linux-kernel - [PATCH] stm class: increase pos if no free channels found

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Date:   Thu,  6 Sep 2018 15:22:10 +0800
From:   Zhi Jin <zhi.jin@...el.com>
To:     alexander.shishkin@...ux.intel.com
Cc:     linux-kernel@...r.kernel.org, Zhi Jin <zhi.jin@...el.com>
Subject: [PATCH] stm class: increase pos if no free channels found

Considering this case in find_free_channels():

bitmap:
       +------------------+-+-+-+-+-+-+-+-+-+-+
       |     ......       |0|0|0|0|0|0|0|1|0|0|
       +------------------+-+-+-+-+-+-+-+-+-+-+

1. Channel #2 has been occupied, so bit #2 is 1, and the others
   are all 0.
2. Another thread tries to find 4 free channels from #0.
3. In the 1st loop, pos starts from 0, and then it checks if the
   following 4 bits are all 0, but fails, as bit#2 is 1.
4. In the 2st loop, pos is not updated, and still starts from 0,
   so nothing changes against loop #1.
5. Dead loop ...

This patch is to update the pos in step #3 to avoid the issue.

Fixes: 7bd1d4093c2f ("stm class: Introduce an abstraction for System Trace Module devices")
Signed-off-by: Zhi Jin <zhi.jin@...el.com>
---
 drivers/hwtracing/stm/core.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/drivers/hwtracing/stm/core.c b/drivers/hwtracing/stm/core.c
index 10bcb5d73f90..c86a979b9bd7 100644
--- a/drivers/hwtracing/stm/core.c
+++ b/drivers/hwtracing/stm/core.c
@@ -244,6 +244,8 @@ static int find_free_channels(unsigned long *bitmap, unsigned int start,
 			;
 		if (i == width)
 			return pos;
+
+		pos += i;
 	}

 	return -1;
-- 
2.7.4