linux-kernel - Re: [RESEND PATCH 01/11] kernel:lockdep:print the shortest dependency chain if finding a circle

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Message-Id: <1247476498.7529.54.camel@twins>
Date:	Mon, 13 Jul 2009 11:14:58 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Ming Lei <tom.leiming@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [RESEND PATCH 01/11] kernel:lockdep:print the shortest
 dependency chain if finding a circle

On Mon, 2009-07-13 at 09:01 +0200, Ingo Molnar wrote:
> 
> It's a nice byproduct, beyond the primary advantage of not being a 
> stack based recursion check.
> 
> I think this patch-set is great, and there's just one more step 
> needed to make it round: it would be nice to remove the limitation 
> of maximum number of locks held per task. (MAX_LOCK_DEPTH)
> 
> The way we could do it is to split out this bit of struct task:
> 
> #ifdef CONFIG_LOCKDEP
> # define MAX_LOCK_DEPTH 48UL
>         u64 curr_chain_key;
>         int lockdep_depth;
>         unsigned int lockdep_recursion;
>         struct held_lock held_locks[MAX_LOCK_DEPTH];
>         gfp_t lockdep_reclaim_gfp;
> #endif
> 
> into a separate 'struct lockdep_state' structure, and allocate it 
> dynamically during fork with a initial pre-set size of say 64 locks 
> depth. If we hit that limit, we'd double the allocation threshold, 
> which would cause a larger structure to be allocated for all newly 
> allocated tasks.

Right, except allocating stuff while in the middle of lockdep is very
hard since it involves taking more locks :-)

I've tried it several times but never quite managed it in a way that I
felt comfortable with.

It would require having a reserve and serializing over that reserve.

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