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Re: [PATCH 5/5] percpu_counter: only disable preemption if needed in add

To: Alex Elder <aelder@xxxxxxx>
Subject: Re: [PATCH 5/5] percpu_counter: only disable preemption if needed in add_unless_lt()
From: Dave Chinner <david@xxxxxxxxxxxxx>
Date: Thu, 23 Dec 2010 17:31:38 +1100
Cc: xfs@xxxxxxxxxxx
In-reply-to: <1293076602.2408.434.camel@doink>
References: <1293076602.2408.434.camel@doink>
User-agent: Mutt/1.5.20 (2009-06-14)
On Wed, Dec 22, 2010 at 09:56:42PM -0600, Alex Elder wrote:
> In __percpu_counter_add_unless_lt() we don't need to disable
> preemption unless we're manipulating a per-cpu variable.  That only
> happens in a limited case, so narrow the scope of that preemption to
> surround that case.  This makes the "out" label rather unnecessary,
> so replace a couple "goto out" calls to just return.
> 
> Signed-off-by: Alex Elder <aelder@xxxxxxx>
> 
> ---
>  lib/percpu_counter.c |   21 ++++++++++-----------
>  1 file changed, 10 insertions(+), 11 deletions(-)
> 
> Index: b/lib/percpu_counter.c
> ===================================================================
> --- a/lib/percpu_counter.c
> +++ b/lib/percpu_counter.c
> @@ -232,8 +232,6 @@ int __percpu_counter_add_unless_lt(struc
>       int     cpu;
>       int     ret = -1;
>  
> -     preempt_disable();
> -
>       /*
>        * Check to see if rough count will be sufficient for
>        * comparison.  First, if the upper bound is too low,
> @@ -241,7 +239,7 @@ int __percpu_counter_add_unless_lt(struc
>        */
>       count = percpu_counter_read(fbc);
>       if (count + error + amount < threshold)
> -             goto out;
> +             return -1;
>  
>       /*
>        * Next, if the lower bound is above the threshold, we can
> @@ -251,12 +249,15 @@ int __percpu_counter_add_unless_lt(struc
>       if (count - error + amount > threshold) {
>               s32 *pcount = this_cpu_ptr(fbc->counters);
>  
> +             preempt_disable();
> +             pcount = this_cpu_ptr(fbc->counters);
>               count = *pcount + amount;
>               if (abs(count) < batch) {
>                       *pcount = count;
> -                     ret = 1;
> -                     goto out;
> +                     preempt_enable();
> +                     return 1;
>               }
> +             preempt_enable();
>       }

Regardless of the other changes, this is not valid. That is:

        amount = -1;
        count = fbc->count;
        .....

        <get preempted>

        <other operations may significantly change fbc->count (i.e
        lots more than error will catch), so the current value of
        count in this context is wrong and cannot be trusted>

        <start running again>

        if (count - error + amount > threshold) {
                <not valid to run this lockless optimisation based
                on a stale count value>
                
                ....
        }

Effectively, if we want to be able to use lockless optimisations, we
need to ensure that the value of the global counter that we read
remains within the given error bounds until we have finished making
the lockless modification. That is done via disabling preemption
across the entire function...

Cheers,

Dave.
-- 
Dave Chinner
david@xxxxxxxxxxxxx

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