linux-kernel - Re: [PATCH] soundwire: stream: fix bad unlock balance

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Message-ID: <2dc66f9d-e508-d457-a7d6-c06c4336e7b8@linaro.org>
Date:   Thu, 23 May 2019 09:43:14 +0100
From:   Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
To:     Pierre-Louis Bossart <pierre-louis.bossart@...ux.intel.com>,
        vkoul@...nel.org
Cc:     sanyog.r.kale@...el.com, alsa-devel@...a-project.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] soundwire: stream: fix bad unlock balance



On 22/05/2019 17:41, Pierre-Louis Bossart wrote:
> 
> 
> On 5/22/19 11:25 AM, Srinivas Kandagatla wrote:
>> This patch fixes below warning due to unlocking without locking.
>>
>>   =====================================
>>   WARNING: bad unlock balance detected!
>>   5.1.0-16506-gc1c383a6f0a2-dirty #1523 Tainted: G        W
>>   -------------------------------------
>>   aplay/2954 is trying to release lock (&bus->msg_lock) at:
>>   do_bank_switch+0x21c/0x480
>>   but there are no more locks to release!
>>
>> Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@...aro.org>
>> ---
>>   drivers/soundwire/stream.c | 3 ++-
>>   1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/drivers/soundwire/stream.c b/drivers/soundwire/stream.c
>> index 544925ff0b40..d16268f30e4f 100644
>> --- a/drivers/soundwire/stream.c
>> +++ b/drivers/soundwire/stream.c
>> @@ -814,7 +814,8 @@ static int do_bank_switch(struct 
>> sdw_stream_runtime *stream)
>>               goto error;
>>           }
>> -        mutex_unlock(&bus->msg_lock);
>> +        if (mutex_is_locked(&bus->msg_lock))
>> +            utex_unlock(&bus->msg_lock);
> 
> Does this even compile? should be mutex_unlock, no?
> 
> We also may want to identify the issue in more details without pushing 
> it under the rug. The locking mechanism is far from simple and it's 
> likely there are a number of problems with it.
> 
msg_lock is taken conditionally during multi link bank switch cases, 
however the unlock is done unconditionally leading to this warning.

Having a closer look show that there could be a dead lock in this path 
while executing sdw_transfer(). And infact there is no need to take 
msg_lock in  multi link switch cases as sdw_transfer should take care of 
this.

Vinod/Sanyog any reason why msg_lock is really required in this path?

--srini

>>       }
>>       return ret;
>>