [PATCH] rcu: Fix rcu_barrier() VS post CPUHP_TEARDOWN_CPU invocation When rcu_barrier() calls rcu_rdp_cpu_online() and observes a CPU off rnp->qsmaskinitnext, it means that all accesses from the offline CPU preceding the CPUHP_TEARDOWN_CPU are visible to RCU barrier, including callbacks expiration and counter updates. However interrupts can still fire after stop_machine() re-enables interrupts and before rcutree_report_cpu_dead(). The related accesses happening between CPUHP_TEARDOWN_CPU and rnp->qsmaskinitnext clearing are _NOT_ guaranteed to be seen by rcu_barrier() without proper ordering, especially when callbacks are invoked there to the end, making rcutree_migrate_callback() bypass barrier_lock. The following theoretical race example can make rcu_barrier() hang: CPU 0 CPU 1 ----- ----- //cpu_down() smpboot_park_threads() //ksoftirqd is parked now rcu_sched_clock_irq() invoke_rcu_core() do_softirq() rcu_core() rcu_do_batch() // callback storm // rcu_do_batch() returns // before completing all // of them // do_softirq also returns early because of // timeout. It defers to ksoftirqd but // it's parked stop_machine() take_cpu_down() rcu_barrier() spin_lock(barrier_lock) // observes rcu_segcblist_n_cbs(&rdp->cblist) != 0 do_softirq() rcu_core() rcu_do_batch() //completes all pending callbacks //smp_mb() implied _after_ callback number dec rcutree_report_cpu_dead() rnp->qsmaskinitnext &= ~rdp->grpmask; rcutree_migrate_callback() // no callback, early return without locking // barrier_lock //observes !rcu_rdp_cpu_online(rdp) rcu_barrier_entrain() rcu_segcblist_entrain() // Observe rcu_segcblist_n_cbs(rsclp) == 0 // because no barrier between reading // rnp->qsmaskinitnext and rsclp->len rcu_segcblist_add_len() smp_mb__before_atomic() // will now observe the 0 count and empty // list, but too late, we enqueue regardless WRITE_ONCE(rsclp->len, rsclp->len + v); // ignored barrier callback // rcu barrier stall... This could be solved with a read memory barrier, enforcing the message passing between rnp->qsmaskinitnext and rsclp->len, matching the full memory barrier after rsclp->len addition in rcu_segcblist_add_len() performed at the end of rcu_do_batch(). However the rcu_barrier() is complicated enough and probably doesn't need too many more subtleties. CPU down is a slowpath and the barrier_lock seldom contended. Solve the issue with unconditionally locking the barrier_lock on rcutree_migrate_callbacks(). This makes sure that either rcu_barrier() sees the empty queue or its entrained callback will be migrated. Signed-off-by: Frederic Weisbecker --- kernel/rcu/tree.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 944e55085262..925e006b64f9 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -4736,11 +4736,15 @@ void rcutree_migrate_callbacks(int cpu) struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu); bool needwake; - if (rcu_rdp_is_offloaded(rdp) || - rcu_segcblist_empty(&rdp->cblist)) - return; /* No callbacks to migrate. */ + if (rcu_rdp_is_offloaded(rdp)) + return; raw_spin_lock_irqsave(&rcu_state.barrier_lock, flags); + if (rcu_segcblist_empty(&rdp->cblist)) { + raw_spin_unlock_irqrestore(&rcu_state.barrier_lock, flags); + return; /* No callbacks to migrate. */ + } + WARN_ON_ONCE(rcu_rdp_cpu_online(rdp)); rcu_barrier_entrain(rdp); my_rdp = this_cpu_ptr(&rcu_data);