linux-kernel - Re: [PATCH 7/7] lockdep: Remove cmpxchg to update nr_chain

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Date:	Thu, 21 Apr 2011 14:50:12 +0800
From:	Yong Zhang <yong.zhang0@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Subject: Re: [PATCH 7/7] lockdep: Remove cmpxchg to update nr_chain_hlocks

On Thu, Apr 21, 2011 at 9:42 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> From: Steven Rostedt <srostedt@...hat.com>
>
> For some reason nr_chain_hlocks is updated with cmpxchg, but this
> is performed inside of the lockdep global "grab_lock()", which also
> makes simple modification of this variable atomic.
>
> Remove the cmpxchg logic for updating nr_chain_hlocks and simplify
> the code.
>
> Acked-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Signed-off-by: Steven Rostedt <rostedt@...dmis.org>
> ---
>  kernel/lockdep.c |   14 ++++----------
>  1 files changed, 4 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/lockdep.c b/kernel/lockdep.c
> index 3b1ac92..60bce52 100644
> --- a/kernel/lockdep.c
> +++ b/kernel/lockdep.c
> @@ -1973,7 +1973,7 @@ static inline int lookup_chain_cache(struct task_struct *curr,
>        struct list_head *hash_head = chainhashentry(chain_key);
>        struct lock_chain *chain;
>        struct held_lock *hlock_curr, *hlock_next;

Maybe we can also remove hlock_next in this function for some cleanup, because
it's just an alias of hlock.

Thanks,
Yong

> -       int i, j, n, cn;
> +       int i, j;
>
>        if (DEBUG_LOCKS_WARN_ON(!irqs_disabled()))
>                return 0;
> @@ -2033,15 +2033,9 @@ cache_hit:
>        }
>        i++;
>        chain->depth = curr->lockdep_depth + 1 - i;
> -       cn = nr_chain_hlocks;
> -       while (cn + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS) {
> -               n = cmpxchg(&nr_chain_hlocks, cn, cn + chain->depth);
> -               if (n == cn)
> -                       break;
> -               cn = n;
> -       }
> -       if (likely(cn + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
> -               chain->base = cn;
> +       if (likely(nr_chain_hlocks + chain->depth <= MAX_LOCKDEP_CHAIN_HLOCKS)) {
> +               chain->base = nr_chain_hlocks;
> +               nr_chain_hlocks += chain->depth;
>                for (j = 0; j < chain->depth - 1; j++, i++) {
>                        int lock_id = curr->held_locks[i].class_idx - 1;
>                        chain_hlocks[chain->base + j] = lock_id;
> --
> 1.7.2.3
>
>
> --
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