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Thu, 19 Mar 2026 12:48:17 -0400 (EDT) Date: Thu, 19 Mar 2026 09:48:16 -0700 From: Boqun Feng To: Sebastian Andrzej Siewior Cc: Joel Fernandes , paulmck@kernel.org, frederic@kernel.org, neeraj.iitr10@gmail.com, urezki@gmail.com, boqun.feng@gmail.com, rcu@vger.kernel.org, Kumar Kartikeya Dwivedi , Tejun Heo , bpf@vger.kernel.org, Alexei Starovoitov , Daniel Borkmann , John Fastabend Subject: Re: Next-level bug in SRCU implementation of RCU Tasks Trace + PREEMPT_RT Message-ID: References: <3c4c5a29-24ea-492d-aeee-e0d9605b4183@nvidia.com> <20260319090315.Ec_eXAg4@linutronix.de> <20260319163350.c7WuYOM9@linutronix.de> Precedence: bulk X-Mailing-List: bpf@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: <20260319163350.c7WuYOM9@linutronix.de> On Thu, Mar 19, 2026 at 05:33:50PM +0100, Sebastian Andrzej Siewior wrote: > On 2026-03-19 09:27:59 [-0700], Boqun Feng wrote: > > On Thu, Mar 19, 2026 at 10:03:15AM +0100, Sebastian Andrzej Siewior wrote: > > > Please just use the queue_delayed_work() with a delay >0. > > > > > > > That doesn't work since queue_delayed_work() with a positive delay will > > still acquire timer base lock, and we can have BPF instrument with timer > > base lock held i.e. calling call_srcu() with timer base lock. > > > > irq_work on the other hand doesn't use any locking. > > Could we please restrict BPF somehow so it does roam free? It is > absolutely awful to have irq_work() in call_srcu() just because it > might acquire locks. > I agree it's not RCU's fault ;-) I guess it'll be difficult to restrict BPF, however maybe BPF can call call_srcu() in irq_work instead? Or a more systematic defer mechanism that allows BPF to defer any lock holding functions to a different context. (We have a similar issue that BPF cannot call kfree_rcu() in some cases IIRC). But we need to fix this in v7.0, so this short-term fix is still needed. Regars, Boqun > > Regards, > > Boqun > > > Sebastian