linux-kernel - [PATCH 03/25] perf intel-pt: Fix potential loop forever

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Date:	Fri, 25 Sep 2015 16:15:34 +0300
From:	Adrian Hunter <adrian.hunter@...el.com>
To:	Arnaldo Carvalho de Melo <acme@...nel.org>
Cc:	Jiri Olsa <jolsa@...hat.com>, linux-kernel@...r.kernel.org
Subject: [PATCH 03/25] perf intel-pt: Fix potential loop forever

TSC packets contain only 7 bytes of TSC.  The 8th byte is assumed
to change so infrequently that its value can be inferred.  However
the logic must cater for a 7 byte wraparound, which it does by
adding 1 to the top byte.

The existing code was doing that with a while loop even though the
addition should only need to be done once.  That logic won't work
(will loop forever) if TSC wraps around at the 8th byte.
Theoretically that would take at least 10 years, unless something
else went wrong.

And what else could go wrong.  Well, if the chunks of trace data
are processed out of order, it will make it look like the 7-byte
TSC has gone backwards (i.e. wrapped).  If that happens 256 times
then stuck in the while loop it will be.

Fix that by getting rid of the unnecessary while loop.

Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
---
 tools/perf/util/intel-pt-decoder/intel-pt-decoder.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c b/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
index 22ba50224319..9409d014b46c 100644
--- a/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
+++ b/tools/perf/util/intel-pt-decoder/intel-pt-decoder.c
@@ -650,7 +650,7 @@ static int intel_pt_calc_cyc_cb(struct intel_pt_pkt_info *pkt_info)
 		if (data->from_mtc && timestamp < data->timestamp &&
 		    data->timestamp - timestamp < decoder->tsc_slip)
 			return 1;
-		while (timestamp < data->timestamp)
+		if (timestamp < data->timestamp)
 			timestamp += (1ULL << 56);
 		if (pkt_info->last_packet_type != INTEL_PT_CYC) {
 			if (data->from_mtc)
@@ -1191,7 +1191,7 @@ static void intel_pt_calc_tsc_timestamp(struct intel_pt_decoder *decoder)
 					timestamp);
 			timestamp = decoder->timestamp;
 		}
-		while (timestamp < decoder->timestamp) {
+		if (timestamp < decoder->timestamp) {
 			intel_pt_log_to("Wraparound timestamp", timestamp);
 			timestamp += (1ULL << 56);
 			decoder->tsc_timestamp = timestamp;
-- 
1.9.1

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