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Date: Thu, 20 May 2010 15:43:58 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Pekka Enberg <penberg@cs.helsinki.fi>
Cc: mingo@redhat.com, hpa@zytor.com, acme@redhat.com, paulus@samba.org,
       linux-kernel@vger.kernel.org, tzanussi@gmail.com,
       a.p.zijlstra@chello.nl, tglx@linutronix.de, mingo@elte.hu,
       linux-tip-commits@vger.kernel.org
Subject: Re: [tip:perf/core] perf: Use read() instead of lseek() in
	trace_event_read.c:skip()
Message-ID: <20100520134356.GA10080@nowhere>
References: <1273032130.6383.28.camel@tropicana> <tip-cbb5cf7ff6b298beacfe23db3386335b0b9c0a2d@git.kernel.org> <AANLkTikqkbScJOVxynoOzejKnaNIucrAczgtGIDqXLCO@mail.gmail.com>
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On Thu, May 20, 2010 at 04:39:37PM +0300, Pekka Enberg wrote:
> On Thu, May 20, 2010 at 3:54 PM, tip-bot for Tom Zanussi
> <tzanussi@gmail.com> wrote:
> > Commit-ID:  cbb5cf7ff6b298beacfe23db3386335b0b9c0a2d
> > Gitweb:     http://git.kernel.org/tip/cbb5cf7ff6b298beacfe23db3386335b0b9c0a2d
> > Author:     Tom Zanussi <tzanussi@gmail.com>
> > AuthorDate: Tue, 4 May 2010 23:02:10 -0500
> > Committer:  Frederic Weisbecker <fweisbec@gmail.com>
> > CommitDate: Thu, 20 May 2010 08:37:17 +0200
> >
> > perf: Use read() instead of lseek() in trace_event_read.c:skip()
> >
> > This is a small fix for a problem affecting live-mode, introduced
> > recently:
> >
> > root@tropicana:~# perf trace rwtop
> > perf trace started with Perl
> > script /root/libexec/perf-core/scripts/perl/rwtop.pl
> >
> >  Fatal: did not read header event
> >
> > commit d00a47cce569a3e660a8c9de5d57af28d6a9f0f7 added a skip()
> > function to skip over e.g. header_page, but this doesn't work for
> > live mode.  This patch re-implements skip() to use read() instead of
> > lseek() to fix that.
> 
> Out of curiosity: why doesn't lseek() work and why can't we fix that?


Because depending on the mode we are, that may or may not work.

We have the normal mode, when the input in a regular file (perf.data)
where it works. And we have the pipe mode, where the seek fails.