linux-kernel - Re: [patch 05/52] lglock: introduce special lglock and brlock spin locks

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Date:	Thu, 24 Jun 2010 20:15:54 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	npiggin@...e.de
cc:	linux-fsdevel@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
	John Stultz <johnstul@...ibm.com>,
	Frank Mayhar <fmayhar@...gle.com>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 05/52] lglock: introduce special lglock and brlock spin
 locks

On Thu, 24 Jun 2010, npiggin@...e.de wrote:

> +#define DEFINE_LGLOCK(name)						\
> +									\
> + DEFINE_PER_CPU(arch_spinlock_t, name##_lock);				\

Uuurgh. You want to make that an arch_spinlock ? Just to avoid the
preempt_count overflow when you lock all cpu locks nested ?

I'm really not happy about that, it's going to be a complete nightmare
for RT. If you wanted to make this a present for RT giving the
scalability stuff massive testing, then you failed miserably :)

I know how to fix it, but can't we go for an approach which
does not require massive RT patching again ?

struct percpu_lock {
       spinlock_t lock;
       unsigned   global_state;
};

And let the lock function do:

    spin_lock(&pcp->lock);
    while (pcp->global_state)
    	  cpu_relax();

So the global lock side can take each single lock, modify the percpu
"global state" and release the lock. On unlock you just need to reset
the global state w/o taking the percpu lock and be done.

I doubt that the extra conditional in the lock path is going to be
relevant overhead, compared to the spin_lock it's noise.

Thanks,

	tglx
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