linux-kernel - Re: [RFC PATCH 5/5 single-thread-version] implement per-domain single-thread state machine call

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Date:	Thu, 8 Mar 2012 22:17:44 +0800
From:	Lai Jiangshan <eag0628@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Lai Jiangshan <laijs@...fujitsu.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, mingo@...e.hu, dipankar@...ibm.com,
	akpm@...ux-foundation.org, mathieu.desnoyers@...ymtl.ca,
	josh@...htriplett.org, niv@...ibm.com, tglx@...utronix.de,
	rostedt@...dmis.org, Valdis.Kletnieks@...edu, dhowells@...hat.com,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	patches@...aro.org
Subject: Re: [RFC PATCH 5/5 single-thread-version] implement per-domain
 single-thread state machine call_srcu()

On Thu, Mar 8, 2012 at 9:04 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Wed, 2012-03-07 at 11:54 +0800, Lai Jiangshan wrote:
>> +static void srcu_advance_batches(struct srcu_struct *sp)
>> +{
>> +       int idx = 1 - (sp->completed & 0x1UL);
>> +
>> +       /*
>> +        * SRCU read-side critical sections are normally short, so check
>> +        * twice after a flip.
>> +        */
>> +       if (!rcu_batch_empty(&sp->batch_check1) ||
>> +           !rcu_batch_empty(&sp->batch_check0)) {
>> +               if (try_check_zero(sp, idx, 1)) {
>> +                       rcu_batch_move(&sp->batch_done, &sp->batch_check1);
>> +                       rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
>> +                       if (!rcu_batch_empty(&sp->batch_check1)) {
>> +                               srcu_flip(sp);
>> +                               if (try_check_zero(sp, 1 - idx, 2)) {
>> +                                       rcu_batch_move(&sp->batch_done,
>> +                                               &sp->batch_check1);
>> +                               }
>> +                       }
>> +               }
>> +       }
>> +}
>

Good, Thanks,

I'm thinking how to introduce the comments(original in __synchronize_srcu())
back. Your code will make it easier. (the comments are still needed to rewrite
before bring them back, need help!)

I will use your code with a little changed.

> static void srcu_advance_batches(struct srcu_struct *sp)
> {
>        int idx = 1 - (sp->completed & 1);

(sp->completed & 1) ^ 1;


>
>        if (rcu_batch_empty(&sp->batch_check0) &&
>            rcu_batch_empty(&sp->batch_check1))
>                return;
>
>        if (!try_check_zero(sp, idx, 1))
>                return;
>
>        rcu_batch_move(&sp->batch_done,   &sp->batch_check1);

....

>        rcu_batch_move(&sp->batch_check1, &sp->batch_check0);
>
>        if (rcu_batch_empty(&sp->batch_check1))
>                return;
>
>        srcu_flip(sp);

        if (rcu_batch_empty(&sp->batch_check0))
                return;

        srcu_flip(sp);
        rcu_batch_move(&sp->batch_check1, &sp->batch_check0);

make it match the changlog:
3) ->batch_check1 after finish its first check_zero and the flip

>
>        if (!try_check_zero(sp, idx^1, 2))
>                return;
>
>        rcu_batch_move(&sp->batch_done, &sp->batch_check1);
> }
>
> Seems like a more readable version.. do check I didn't mess up the logic
> though.
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