linux-kernel - Re: [PATCH] tracepoints: Update static_call before tp

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Message-ID: <YP/xjnGx+CRYr5RR@hirez.programming.kicks-ass.net>
Date:   Tue, 27 Jul 2021 13:44:14 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-trace-devel <linux-trace-devel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stefan Metzmacher <metze@...ba.org>,
        io-uring <io-uring@...r.kernel.org>, paulmck <paulmck@...nel.org>
Subject: Re: [PATCH] tracepoints: Update static_call before tp_funcs when
 adding a tracepoint

On Mon, Jul 26, 2021 at 02:49:03PM -0400, Steven Rostedt wrote:
> OK. I see the issue you are saying. And this came from my assumption
> that the tracepoint code did a synchronization when unregistering the
> last callback. But of course it wont because that would make a lot of
> back to back synchronizations of a large number of tracepoints being
> unregistered at once.
> 
> And doing it for all 0->1 or 1->0 or even a 1->0->1 can be costly.
> 
> One way to handle this is when going from 1->0, set off a worker that
> will do the synchronization asynchronously, and if a 0->1 comes in,
> have that block until the synchronization is complete. This should
> work, and not have too much of an overhead.
> 
> If one 1->0 starts the synchronization, and one or more 1->0
> transitions happen, it will be recorded where the worker will do
> another synchronization, to make sure all 1->0 have went through a full
> synchronization before a 0->1 can happen.
> 
> If a 0->1 comes in while a synchronization is happening, it will note
> the current "number" for the synchronizations (if another one is
> queued, it will wait for one more), before it can begin. As locks will
> be held while waiting for synchronizations to finish, we don't need to
> worry about another 1->0 coming in while a 0->1 is waiting.

Wouldn't get_state_synchronize_rcu() and cond_synchronize_rcu() get you
what you need?