linux-kernel - Re: [PATCH 7/7] staging: comedi: comedi_fops: extend spin-lock scope in comedi

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Date:	Tue, 31 Mar 2015 10:42:58 +0100
From:	Ian Abbott <abbotti@....co.uk>
To:	Hartley Sweeten <HartleyS@...ionengravers.com>,
	"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 7/7] staging: comedi: comedi_fops: extend spin-lock scope
 in comedi_event()

On 30/03/15 17:47, Hartley Sweeten wrote:
> On Friday, March 27, 2015 8:13 AM, Ian Abbott wrote:
>> `comedi_event()` is called from low-level drivers to handle comedi
>> asynchronous command event flags.  As a safety check, it checks the
>> subdevice's "run" flags to make sure an asynchronous command is running.
>> It can also change the run flags to mark the command as no longer
>> running (possibly also marking it as terminated with an error).
>> Checking the runflags and modifying them involves two uses of the
>> subdevice's spin-lock.  It seems better to do it with a single use of
>> the spin-lock.  This also avoids possible interactions with
>> `do_become_nonbusy()`.
>>
>> Acquire the subdevice's spin-lock at the start of `comedi_event()` and
>> release it near the end, before a possible call to `kill_fasync()` (but
>> after it's parameter values have been determined).
>>
>> Add and make use of few new inline helper functions:
>>
>> * `__comedi_clear_subdevice_runflags()` -- clears some run flags without
>>    using the spin-lock
>> * `__comedi_set_subdevice_runflags()` -- sets some run flags without
>>    using the spin-lock
>> * `__comedi_get_subdevice_runflags()` -- a spin-lockless version of
>>    `comedi_get_subdevice_runflags()
>> * `__comedi_is_subdevice_running()` -- a spin-lockless version of
>> * `comedi_is_subdevice_running()`
>>
>> Signed-off-by: Ian Abbott <abbotti@....co.uk>
>
> Ian,
>
> For completeness, the comedi_alloc_spriv() helper should probably use
> __comedi_set_subdevice_runflags() to set the COMEDI_SRF_FREE_SPRIV
> bit.
>
> Regards,
> Hartley
>

Good point.  "drivers/staging/comedi/drivers.c" also reads the runflags 
directly, so perhaps __comedi_clear_subdevice_runflags(), 
__comedi_set_subdevice_runflags() and __comedi_get_subdevice_runflags() 
should be placed in "drivers/staging/comedi/comedi_internal.h".  Or we 
could ditch all three of those inline functions as they are just simple 
one-liners.

-- 
-=( Ian Abbott @ MEV Ltd.    E-mail: <abbotti@....co.uk> )=-
-=(                          Web: http://www.mev.co.uk/  )=-
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