linux-kernel - Re: [PATCH v1 1/4] spidev: Do not use atomic bit operations when allocating minor

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Message-ID: <Yjw4yjgordnSo+7M@smile.fi.intel.com>
Date:   Thu, 24 Mar 2022 11:24:26 +0200
From:   Andy Shevchenko <andriy.shevchenko@...ux.intel.com>
To:     Mark Brown <broonie@...nel.org>
Cc:     linux-spi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/4] spidev: Do not use atomic bit operations when
 allocating minor

On Wed, Mar 23, 2022 at 07:06:25PM +0000, Mark Brown wrote:
> On Wed, Mar 23, 2022 at 07:22:52PM +0200, Andy Shevchenko wrote:
> > On Wed, Mar 23, 2022 at 04:39:01PM +0000, Mark Brown wrote:
> 
> > > There's no *need* but the __ looks suspicious...  what's the upside
> > > here?
> 
> > It's exactly what is written in the commit message
> 
> > __*_bit() are non-atomic
> > *_bit() are atomic
> 
> > Since they are wrapped by mutex, the atomic ones are not needed.
> 
> Yes, it's not needed but what meaningful harm does it do?

There are basically two points:

1) in one driver the additional lock may not be influential, but
   if many drivers will do the same, it will block CPUs for no
   purpose;

2) derived from the above, if one copies'n'pastes the code, esp.
   using spin locks, it may become an unneeded code and performance
   degradation.



-- 
With Best Regards,
Andy Shevchenko