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

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Date:	Fri, 29 Jun 2007 13:48:19 +0200
From:	Duncan Sands <duncan.sands@...h.u-psud.fr>
To:	Alexey Kuznetsov <kuznet@....inr.ac.ru>
Cc:	Ingo Molnar <mingo@...e.hu>, Jeff Garzik <jeff@...zik.org>,
	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
Subject: Re: [RFC PATCH 0/6] Convert all tasklets to workqueues

Hi,

> > Just look at the tasklet_disable() logic.
> 
> Do not count this.
> 
> Done this way because nobody needed that thing, except for _one_ place
> in keyboard/console driver, which was very difficult to fix that time,
> when vt code was utterly messy and not smp safe at all.
> 
> start_bh_atomic() was successfully killed, but we had to preserve analogue
> of disable_bh() with the same semantics for some time.
> It is deliberately implemented in a way, which does not impact hot paths
> and is easy to remove.
> 
> It is sad that some usb drivers started to use this creepy and
> useless thing.

the usbatm USB driver uses it in the methods for opening and closing a new network
connection, and on device disconnect.  Yes, tasklet_disable could be eliminated by
adding a spinlock.  However this would mean taking the lock every time a packet is
received or transmitted.  As it is, typically open occurs once, when the computer
boots, and close and disconnect also occur once each, when the computer shuts down.
I felt that three calls to tasklet_disable were better than a gazillion calls to
spin_(un)lock.  Please feel free to educate me :)

Ciao,

Duncan.
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