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

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Date:   Thu, 22 Feb 2018 18:46:54 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Andrea Parri <parri.andrea@...il.com>
Subject: Re: [RFC tip/locking/lockdep v5 08/17] lockdep: Fix recursive read
 lock related safe->unsafe detection

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?