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 D6FF5253B73 for ; Thu, 19 Mar 2026 01:13:14 +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=1773882794; cv=none; b=NakIh8PIiueFKh+lXEm1/SAEGK2mGhlQC+5POr//JXliza/vmycsCJukI6u+BpKZiSs/Hh/pUnQxTJyQtwQ+Mff1hTVeFo2Kt+5rKg92v+aHn9SQVnBt0JpctC8KueGyQGIs9jAXgfPvVA2hnB4d1dRZMO9MkAvURVbS+j/4mTw= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1773882794; c=relaxed/simple; bh=hzBqHsA1YjsRNpi0dcHwTL34qLAtqbFIQeRl6wwTM40=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=fzxiVBVLyd2i8FQYe7avwZkO2veLC2LK5sMPYPbwygPL7vmVyMlYBHN836Z87TWaMebdA9TfB8QTbVAUyhbr2aQzAFWGkypvrVDT+8ufZz056uyhc4ne6dfohEs0BhZt6La3GQovT79PVRxOrX9RJhka1lJdqgxdjeQC9pyP9Bw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=E/63YZSj; 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="E/63YZSj" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 18B9AC2BC87; Thu, 19 Mar 2026 01:13:13 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1773882794; bh=hzBqHsA1YjsRNpi0dcHwTL34qLAtqbFIQeRl6wwTM40=; h=Date:From:To:Cc:Subject:References:In-Reply-To:From; b=E/63YZSjYIyY/bCT13PrTbODW53pAiJceJLGzDpOjMCwjgAwSv+hTa4BGiHMH8Wxw gURmhI+TnBzxOo6nyx3+m2EjF7H3HzgL6LST88x9c/ZofmHEfsfbTEnXw6roqLe7gZ JgLIJJsD4Blvf0lV3PpYWFC7umCR4z3A8iFKqdztfHvRgpf+IB48odF8A0VuEvfdbP w8y4Bovnga6CGc5covmjYy0uETakb5QI4x31HrTZCmtCo7FudjiWoFEKgMf25xWL46 xV3mRzkcM2KZvRJ5aXB4Ut5WKFEmPLaFkZsulrCMHJWrLX+8iXfkA1+ki5gOpQ1v3z M8EYvmvuVx3QQ== Received: from phl-compute-02.internal (phl-compute-02.internal [10.202.2.42]) by mailfauth.phl.internal (Postfix) with ESMTP id D5E11F4006A; Wed, 18 Mar 2026 21:13:12 -0400 (EDT) Received: from phl-frontend-03 ([10.202.2.162]) by phl-compute-02.internal (MEProxy); Wed, 18 Mar 2026 21:13:12 -0400 X-ME-Sender: X-ME-Received: X-ME-Proxy-Cause: gggruggvucftvghtrhhoucdtuddrgeefgedrtddtgdeftdehieeiucetufdoteggodetrf dotffvucfrrhhofhhilhgvmecuhfgrshhtofgrihhlpdfurfetoffkrfgpnffqhgenuceu rghilhhouhhtmecufedttdenucenucfjughrpeffhffvvefukfhfgggtuggjsehttdertd dttddvnecuhfhrohhmpeeuohhquhhnucfhvghnghcuoegsohhquhhnsehkvghrnhgvlhdr ohhrgheqnecuggftrfgrthhtvghrnhepleeuheethfdttdfgjedvjeeuhefhkeetveeuue eukeegteeigeeghedvffehhffhnecuffhomhgrihhnpehkvghrnhgvlhdrohhrghenucev lhhushhtvghrufhiiigvpedtnecurfgrrhgrmhepmhgrihhlfhhrohhmpegsohhquhhnod hmvghsmhhtphgruhhthhhpvghrshhonhgrlhhithihqdduieejtdelkeegjeduqddujeej keehheehvddqsghoqhhunheppehkvghrnhgvlhdrohhrghesfhhigihmvgdrnhgrmhgvpd hnsggprhgtphhtthhopeduuddpmhhouggvpehsmhhtphhouhhtpdhrtghpthhtohepqhhi rghnghdriihhrghngheslhhinhhugidruggvvhdprhgtphhtthhopehmvghmgihorhesgh hmrghilhdrtghomhdprhgtphhtthhopehjohgvlhgrghhnvghlfhesnhhvihguihgrrdgt ohhmpdhrtghpthhtohepphgruhhlmhgtkheskhgvrhhnvghlrdhorhhgpdhrtghpthhtoh epsghighgvrghshieslhhinhhuthhrohhnihigrdguvgdprhgtphhtthhopehfrhgvuggv rhhitgeskhgvrhhnvghlrdhorhhgpdhrtghpthhtohepnhgvvghrrghjrdhiihhtrhdutd esghhmrghilhdrtghomhdprhgtphhtthhopehurhgviihkihesghhmrghilhdrtghomhdp rhgtphhtthhopegsohhquhhnrdhfvghnghesghhmrghilhdrtghomh X-ME-Proxy: Feedback-ID: i8dbe485b:Fastmail Received: by mail.messagingengine.com (Postfix) with ESMTPA; Wed, 18 Mar 2026 21:13:12 -0400 (EDT) Date: Wed, 18 Mar 2026 18:13:11 -0700 From: Boqun Feng To: Zqiang Cc: Kumar Kartikeya Dwivedi , Joel Fernandes , paulmck@kernel.org, Sebastian Andrzej Siewior , frederic@kernel.org, neeraj.iitr10@gmail.com, urezki@gmail.com, boqun.feng@gmail.com, rcu@vger.kernel.org Subject: Re: Next-level bug in SRCU implementation of RCU Tasks Trace + PREEMPT_RT Message-ID: References: <20260318144305.xI6RDtzk@linutronix.de> <214fb140-041d-4fd1-8694-658547209b84@paulmck-laptop> <3c4c5a29-24ea-492d-aeee-e0d9605b4183@nvidia.com> Precedence: bulk X-Mailing-List: rcu@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: On Thu, Mar 19, 2026 at 12:26:38AM +0000, Zqiang wrote: > > > > On Wed, 18 Mar 2026 at 23:15, Boqun Feng wrote: > > > > > > > > On Wed, Mar 18, 2026 at 02:55:48PM -0700, Boqun Feng wrote: > > > On Wed, Mar 18, 2026 at 02:52:48PM -0700, Boqun Feng wrote: > > > [...] > > > > > Ah so it is an ABBA deadlock, not a ABA self-deadlock. I guess this is a > > > > > different issue, from the NMI issue? It is more of an issue of calling > > > > > call_srcu API with scheduler locks held. > > > > > > > > > > Something like below I think: > > > > > > > > > > CPU A (BPF tracepoint) CPU B (concurrent call_srcu) > > > > > ---------------------------- ------------------------------------ > > > > > [1] holds &rq->__lock > > > > > [2] > > > > > -> call_srcu > > > > > -> srcu_gp_start_if_needed > > > > > -> srcu_funnel_gp_start > > > > > -> spin_lock_irqsave_ssp_content... > > > > > -> holds srcu locks > > > > > > > > > > [4] calls call_rcu_tasks_trace() [5] srcu_funnel_gp_start (cont..) > > > > > -> queue_delayed_work > > > > > -> call_srcu() -> __queue_work() > > > > > -> srcu_gp_start_if_needed() -> wake_up_worker() > > > > > -> srcu_funnel_gp_start() -> try_to_wake_up() > > > > > -> spin_lock_irqsave_ssp_contention() [6] WANTS rq->__lock > > > > > -> WANTS srcu locks > > > > > > > > I see, we can also have a self deadlock even without CPU B, when CPU A > > > > is going to try_to_wake_up() the a worker on the same CPU. > > > > > > > > An interesting observation is that the deadlock can be avoided in > > > > queue_delayed_work() uses a non-zero delay, that means a timer will be > > > > armed instead of acquiring the rq lock. > > > > > > > > > > If my observation is correct, then this can probably fix the deadlock > > > issue with runqueue lock (untested though), but it won't work if BPF > > > tracepoint can happen with timer base lock held. > > > > > Unfortunately it can be, there is at least one tracepoint that is > > invoked with hrtimer base lock held. > > Alexei ended up fixing this in the recent past [0]. So I think this > > would cause trouble too. > > > > hrtimer_start_range_ns() -> __hrtimer_start_range_ns() -> > > remove_timer() -> __remove_hrtimer() -> debug_deactivate() -> > > trace_hrtimer_cancel(). > > BPF can attach to such a tracepoint. > > Is it possible to use irq_work_queue() to trigger queue_delay_work() > by checking if ssp == &rcu_tasks_trace_srcu_struct ? > Good call! I didn't do the exact == check, but close: https://lore.kernel.org/rcu/abtMhd_LVp3uL_pA@tardis.local/ ;-) Regards, Boqun > Thanks > Zqiang > > > > > > [0]: https://lore.kernel.org/bpf/20260204040834.22263-2-alexei.starovoitov@gmail.com > > > > > > > > Regards, > > > Boqun > > > > > > ------> > > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c > > > index 2328827f8775..a5d67264acb5 100644 > > > --- a/kernel/rcu/srcutree.c > > > +++ b/kernel/rcu/srcutree.c > > > @@ -1061,6 +1061,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, > > > struct srcu_node *snp_leaf; > > > unsigned long snp_seq; > > > struct srcu_usage *sup = ssp->srcu_sup; > > > + bool irqs_were_disabled; > > > > > > /* Ensure that snp node tree is fully initialized before traversing it */ > > > if (smp_load_acquire(&sup->srcu_size_state) < SRCU_SIZE_WAIT_BARRIER) > > > @@ -1098,6 +1099,7 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, > > > > > > /* Top of tree, must ensure the grace period will be started. */ > > > raw_spin_lock_irqsave_ssp_contention(ssp, &flags); > > > + irqs_were_disabled = irqs_disabled_flags(flags); > > > if (ULONG_CMP_LT(sup->srcu_gp_seq_needed, s)) { > > > /* > > > * Record need for grace period s. Pair with load > > > @@ -1118,9 +1120,16 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, > > > // it isn't. And it does not have to be. After all, it > > > // can only be executed during early boot when there is only > > > // the one boot CPU running with interrupts still disabled. > > > + // > > > + // If irq was disabled when call_srcu() is called, then we > > > + // could be in the scheduler path with a runqueue lock held, > > > + // delay the process_srcu() work 1 more jiffies so we don't go > > > + // through the kick_pool() -> wake_up_process() path below, and > > > + // we could avoid deadlock with runqueue lock. > > > if (likely(srcu_init_done)) > > > queue_delayed_work(rcu_gp_wq, &sup->work, > > > - !!srcu_get_delay(ssp)); > > > + !!srcu_get_delay(ssp) + > > > + !!irqs_were_disabled); > > > else if (list_empty(&sup->work.work.entry)) > > > list_add(&sup->work.work.entry, &srcu_boot_list); > > > } > > > > >