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Date: Wed, 2 Apr 2025 12:57:46 +0200
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From: Sebastian Sewior <bigeasy@linutronix.de>
To: Oleg Nesterov <oleg@redhat.com>
Cc: Andrii Nakryiko <andrii.nakryiko@gmail.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Peter Ziljstra <peterz@infradead.org>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Andrii Nakryiko <andrii@kernel.org>, bpf <bpf@vger.kernel.org>,
	Jiri Olsa <jolsa@kernel.org>,
	Masami Hiramatsu <mhiramat@kernel.org>
Subject: Re: uprobe splat in PREEMP_RT
Message-ID: <20250402105746.FMPvRBwL@linutronix.de>
References: <CAADnVQLLOHZmPO4X_dQ+cTaSDvzdWHzA0qUqQDhLFYL3D6xPxg@mail.gmail.com>
 <20250401172225.06b01b22@gandalf.local.home>
 <CAEf4BzbVmUfDVEs1ndy5hr2YYA5xgt7NODjNhy4x+Syfbr1yaA@mail.gmail.com>
 <20250402103326.GD22091@redhat.com>
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In-Reply-To: <20250402103326.GD22091@redhat.com>

On 2025-04-02 12:33:55 [+0200], Oleg Nesterov wrote:
>=20
> I still didn't read this code, but after the quick glance I don't
> understand why do we actually need utask->ri_seqcount.

Same here but on it=E2=80=A6

> The "writer" ri_timer() can't race with itself, right?

On PREEMPT_RT the timer could be preempted by a task with higher
priority and invoke hprobe_expire() somewhere else.

> The "reader" free_ret_instance() uses raw_seqcount_try_begin() without
> the "retry" logic.
>=20
> I have no idea if this logic is correct or not, but it seems that (apart
> from the necessary barriers) we could use the utask->ri_timer_is_running
> boolean instead with the same effect? Set/cleared in ri_timer(), checked
> in free_ret_instance().
>=20
> I must have missed something...

Let me try to study this  before I can make a statement=E2=80=A6

> Oleg.

Sebastian