linux-kernel - Re: [PATCH v6] Unified trace buffer

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Message-ID: <alpine.DEB.1.10.0809261724000.21618@gandalf.stny.rr.com>
Date:	Fri, 26 Sep 2008 17:26:15 -0400 (EDT)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Masami Hiramatsu <mhiramat@...hat.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	prasad@...ux.vnet.ibm.com,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Mathieu Desnoyers <compudj@...stal.dyndns.org>,
	"Frank Ch. Eigler" <fche@...hat.com>,
	David Wilder <dwilder@...ibm.com>, hch@....de,
	Martin Bligh <mbligh@...gle.com>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <srostedt@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>
Subject: Re: [PATCH v6] Unified trace buffer


On Fri, 26 Sep 2008, Masami Hiramatsu wrote:

> Peter Zijlstra wrote:
> > On Fri, 2008-09-26 at 14:05 -0400, Steven Rostedt wrote:
> >> +static void
> >> +rb_remove_pages(struct ring_buffer_per_cpu *cpu_buffer, unsigned
> >> nr_pages)
> >> +{
> >> +       struct page *page;
> >> +       struct list_head *p;
> >> +       unsigned i;
> >> +
> >> +       atomic_inc(&cpu_buffer->record_disabled);
> > 
> > You probably want synchronize_sched() here (and similar other places) to
> > ensure any active writer on the corresponding cpu is actually stopped.
> 
> Would it really be done in the buffer layer?
> I think it should be done by each tracer, because buffer layer
> can't ensure truly active writers have stopped.
> 

Actually it can ;-)

Since all writes to the buffer at least disable preemption, by issuing a 
synchronize_sched, we can guarantee that after disabling the record, all 
activity will be done.

-- Steve

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