linux-kernel - Re: [PATCH v12 15/17] counter: Implement events_queue

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Message-ID: <f1c57abc-cb68-c00c-3037-dc53c0adab1a@lechnology.com>
Date:   Sat, 10 Jul 2021 12:53:54 -0500
From:   David Lechner <david@...hnology.com>
To:     William Breathitt Gray <vilhelm.gray@...il.com>,
        Jarkko Nikula <jarkko.nikula@...ux.intel.com>
Cc:     jic23@...nel.org, linux-stm32@...md-mailman.stormreply.com,
        kernel@...gutronix.de, a.fatoum@...gutronix.de,
        kamel.bouhara@...tlin.com, gwendal@...omium.org,
        alexandre.belloni@...tlin.com, linux-iio@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        syednwaris@...il.com, patrick.havelange@...ensium.com,
        fabrice.gasnier@...com, mcoquelin.stm32@...il.com,
        alexandre.torgue@...com, o.rempel@...gutronix.de
Subject: Re: [PATCH v12 15/17] counter: Implement events_queue_size sysfs
 attribute

On 7/10/21 11:08 AM, David Lechner wrote:
> On 7/10/21 10:43 AM, David Lechner wrote:
>> On 7/10/21 5:25 AM, William Breathitt Gray wrote:
>>>> and after that
>>>>
>>>> [   16.564403] ================================================
>>>> [   16.570725] WARNING: lock held when returning to user space!
>>>> [   16.577044] 5.13.0-next-20210706+ #4 Not tainted
>>>> [   16.582198] ------------------------------------------------
>>>> [   16.588507] cat/331 is leaving the kernel with locks still held!
>>>> [   16.595214] 1 lock held by cat/331:
>>>> [   16.599103]  #0: ffff888102bb3630
>>>> (&counter->chrdev_lock){+.+.}-{3:3}, at: counter_chrdev_open+0x21/0x60
>>>> [counter]
>>>>
>>>> Jarkko
>>> I'm not sure how to resolve this warning. The purpose of this lock is to
>>> limit chrdev to a single open at a time. To accomplish this I grab this
>>> lock in counter_chrdev_open() and hold it until counter_chrdev_release()
>>> is called. Is there a better way to accomplish this?
>>
>> How about using an atomic flag, e.g test_and_set_bit()?
> 
> Another option could be to rethink it at a higher level and avoid the
> need for a lock (and sysfs attribute) altogether. For example, would it
> work to (re)allocate the kfifo buffer in the COUNTER_ENABLE_EVENTS_IOCTL
> callback and add a parameter to that ioctl to specify the buffer size
> (with units of events rather than bytes)?
> 

Thinking about it a bit more, this probably isn't a good suggestion.
It would create issues with being able to start reading from the chrdev
before enabling events.