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

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

[Zhi Jin]

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@intel.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

RE: [PATCH] stm class: increase pos if no free channels found

[Jin, Zhi]

Hi Alexander,

Any comments about this patch?

BRs
Zhi Jin

> -----Original Message-----
> From: Jin, Zhi
> Sent: Thursday, September 06, 2018 3:22 PM
> To: alexander.shishkin@linux.intel.com
> Cc: linux-kernel@vger.kernel.org; Jin, Zhi <zhi.jin@intel.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@intel.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

Re: [PATCH] stm class: increase pos if no free channels found

[Alexander Shishkin]

Zhi Jin <zhi.jin@intel.com> writes:

> 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.

The description is slightly confusing, but the patch looks correct and
the original code is clearly wrong. Thank you for finding this!

Basically, if you request 1 channel 3 times, release the first two and
then request 4 channels, you'll be stuck, right?

Thanks,
--
Alex

RE: [PATCH] stm class: increase pos if no free channels found

[Jin, Zhi]

> -----Original Message-----
> From: Alexander Shishkin [mailto:alexander.shishkin@linux.intel.com]
> Sent: Monday, October 15, 2018 3:43 PM
> To: Jin, Zhi <zhi.jin@intel.com>
> Cc: linux-kernel@vger.kernel.org; Jin, Zhi <zhi.jin@intel.com>
> Subject: Re: [PATCH] stm class: increase pos if no free channels found
> 
> Zhi Jin <zhi.jin@intel.com> writes:
> 
> > 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.
> 
> The description is slightly confusing, but the patch looks correct and
> the original code is clearly wrong. Thank you for finding this!
> 
> Basically, if you request 1 channel 3 times, release the first two and
> then request 4 channels, you'll be stuck, right?

Yes, you are right.
But the real case that I reproduced the issue is a little different:
I have 2 stp-policy:
"console": masters "256 259"  channels "7 10"
"user"   : masters "256 1024" channels "0 127"
I understand the policies should not be overlapped, which is caused by some
other issues.
So if someone uses "console" to request a channel (who will get Channel #7)
and then another uses "user" to request more than 8 channels, it will be stuck.
The commit message is what I trying to abstract the above case, sorry for the
confusion.

> 
> Thanks,
> --
> Alex