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From: Dario Faggioli <dario.faggioli@citrix.com>
To: Jan Beulich <JBeulich@suse.com>
Cc: George Dunlap <george.dunlap@eu.citrix.com>,
	xen-devel@lists.xenproject.org
Subject: Re: [PATCH 02/15] xen: tracing: avoid checking tb_init_done multiple times.
Date: Wed, 7 Jun 2017 17:55:31 +0200	[thread overview]
Message-ID: <1496850931.9462.5.camel@citrix.com> (raw)
In-Reply-To: <59382DDF0200007800160770@prv-mh.provo.novell.com>


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On Wed, 2017-06-07 at 08:46 -0600, Jan Beulich wrote:
> > > > On 01.06.17 at 19:33, <dario.faggioli@citrix.com> wrote:
> > 
> > In fact, when calling __trace_var() directly, we can
> > assume that tb_init_done has been checked to be true,
> > and the if is hence redundant.
> 
> The "assume" here worries me: What if there's a single place
> somewhere that a grep can't easily find where no check is
> present? Is it certain that ...
> 
> > @@ -691,7 +691,8 @@ void __trace_var(u32 event, bool_t cycles,
> > unsigned int extra,
> >      unsigned int extra_word;
> >      bool_t started_below_highwater;
> >  
> > -    if( !tb_init_done )
> > +    /* If the event is not interesting, bail, as early as possible
> > */
> > +    if ( (tb_event_mask & event) == 0 )
> >          return;
> 
> ... this check would always be false then (i.e. tb_event_mask is
> always zero) in that case?
> 
As said when replying to Andrew, I originally put an ASSERT() there.

That made me realize, though, that the existing if(!tb_init_done) is
ineffective and potentially racy (or, if you want to be kind "it's a
best effort kind of measure") already.

In fact, even right now, without my patches, we don't hold the tracing
lock when we execute that if. Nothing prevents tb_init_done to become 0
_right_after_ we saw it being 1 and decide to go ahead.

This, to me, looks like an even more compelling reason to remove it.
But I think I can improve the commit message so that it explains this
thing that I just said above too.

Thanks and Regards,
Dario
-- 
<<This happens because I choose it to happen!>> (Raistlin Majere)
-----------------------------------------------------------------
Dario Faggioli, Ph.D, http://about.me/dario.faggioli
Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK)

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  reply	other threads:[~2017-06-07 15:56 UTC|newest]

Thread overview: 68+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2017-06-01 17:33 [PATCH 00/15] xen/tools: add tracing to various Xen subsystems Dario Faggioli
2017-06-01 17:33 ` [PATCH 01/15] xen: in do_softirq() sample smp_processor_id() once and for all Dario Faggioli
2017-06-07 14:38   ` Jan Beulich
2017-06-08 14:12     ` George Dunlap
2017-06-08 14:20   ` George Dunlap
2017-06-08 14:42     ` Jan Beulich
2017-06-01 17:33 ` [PATCH 02/15] xen: tracing: avoid checking tb_init_done multiple times Dario Faggioli
2017-06-01 17:53   ` Andrew Cooper
2017-06-01 23:08     ` Dario Faggioli
2017-06-07 14:46   ` Jan Beulich
2017-06-07 15:55     ` Dario Faggioli [this message]
2017-06-07 16:06       ` Jan Beulich
2017-06-08 14:34         ` George Dunlap
2017-06-08 14:37   ` George Dunlap
2017-06-01 17:33 ` [PATCH 03/15] xen/tools: tracing: several improvements on IRQs tracing Dario Faggioli
2017-06-01 18:02   ` Andrew Cooper
2017-06-01 23:12     ` Dario Faggioli
2017-06-07 15:05   ` Jan Beulich
2017-06-07 15:45     ` Dario Faggioli
2017-06-07 15:58       ` Jan Beulich
2017-06-08 14:53         ` George Dunlap
2017-06-08 15:34           ` Jan Beulich
2017-06-08 14:59   ` George Dunlap
2017-06-01 17:34 ` [PATCH 04/15] tools: xenalyze: fix dumping of PM_IDLE events Dario Faggioli
2017-06-08 15:06   ` George Dunlap
2017-06-01 17:34 ` [PATCH 05/15] xen: make it possible to disable tracing in Kconfig Dario Faggioli
2017-06-01 18:43   ` Andrew Cooper
2017-06-07 11:01     ` Julien Grall
2017-06-07 15:14   ` Jan Beulich
2017-06-08 15:16     ` George Dunlap
2017-06-08 15:35       ` Jan Beulich
2017-06-08 15:37         ` George Dunlap
2017-06-08 15:44           ` Jan Beulich
2017-06-08 15:17   ` George Dunlap
2017-06-01 17:34 ` [PATCH 06/15] xen: trace IRQ enabling/disabling Dario Faggioli
2017-06-01 19:08   ` Andrew Cooper
2017-06-01 23:42     ` Dario Faggioli
2017-06-08 15:51       ` George Dunlap
2017-06-08 16:05       ` Jan Beulich
2017-06-07 11:16   ` Julien Grall
2017-06-07 15:22     ` Dario Faggioli
2017-06-09 10:51       ` Julien Grall
2017-06-09 10:53         ` Julien Grall
2017-06-09 10:55         ` George Dunlap
2017-06-09 11:00           ` Julien Grall
2017-06-08 16:01   ` George Dunlap
2017-06-08 16:11     ` Dario Faggioli
2017-06-09 10:41   ` Jan Beulich
2017-06-01 17:34 ` [PATCH 07/15] tools: tracing: handle IRQs on/off events in xentrace and xenalyze Dario Faggioli
2017-06-13 15:58   ` George Dunlap
2017-06-01 17:34 ` [PATCH 08/15] xen: trace RCU behavior Dario Faggioli
2017-06-09 10:48   ` Jan Beulich
2017-06-13 16:05   ` George Dunlap
2017-06-01 17:34 ` [PATCH 09/15] tools: tracing: handle RCU events in xentrace and xenalyze Dario Faggioli
2017-06-13 16:12   ` George Dunlap
2017-06-01 17:34 ` [PATCH 10/15] xen: trace softirqs Dario Faggioli
2017-06-09 10:51   ` Jan Beulich
2017-06-01 17:34 ` [PATCH 11/15] tools: tracing: handle RCU events in xentrace and xenalyze Dario Faggioli
2017-06-01 17:35 ` [PATCH 12/15] xen: trace tasklets Dario Faggioli
2017-06-09 10:59   ` Jan Beulich
2017-06-09 11:17     ` Dario Faggioli
2017-06-09 11:29       ` Jan Beulich
2017-06-01 17:35 ` [PATCH 13/15] tools: tracing: handle tasklets events in xentrace and xenalyze Dario Faggioli
2017-06-01 17:35 ` [PATCH 14/15] xen: trace timers Dario Faggioli
2017-06-01 17:35 ` [PATCH 15/15] tools: tracing: handle timers events in xentrace and xenalyze Dario Faggioli
2017-06-07 14:13 ` [PATCH 00/15] xen/tools: add tracing to various Xen subsystems Konrad Rzeszutek Wilk
2017-06-08 16:45   ` Dario Faggioli
2017-06-13 16:34 ` George Dunlap

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