linux-kernel - Re: [GIT PULL v2] Preparation for BKL'ed ioctl removal

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Date:	Tue, 27 Apr 2010 11:25:30 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Al Viro <viro@...iv.linux.org.uk>,
	Jan Blunck <jblunck@...e.de>, John Kacur <jkacur@...hat.com>
Subject: Re: [GIT PULL v2] Preparation for BKL'ed ioctl removal

* Arnd Bergmann <arnd@...db.de> wrote:

> On Monday 26 April 2010, Ingo Molnar wrote:
> > This could be done all automated for a hundred old drivers if need to be. 
> > There would be no bkl_ioctl's left.
> 
> I don't think it can be fully automated. [...]

Corner cases are not a problem as long as the risk of them going unnoticed is 
lower than the risk of a manual conversion introducing bugs.

> [...] For the majority of the modules, your approach would work fine, but 
> there are still the well-known pitfalls in corner cases:
> 
> - recursive uses in functions outside of ioctl (possibly none left
>   after the TTY layer is done, but who knows)

Not a problem even if there's any such usage left: lockdep will sort those out 
very quickly.

> - lock-order problems with other mutexes (see DRM)

This too will be mapped out very quickly via lockdep.

> - code that depends on autorelease to allow one ioctl while another
>   is sleeping. (a small number of drivers)

This is a real issue, and in fact it's an unknown: there may be an unknown 
number of random sleep points within BKL codepaths that is being relied on in 
creative ways.

Note that by introducing a mutex we (in most cases) make the locking 
_stricter_, so the biggest risk from that is a lockup - which will be 
debuggable via lockdep.

	Ingo
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