linux-kernel - Re: [PATCH v9 19/32] timers: add_timer_on(): Make sure TIMER

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Message-ID: <20231205182924.SFCmSKXe@linutronix.de>
Date:   Tue, 5 Dec 2023 19:29:24 +0100
From:   Sebastian Siewior <bigeasy@...utronix.de>
To:     Anna-Maria Behnsen <anna-maria@...utronix.de>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Eric Dumazet <edumazet@...gle.com>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Giovanni Gherdovich <ggherdovich@...e.cz>,
        Lukasz Luba <lukasz.luba@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Srinivas Pandruvada <srinivas.pandruvada@...el.com>,
        K Prateek Nayak <kprateek.nayak@....com>
Subject: Re: [PATCH v9 19/32] timers: add_timer_on(): Make sure TIMER_PINNED
 flag is set

On 2023-12-01 10:26:41 [+0100], Anna-Maria Behnsen wrote:
> When adding a timer to the timer wheel using add_timer_on(), it is an
> implicitly pinned timer. With the timer pull at expiry time model in place,
> TIMER_PINNED flag is required to make sure timers end up in proper base.
> 
> Add TIMER_PINNED flag unconditionally when add_timer_on() is executed.

This is odd. I have some vague memory that this was already the case.
Otherwise all add_timer_on() users without TIMER_PINNED may get it wrong
due to optimisation. Looking at git history it was never the case and I
can't confuse it with hrtimer since it never supported the "_on()"
feature…
At least the mix timer in drivers/char/random.c is not using the PINNED
flag with _on(). So this was wrong then?…

Sebastian