linux-kernel - RE: [PATCH 6/6] staging: comedi: don't use mutex when polling file

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Date:	Fri, 9 Oct 2015 17:24:46 +0000
From:	Hartley Sweeten <HartleyS@...ionengravers.com>
To:	Ian Abbott <abbotti@....co.uk>,
	"driverdev-devel@...uxdriverproject.org" 
	<driverdev-devel@...uxdriverproject.org>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 6/6] staging: comedi: don't use mutex when polling file

On Friday, October 09, 2015 4:27 AM, Ian Abbott wrote:
> The main mutex in a comedi device can get held for quite a while when
> processing comedi instructions, so for performance reasons, the "read"
> and "write" file operations do not use it; they use use the
> `attach_lock` rwsemaphore to protect against the comedi device becoming
> detached at an inopportune moment.  Do the same for the "poll" file
> operation.
>
> Signed-off-by: Ian Abbott <abbotti@....co.uk>
> ---
>  drivers/staging/comedi/comedi_fops.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/staging/comedi/comedi_fops.c b/drivers/staging/comedi/comedi_fops.c
> index 07bb197..88e9334 100644
> --- a/drivers/staging/comedi/comedi_fops.c
> +++ b/drivers/staging/comedi/comedi_fops.c
> @@ -2264,7 +2264,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
>  	struct comedi_device *dev = cfp->dev;
>  	struct comedi_subdevice *s, *s_read;
>  
> -	mutex_lock(&dev->mutex);
> +	down_read(&dev->attach_lock);
>  
>  	if (!dev->attached) {
>  		dev_dbg(dev->class_dev, "no driver attached\n");
> @@ -2294,7 +2294,7 @@ static unsigned int comedi_poll(struct file *file, poll_table *wait)
>  	}
>  
>  done:
> -	mutex_unlock(&dev->mutex);
> +	up_read(&dev->attach_lock);
>  	return mask;
>  }

Ian,

No issues with this patch, just a comment:

checkpatch.pl reports some issue about the spinlock_t and mutex definitions
in comedidev.h:

CHECK: spinlock_t definition without comment
#177: FILE: drivers/staging/comedi/comedidev.h:177:
+       spinlock_t spin_lock;

CHECK: spinlock_t definition without comment
#540: FILE: drivers/staging/comedi/comedidev.h:540:
+       spinlock_t spinlock;

CHECK: struct mutex definition without comment
#541: FILE: drivers/staging/comedi/comedidev.h:541:
+       struct mutex mutex;

I know these are documented in the docbook comment for the structs but would
you mind adding some comments to the definitions to quiet checkpatch.pl?

Thanks,
Hartley


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