linux-kernel - Re: [tip:perf/core] perf: Use read() instead of lseek() in trace_event

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Message-ID: <4BF53C1E.9040305@zytor.com>
Date:	Thu, 20 May 2010 06:41:50 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Pekka Enberg <penberg@...helsinki.fi>
CC:	mingo@...hat.com, acme@...hat.com, paulus@...ba.org,
	linux-kernel@...r.kernel.org, tzanussi@...il.com,
	a.p.zijlstra@...llo.nl, fweisbec@...il.com, tglx@...utronix.de,
	mingo@...e.hu, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:perf/core] perf: Use read() instead of lseek() in 	trace_event_read.c:skip()

On 05/20/2010 06:39 AM, Pekka Enberg wrote:
> On Thu, May 20, 2010 at 3:54 PM, tip-bot for Tom Zanussi
> <tzanussi@...il.com> wrote:
>> Commit-ID:  cbb5cf7ff6b298beacfe23db3386335b0b9c0a2d
>> Gitweb:     http://git.kernel.org/tip/cbb5cf7ff6b298beacfe23db3386335b0b9c0a2d
>> Author:     Tom Zanussi <tzanussi@...il.com>
>> AuthorDate: Tue, 4 May 2010 23:02:10 -0500
>> Committer:  Frederic Weisbecker <fweisbec@...il.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@...picana:~# 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?

Presumably because we're reading from a nonseekable stream?

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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