linux-kernel - Re: [PATCH RFC] rcu: Make __rcu_read

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Date:	Tue, 27 Mar 2012 16:06:08 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	paulmck@...ux.vnet.ibm.com
CC:	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	peterz@...radead.org, rostedt@...dmis.org, Valdis.Kletnieks@...edu,
	dhowells@...hat.com, eric.dumazet@...il.com, darren@...art.com,
	fweisbec@...il.com, patches@...aro.org,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH RFC] rcu: Make __rcu_read_lock() inlinable

On 03/26/2012 04:52 AM, Paul E. McKenney wrote:

> +void rcu_switch_from(void)
>  {
> -	current->rcu_read_lock_nesting++;
> -	barrier();  /* needed if we ever invoke rcu_read_lock in rcutree.c */
> +	current->rcu_read_lock_nesting_save =
> +		__this_cpu_read(rcu_read_lock_nesting);
> +	barrier();
> +	__this_cpu_write(rcu_read_lock_nesting, 0);

-	__this_cpu_write(rcu_read_lock_nesting, 0);
+	__this_cpu_write(rcu_read_lock_nesting, 1);

if prev or next task has non-zero rcu_read_unlock_special,
"__this_cpu_write(rcu_read_lock_nesting, 1)" will prevent wrong qs reporting
when rcu_read_unlock() is called in any interrupt/tracing while doing switch_to().

> +}
> +
> +/*
> + * Restore the incoming task's value for rcu_read_lock_nesting at the
> + * end of a context switch.
> + */
> +void rcu_switch_to(void)
> +{
> +	__this_cpu_write(rcu_read_lock_nesting,
> +			 current->rcu_read_lock_nesting_save);
> +	barrier();
> +	current->rcu_read_lock_nesting_save = 0;
>  }

-	barrier();
-	current->rcu_read_lock_nesting_save = 0;

rcu_read_lock_nesting_save is set but not used before next set here, just remove it.


I don't like it hooks too much into scheduler.

Approaches:
0) stay using function call
1) hook into kbuild(https://lkml.org/lkml/2011/3/27/170,https://lkml.org/lkml/2011/3/27/171)
2) hook into scheduler(still need more works for rcu_read_unlock())
3) Add rcu_read_lock_nesting to thread_info like preempt_count
4) resolve header-file dependence

For me
3=4>1>2>0

Thanks,
Lai
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