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

[Jin, Zhi]

> -----Original Message-----
> From: Alexander Shishkin [mailto:alexander.shishkin@xxxxxxxxxxxxxxx]
> Sent: Monday, October 15, 2018 3:43 PM
> To: Jin, Zhi <zhi.jin@xxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx; Jin, Zhi <zhi.jin@xxxxxxxxx>
> Subject: Re: [PATCH] stm class: increase pos if no free channels found
> 
> Zhi Jin <zhi.jin@xxxxxxxxx> 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