Re: [RFC tip/locking/lockdep v5 08/17] lockdep: Fix recursive read lock related safe->unsafe detection

[Boqun Feng]

On Fri, Feb 23, 2018 at 04:21:34PM +0800, Boqun Feng wrote:
> On Thu, Feb 22, 2018 at 06:46:54PM +0100, Peter Zijlstra wrote:
> > On Thu, Feb 22, 2018 at 03:08:55PM +0800, Boqun Feng wrote:
> > > There are four cases for recursive read lock realted deadlocks:
> > > 
> > > (--(X..Y)--> means a strong dependency path starts with a --(X*)-->
> > > dependency and ends with a --(*Y)-- dependency.)
> > > 
> > > 1.	An irq-safe lock L1 has a dependency --(*..*)--> to an
> > > 	irq-unsafe lock L2.
> > > 
> > > 2.	An irq-read-safe lock L1 has a dependency --(N..*)--> to an
> > > 	irq-unsafe lock L2.
> > > 
> > > 3.	An irq-safe lock L1 has a dependency --(*..N)--> to an
> > > 	irq-read-unsafe lock L2.
> > > 
> > > 4.	An irq-read-safe lock L1 has a dependency --(N..N)--> to an
> > > 	irq-read-unsafe lock L2.
> > > 
> > > The current check_usage() only checks 1) and 2), so this patch adds
> > > checks for 3) and 4) and makes sure when find_usage_{back,for}wards find
> > > an irq-read-{,un}safe lock, the traverse path should ends at a
> > > dependency --(*N)-->. Note when we search backwards, --(*N)--> indicates
> > > a real dependency --(N*)-->.
> > 
> > This adds 4 __bfs() searches for every new link.
> > 
> > Can't we make the existing traversals smarter?
> 
> Haven't really thought this one through, I will try. But as you said, we

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 ;-)

Regards,
Boqun

> only need to do more searchs for _new_ links, so I think it's the slow
> path, would the performance matter that much?
> 
> Regards,
> Boqun

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