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To: Ingo Molnar <mingo@elte.hu>, linux-kernel@vger.kernel.org
Subject: On some configs, sparse spinlock balance checking is broken
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From: Roland Dreier <rdreier@cisco.com>
Date: Tue, 16 Jan 2007 15:47:42 -0800
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(Ingo -- you seem to be the last person to touch all this stuff, and I
can't untangle what you did, hence I'm sending this email to you)

On at least some of my configs on x86_64, when running sparse, I see
bogus 'warning: context imbalance in '<func>' - wrong count at exit'.

This seems to be because I have CONFIG_SMP=y, CONFIG_DEBUG_SPINLOCK=n
and CONFIG_PREEMPT=n.  Therefore, <linux/spinlock.h> does

	#define spin_lock(lock)			_spin_lock(lock)

which picks up

	void __lockfunc _spin_lock(spinlock_t *lock)		__acquires(lock);

from <linux/spinlock_api_smp.h>, but <linux/spinlock.h> also has:

	#if defined(CONFIG_DEBUG_SPINLOCK) || defined(CONFIG_PREEMPT) || \
		!defined(CONFIG_SMP)
	//...
	#else
	# define spin_unlock(lock)		__raw_spin_unlock(&(lock)->raw_lock)

and <asm-x86_64/spinlock.h> has:

	static inline void __raw_spin_unlock(raw_spinlock_t *lock)
	{
		asm volatile("movl $1,%0" :"=m" (lock->slock) :: "memory");
	}

so sparse doesn't see any __releases() to match the __acquires.

This all seems to go back to commit bda98685 ("x86: inline spin_unlock
if !CONFIG_DEBUG_SPINLOCK and !CONFIG_PREEMPT") but I don't know what
motivated that change.

Anyway, Ingo or anyone else, what's the best way to fix this?  Maybe
the right way to fix this is just to define away __acquires/__releases
unless CONFIG_DEBUG_SPINLOCK is set, but that seems suboptimal.

Thanks,
  Roland