linux-kernel - [PATCH] perf_events: fix errors path in perf_output

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Message-ID: <4bf13099.698fd80a.503a.ffffacd5@mx.google.com>
Date:	Mon, 17 May 2010 12:46:01 +0200
From:	Stephane Eranian <eranian@...gle.com>
To:	linux-kernel@...r.kernel.org
Cc:	peterz@...radead.org, mingo@...e.hu, paulus@...ba.org,
	davem@...emloft.net, fweisbec@...il.com, acme@...radead.org,
	perfmon2-devel@...ts.sf.net, eranian@...il.com, eranian@...gle.com,
	jkacur@...hat.com
Subject: [PATCH] perf_events: fix errors path in perf_output_begin() (take 2)

In case the sampling buffer has no "payload" pages, nr_pages is 0.
The problem is that the error path in perf_output_begin() skips to
a label which assumes perf_output_lock() has been issued which is
not the case. That triggers a WARN_ON() is perf_output_unlock().

This patch fixes the problem by skipping perf_output_unlock()
in case data->nr_pages is 0 while maintaining the accounting
for data->lost.

Signed-off-by: Stephane Eranian <eranian@...gle.com>

--
diff --git a/kernel/perf_event.c b/kernel/perf_event.c
index a4fa381..95137b6 100644
--- a/kernel/perf_event.c
+++ b/kernel/perf_event.c
@@ -3035,8 +3035,10 @@ int perf_output_begin(struct perf_output_handle *handle,
 	handle->nmi	= nmi;
 	handle->sample	= sample;
 
-	if (!data->nr_pages)
-		goto fail;
+	if (!data->nr_pages) {
+		atomic_inc(&data->lost);
+		goto out;
+	}
 
 	have_lost = atomic_read(&data->lost);
 	if (have_lost)
--
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