Messages in this thread | | | Date | Fri, 23 Feb 2018 16:58:12 +0800 | From | Boqun Feng <> | 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 04:21:34PM +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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