linux-kernel - Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

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Date:	Fri, 22 Jun 2007 15:43:16 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Christoph Hellwig <hch@...radead.org>,
	john stultz <johnstul@...ibm.com>,
	Oleg Nesterov <oleg@...sign.ru>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	"David S. Miller" <davem@...emloft.net>, matthew.wilcox@...com,
	kuznet@....inr.ac.ru
Subject: Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

 > > [ and on a similar notion, i still havent given up on seeing all BKL 
 > >   use gone from the kernel. I expect it to happen any decade now ;-) ]

 > 2.6.21 had 476 lock_kernel() calls. 2.6.22-git has 473 lock_kernel() 
 > calls currently. With that kind of flux we'll see the BKL gone in about 
 > 40 years =B-)

 > 'struct semaphore' use on the other hand has gone down by 10% in this 
 > release, which is a good rate. I guess the lack of lockdep coverage for 
 > semaphores might be one of the driving forces? ;-)

The problem with removing uses of the BKL is that a "lock_kernel()"
gives no clue about what it is protecting against, and so it requires
a lot of very difficult auditing to replace with appropriate locking.

To take a couple of examples at random: fs/ext4/ioctl.c takes the BKL
in ext4_compat_ioctl() around the call to ext4_ioctl().  Kind of sad
that a "next-generation" FS still uses the BKL, but who understands
things well enough to say how all the cases in ext4_ioctl() are
relying on being called with the BKL held?

As a second example, msr_seek() in arch/i386/kernel/msr.c... is the
inode semaphore enough or not?  Who understands the implications well
enough to say?

Most semaphores on the other hand can be replaced by mutexes or
completions in a fairly straightforward way.

 - R.
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