linux-kernel - Re: [PATCH 1/3] drivers: misc: add omap

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Message-Id: <201010211104.28555.arnd@arndb.de>
Date:	Thu, 21 Oct 2010 11:04:28 +0200
From:	Arnd Bergmann <arnd@...db.de>
To:	"Ohad Ben-Cohen" <ohad@...ery.com>
Cc:	linux-arm-kernel@...ts.infradead.org,
	Hari Kanigeri <h-kanigeri2@...com>, Suman Anna <s-anna@...com>,
	Benoit Cousson <b-cousson@...com>,
	Tony Lindgren <tony@...mide.com>, Greg KH <greg@...ah.com>,
	linux-kernel@...r.kernel.org,
	Grant Likely <grant.likely@...retlab.ca>,
	akpm@...ux-foundation.org, linux-omap@...r.kernel.org,
	"Krishnamoorthy, Balaji T" <balajitk@...com>
Subject: Re: [PATCH 1/3] drivers: misc: add omap_hwspinlock driver

On Thursday 21 October 2010, Ohad Ben-Cohen wrote:
> This sounds like adding a set of API that resembles spin_{unlock,lock}_irq.
> 
> My gut feeling here is that while this may be useful and simple at
> certain places, it is somewhat error prone; a driver which would
> erroneously use this at the wrong place will end up enabling
> interrupts where it really shouldn't.
> 
> Don't you feel that proving a simple spin_lock_irqsave-like API is
> actually safer and less error prone ?
> 
> I guess that is one of the reasons why spin_lock_irqsave is much more
> popular than spin_lock_irq - people just know it will never screw up.

People can screw that up in different ways, e.g.

	spin_lock_irqsave(&lock_a, flags);
	spin_lock_irqsave(&lock_b, flags); /* overwrites flags */

	spin_lock_irqrestore(&lock_b, flags);
	spin_lock_irqrestore(&lock_a, flags); /* still disabled! */

I use the presence of spin_lock_irqsave in driver code as an indication
of whether the author had any clue about locking. Most experienced
coders use the right version intuitively, while beginners often just
use _irqsave because they didn't understand the API and they were told
that using this is safe.

	Arnd
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