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From: Frederic Weisbecker <frederic@kernel.org>
To: Josef Bacik <josef@toxicpanda.com>
Cc: "Paul E. McKenney" <paulmck@kernel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@kernel.org>,
	Joel Fernandes <joelagnelf@nvidia.com>,
	Boqun Feng <boqun@kernel.org>, Thomas Gleixner <tglx@kernel.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Steven Rostedt <rostedt@goodmis.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>, Jiri Olsa <jolsa@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>,
	x86@kernel.org, Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Puranjay Mohan <puranjay@kernel.org>,
	Xu Kuohai <xukuohai@huaweicloud.com>,
	Andy Lutomirski <luto@kernel.org>,
	Josh Triplett <josh@joshtriplett.org>,
	Uladzislau Rezki <urezki@gmail.com>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	Zqiang <qiang.zhang@linux.dev>, Juergen Gross <jgross@suse.com>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Ihor Solodrai <ihor.solodrai@linux.dev>,
	linux-kernel@vger.kernel.org, rcu@vger.kernel.org,
	linux-trace-kernel@vger.kernel.org, bpf@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH RFC v2 03/15] rcu-tasks: Hold trampoline nesting across irq-exit preemption in trampoline text
Date: Tue, 15 Sep 2026 15:13:23 +0200	[thread overview]
Message-ID: <aqlEcz5g5VNjHWzP@localhost.localdomain> (raw)
In-Reply-To: <20260911-b4-rcu-tasks-preempt-qs-v2-3-eaaa61ed2da4@toxicpanda.com>

Le Fri, Sep 11, 2026 at 02:08:41PM +0000, Josef Bacik a écrit :
> A trampoline's own rcu_tramp_nesting increment and decrement live inside
> the trampoline, so there is a window of a few instructions on entry and
> exit where the count is zero while the CPU is executing trampoline text
> (or text on the way into one, such as a static ftrace stub holding a
> direct-call target).  In that window the task has not called out, so it
> can only be preempted from an interrupt, and the interrupted instruction
> pointer identifies where it is.

I haven't read the whole patchset yet but what happens if the count is zero
on trampoline exit but the trampoline isn't finished yet and RCU tasks does a
fully quiescent scan of all tasks during that short window? Isn't that ending
with a too short grace period?

Thanks.

-- 
Frederic Weisbecker
SUSE Labs

  reply	other threads:[~2026-09-15 13:13 UTC|newest]

Thread overview: 32+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-09-11 14:08 [PATCH RFC v2 00/15] rcu-tasks: let preemption outside trampolines be a quiescent state Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 01/15] rcu-tasks: Add per-task trampoline nesting count Josef Bacik
2026-09-11 17:23   ` Paul E. McKenney
2026-09-11 14:08 ` [PATCH RFC v2 02/15] entry: Pass pt_regs to irqentry_exit_cond_resched() Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 03/15] rcu-tasks: Hold trampoline nesting across irq-exit preemption in trampoline text Josef Bacik
2026-09-15 13:13   ` Frederic Weisbecker [this message]
2026-09-11 14:08 ` [PATCH RFC v2 04/15] kprobes: Let Tasks RCU recognise tasks preempted in an optprobe jump window Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 05/15] ftrace: Mark modules hosting direct-call trampolines for Tasks RCU Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 06/15] x86/ftrace: Maintain Tasks RCU trampoline nesting in ftrace_caller Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 07/15] x86/kprobes: Maintain Tasks RCU trampoline nesting in the optprobe template Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 08/15] bpf, x86: Maintain Tasks RCU trampoline nesting in the BPF trampoline Josef Bacik
2026-09-12  3:27   ` Alexei Starovoitov
2026-09-12  5:10     ` Paul E. McKenney
2026-09-12 17:18       ` Alexei Starovoitov
2026-09-12 18:03         ` Paul E. McKenney
2026-09-12 19:40           ` Alexei Starovoitov
2026-09-12 22:28             ` Paul E. McKenney
2026-09-12 23:59               ` Alexei Starovoitov
2026-09-13  3:07                 ` Paul E. McKenney
2026-09-12 21:14           ` David Laight
2026-09-12 22:31             ` Paul E. McKenney
2026-09-13 11:28               ` David Laight
2026-09-13 18:20                 ` Paul E. McKenney
2026-09-11 14:08 ` [PATCH RFC v2 09/15] arm64: ftrace: Maintain Tasks RCU trampoline nesting in ftrace_caller Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 10/15] bpf, arm64: Maintain Tasks RCU trampoline nesting in the BPF trampoline Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 11/15] samples: ftrace: Maintain Tasks RCU trampoline nesting in direct-call trampolines Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 12/15] rcutorture: Bracket Tasks RCU readers with trampoline nesting Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 13/15] rcu-tasks: Treat preemption outside trampolines as a quiescent state Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 14/15] rcu-tasks: Retire switched-out tasks with no trampoline nesting at scan time Josef Bacik
2026-09-11 14:08 ` [PATCH RFC v2 15/15] rcu-tasks: Kick running holdouts through the scheduler Josef Bacik
2026-09-11 18:46   ` Paul E. McKenney
2026-09-13  7:13 ` [PATCH RFC v2 00/15] rcu-tasks: let preemption outside trampolines be a quiescent state Yafang Shao

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