linux-kernel - Re: [PATCH v16 2/6] ring-buffer: Introducing ring-buffer mapping functions

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Date: Mon, 12 Feb 2024 16:47:23 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Vincent Donnefort <vdonnefort@...gle.com>
Cc: mhiramat@...nel.org, linux-kernel@...r.kernel.org,
 linux-trace-kernel@...r.kernel.org, mathieu.desnoyers@...icios.com,
 kernel-team@...roid.com
Subject: Re: [PATCH v16 2/6] ring-buffer: Introducing ring-buffer mapping
 functions

On Mon, 12 Feb 2024 10:44:26 +0000
Vincent Donnefort <vdonnefort@...gle.com> wrote:

> > >  static void
> > >  rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> > >  {
> > > @@ -5204,6 +5227,9 @@ rb_reset_cpu(struct ring_buffer_per_cpu *cpu_buffer)
> > >  	cpu_buffer->lost_events = 0;
> > >  	cpu_buffer->last_overrun = 0;
> > >  
> > > +	if (READ_ONCE(cpu_buffer->mapped))  
> > 
> > Isn't the buffer_mutex held when we modify mapped? I believe it's held
> > here. I don't think we need a READ_ONCE() here. Is there a reason for it?
> > 
> > Hmm, looking down, it looks like you take the buffer->mutex after
> > setting mapped, is that necessary? If we take the buffer->mutex we can
> > sync the reset with mapping.  
> 
> The idea was to not take any of the buffer mutex, reader lock if the refcount is
> simply inc/dec. Locks are only used if the meta-page is
> installed/uninstalled.

Does it matter? The inc/dec is only done at initial mapping, right? That's
a very slow path.

> 
> The WRITE_ONCE/READ_ONCE is there only to make sure no compiler optimisation
> could lead a reader to wrongly interpret that refcount while it is inc/dec. That
> is probably not necessary and I'm happy to either drop it completely or replace
> it by taking buffer mutex and reader lock whenever the refcount is inc/dec.

Yeah, probably best to drop it and keep updates within the mutex.

-- Steve