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Date: Thu, 22 Oct 2009 00:16:21 +0200
From: Frederic Weisbecker <fweisbec@gmail.com>
To: John Kacur <jkacur@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>, LKML <linux-kernel@vger.kernel.org>,
       Ingo Molnar <mingo@elte.hu>, Jonathan Corbet <corbet@lwn.net>,
       Benjamin Herrenschmidt <benh@kernel.crashing.org>,
       linuxppc-dev@ozlabs.org, Alan Cox <alan@lxorguk.ukuu.org.uk>,
       Arnd Bergmann <arndbergmann@googlemail.com>
Subject: Re: [PATCH] macintosh: Explicitly set llseek to no_llseek in
	ans-lcd
Message-ID: <20091021221619.GF4880@nowhere>
References: <20091010153314.827301943@linutronix.de> <20091010153349.966159859@linutronix.de> <alpine.LFD.2.00.0910212305090.3526@localhost.localdomain> <20091021212137.GB4880@nowhere> <alpine.LFD.2.00.0910212327160.3526@localhost.localdomain> <20091021214537.GD4880@nowhere> <alpine.LFD.2.00.0910212350210.3526@localhost.localdomain>
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On Wed, Oct 21, 2009 at 11:53:21PM +0200, John Kacur wrote:
> > No problem with that. Setting no_llseek or generic_file_llseek_unlocked,
> > depending on the context is the right thing to do.
> > 
> > What I'm wondering about concerns the future code that will have
> > no llsek() implemented in their fops.
> > 
> > We can't continue to use default_llseek() for future code unless we
> > want to continue these post reviews and fixes forever.
> > 
> 
> I'm thinking that the simplier approach, would be to make the 
> default_llseek the unlocked one. Then you only have to audit the drivers 
> that have the BKL - ie the ones we are auditing anyway, and explicitly set 
> them to the bkl locked llseek.
> 
> There might be a hidden interaction though between the non-unlocked 
> variety of ioctls and default llseek though.


I fear that won't work because the bkl in default_llseek() does not
only synchronizes with others uses of the bkl in a driver, it also
synchronizes lseek() itself.

As an example offset change is not atomic. This is a long long, so
updating its value is not atomic in 32 bits archs.