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    SubjectRe: [PATCH 1/4] fs: Improve filesystem freezing handling
    On Thu, Jan 12, 2012 at 02:20:50AM +0100, Jan Kara wrote:
    > + *
    > + * Decrement number of writers to the filesystem and wake up possible
    > + * waiters wanting to freeze the filesystem.
    > + */
    > +void sb_end_write(struct super_block *sb)
    > +{
    > +#ifdef CONFIG_SMP
    > + this_cpu_dec(sb->s_writers);
    > +#else
    > + preempt_disable();
    > + sb->s_writers--;
    > + preempt_enable();
    > +#endif

    I really dislike this type of open coded per-cpu counter
    implementation. I can't see that there is no good reason to use it
    over percpu_counters here which abstract all this mess away.

    i.e. it is relatively rare that the per-cpu count will nest
    greater than the percpu_counter batch size (needs more than 32
    concurrent blocked active writes per CPU), so there is no
    significant overhead to using the percpu_counters here.

    Indeed, if there are that many blocked writes per CPU, then the
    overhead of an occasional global counter update is going to be lost
    in the noise of everything else that is going on.

    Cheers,

    Dave.
    --
    Dave Chinner
    david@fromorbit.com


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