linux-kernel - Is spin_is_locked() safe to use with BUG_ON()/WARN

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Date:	Thu, 23 May 2013 15:50:36 +0100
From:	David Howells <dhowells@...hat.com>
To:	torvalds@...ux-foundation.org, mingo@...nel.org
cc:	dhowells@...hat.com, milosz@...in.com, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Is spin_is_locked() safe to use with BUG_ON()/WARN_ON()?

We are using spin_is_locked() in a few places to give a warning or an oops if
either a spinlock is not held or if it is held.  I'm not sure all of these are
safe.

Take uas_try_complete() in drivers/usb/storage/uas.c which does:

	WARN_ON(!spin_is_locked(&devinfo->lock));

or fscache_start_operations() which does:

	ASSERT(spin_is_locked(&object->lock));

These will unconditionally fail under sometimes because under certain
conditions spin_is_locked() is hardwired to 0 (ie. not locked) when actually
we're in a place where the spinlock _should_ be locked, and we should get a
non-zero return.

Would it be reasonable to add a spin_is_not_locked() function for use when we
expect it not to be locked and then use spin_is_locked() only when we expect it
to be locked?

Thanks to Milosz Tanski for spotting this one.

David
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