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Date:   Fri, 23 Feb 2018 12:36:43 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Boqun Feng <boqun.feng@gmail.com>
Cc:     linux-kernel@vger.kernel.org, Ingo Molnar <mingo@redhat.com>,
        Andrea Parri <parri.andrea@gmail.com>
Subject: Re: [RFC tip/locking/lockdep v5 08/17] lockdep: Fix recursive read
 lock related safe->unsafe detection
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On Fri, Feb 23, 2018 at 04:58:12PM +0800, Boqun Feng wrote:
> Hmm... think again, maybe I can combine case 1 with 3, and case 2 with
> 4, because each of them could share the same find_usage_backwards(), and
> find_usage_forwards() uses a usage_match_forwards() as follow for the
> match function:
> 
> 	static inline int usage_match_forwards(struct lock_list *entry, void *bit)
> 	{
> 		enum lock_usage_bit ub = (enum lock_usage_bit)bit;
> 		unsigned long mask;
> 		unsigned long read_mask;
> 
> 		/* mask out the read bit */
> 		ub &= ~1;
> 
> 		mask = 1ULL << ub;
> 		read_mask = 1ULL << (ub + 1);
> 
> 		return (entry->class->usage_mask & mask) ||  // *-> L2 and L2 is an irq-unsafe lock
> 		       ((entry->class->usage_mask & read_mask) && !entry->is_rr); // N-> L2 and L2 is an irq-read-unsafe lock
> 	}
> 
> Got a bus to catch, I can explain this later, if you need ;-)

Right, that's about what I was thinking of. Clearly that needs a wee
comment but it's much better.