linux-kernel - Re: [PATCH 11/13] Add a discussion on why spin_is

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Message-ID: <20120328090707.GW22197@one.firstfloor.org>
Date:	Wed, 28 Mar 2012 11:07:07 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Wolfram Sang <w.sang@...gutronix.de>
Cc:	Andi Kleen <andi@...stfloor.org>, akpm@...ux-foundation.org,
	linux-kernel@...r.kernel.org, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 11/13] Add a discussion on why spin_is_locked() is bad to spinlocks.txt

> > +
> > +or some variant of those.
> > +
> > +This does not work on uniprocessor kernels because they will always fail.
> > +While there are ways around that they are ugly and not recommended.
> > +Better use lockdep_assert_held(). This also only checks on a lock debugging
> > +kernel (which you should occasionally run on your code anyways because
> > +it catches many more problems). 
> > +
> > +In generally this would be better done with static annotation anyways 
> > +(there's some support for it in sparse)
> > +
> > +	BUG_ON(spin_is_locked(obj->lock));
> > +	kfree(obj);
> > +
> > +Another usage is checking whether a lock is not hold when freeing an object.
> 
> I'd suggest to move this sentence above the code example. On first read,
> I was confused what the code should tell me regarding annotations :)

Both fixed.

> 
> > +However this is redundant because lock debugging supports this anyways
> > +without explicit code. Just delete the BUG_ON.
> > +
> > +A third usage is to check in a console function if a lock is hold, to get
> > +a panic crash dump out even when some other thread died in it.
> > +This is better implemented with spin_try_lock() et.al. and a timeout.
> > +
> > +Other usages are usually simply races.
> > +
> > +In summary just don't use it.
> 
> At this point, I was wondering when it actually can be used? Otherwise
> it probably would have been removed from the kernel or marked
> deprecated, I'd think?

Even after the patchkit there are a few users left (mostly third category
and some print outs). Eventually these should be fixed or removed too.
Then the function could be truly deprected.

lockdep still needs it I believe, but it should probably use some 
private interface. 

-Andi
-- 
ak@...ux.intel.com -- Speaking for myself only.
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