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

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Message-ID: <87h8hn64gi.fsf@ashishki-desk.ger.corp.intel.com>
Date:   Mon, 15 Oct 2018 10:42:37 +0300
From:   Alexander Shishkin <alexander.shishkin@...ux.intel.com>
To:     Zhi Jin <zhi.jin@...el.com>
Cc:     linux-kernel@...r.kernel.org, Zhi Jin <zhi.jin@...el.com>
Subject: Re: [PATCH] stm class: increase pos if no free channels found

Zhi Jin <zhi.jin@...el.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