linux-kernel - Re: [RFC][PATCH 3/7] lockdep: Annotate read/write states

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Date:	Mon, 18 Apr 2011 21:34:05 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [RFC][PATCH 3/7] lockdep: Annotate read/write states

On Sun, Apr 17, 2011 at 11:45:08AM +0200, Peter Zijlstra wrote:
> From: Gautham R Shenoy <ego@...ibm.com>
> 
> Currently we do not save the recursive read dependencies in the dependency
> chain. As a result, a deadlock caused by the following chains are not spotted,
> since we never have the chain 1 in our dependency list:
> 
>  1: Rlock(A) --> lock(B)
>  2: lock(B)  --> Wlock(A), where A is a recursive read lock.
> 
> Before adding the Recursive Read locks to the dependency chains, we need to
> distinguish them from the normal read locks since the conflicting states for
> these two are quite different.
> 
> Currently the read/write status of a lock while it's acquired is denoted by a
> monotonically increasing variable where:
> 
>  0 - WRITE
>  1 - READ
>  2 - RECURSIVE READ
> 
> In this patch, we propose to modify this distinction from a monotonically
> increasing variable to a bit mask where:
> 
>  0x1 - WRITE
>  0x2 - READ
>  0x4 - RECURSIVE READ
> 
> This helps us to define the conflicting states for each lock with ease:
> Thereby, the conflicting states for a given states are defined as follows:
> 
>  Conflicting_states(WRITE):             RECURSIVE_READ | READ | WRITE
>  Conflicting_states(READ):                               READ | WRITE
>  Conflicting_states(RECURSIVE_READ):                            WRITE
> 
> Also, we use one more bit in the bitmask to distinguish the first recursive
> read in the current chain from the others, since it is sufficient to add only
> this dependency to the dependency list.
> 
> Signed-off-by: Gautham R Shenoy <ego@...ibm.com>
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  include/linux/lockdep.h |  107 ++++++++++++++++++++++++++++++++++++------------
>  kernel/lockdep.c        |   46 ++++++++++----------
>  2 files changed, 105 insertions(+), 48 deletions(-)
> 
> @@ -2273,7 +2273,7 @@ mark_held_locks(struct task_struct *curr
>  		hlock = curr->held_locks + i;
>  
>  		usage_bit = 2 + (mark << 2); /* ENABLED */
> -		if (hlock->read)
> +		if (hlock->rw_state)

		is_read(hlock->rw_state) ?

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