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Message-ID: <653402b90609281220s449ed538k41043348f4b11e77@mail.gmail.com>
Date: Thu, 28 Sep 2006 21:20:22 +0200
From: "Miguel Ojeda" <maxextreme@gmail.com>
To: "Andrew Morton" <akpm@osdl.org>
Subject: Re: [PATCH 2.6.18 real-V5] drivers: add lcd display support
Cc: linux-kernel@vger.kernel.org
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	 <653402b90609271358m58950e96k99b2314f9732b5ef@mail.gmail.com>
	 <20060927154208.149069e3.akpm@osdl.org>
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On 9/28/06, Andrew Morton <akpm@osdl.org> wrote:
> On Wed, 27 Sep 2006 20:58:24 +0000
> "Miguel Ojeda" <maxextreme@gmail.com> wrote:
>
> > On 9/26/06, Andrew Morton <akpm@osdl.org> wrote:
> > > It's also probably-incorrect on big-endian CPUs.
> > > Perhaps you should not
> > > use bitops at all for this driver, use open-coded |
> > > and &/~ instead?
> >
> > Uhm, someone told me that they shouldn't be used because the kernel
> > has generic functions for that.
>
> You can't believe everything you read on the internet ;)
>
> set_bit() and friends are a) atomic wrt other CPUs on SMP and b) only to be
> performed on unsigned longs.
>
> > I researched the kernel sources, and looking at bitops.h I found
> > setbit(), so I tried to use it in the driver, althought I prefer
> > standard |= and &=.
>
> Those are the appropriate operations to use in this driver.
>
>

Ok, I have fixed all the mistakes you found. However, I have a
question about locking.

I have used 1 global mutex for all the fops: seek, write and ioctl.
They call down_interruptable() at the very beggining and up() just
before returning. I think that it's right.

However, what about the exported symbols? I have exported functions at
the ks0108 driver like EXPORT_SYMBOL_GPL(ks0108_writedata); The
cfag12864b drivers exports more symbols also, like:
EXPORT_SYMBOL_GPL(cfag12864b_write);

Should use the mutex all this functions too?

For example: Another driver can have a fops which isn't perform any
kind of locking. Such function is being executed twice at the same
time. And it calls some of my exported symbols. Then, I will have a
race condition in my driver.

(I couldn't find anything specific about that at LDD3, just for fops).

Thanks,
     Miguel Ojeda
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