On Tue, 29 May 2007 01:24:06 +0900,
Tejun Heo <[email protected]> wrote:
> Make sysfs_dirent->s_active an atomic_t instead of rwsem. This
> reduces the size of sysfs_dirent from 136 to 104 on 64bit and from 76
> to 60 on 32bit with lock debugging turned off. With lock debugging
> turned on the reduction is much larger.
Cool.
> s_active starts at zero and each active reference increments s_active.
> Putting a reference decrements s_active. Deactivation subtracts
> SD_DEACTIVATED_BIAS which is currently INT_MIN and assumed to be small
> enough to make s_active negative. If s_active is negative,
> sysfs_get() no longer grants new references. Deactivation succeeds
> immediately if there is no active user; otherwise, it waits using a
> completion for the last put.
>
> Due to the removal of lockdep tricks, this change makes things less
> trickier in release_sysfs_dirent(). As all the complexity is
> contained in three s_active functions, I think it's more readable this
> way.
Agreed.
> @@ -32,11 +33,24 @@ static DEFINE_IDA(sysfs_ino_ida);
> */
> struct sysfs_dirent *sysfs_get_active(struct sysfs_dirent *sd)
> {
> - if (sd) {
> - if (unlikely(!down_read_trylock(&sd->s_active)))
> - sd = NULL;
> + if (unlikely(!sd))
> + return NULL;
> +
> + while (1) {
> + int v, t;
> +
> + v = atomic_read(&sd->s_active);
> + if (unlikely(v < 0))
> + return NULL;
> +
> + t = atomic_cmpxchg(&sd->s_active, v, v + 1);
> + if (likely(t == v))
> + return sd;
> + if (t < 0)
> + return NULL;
> +
> + cpu_relax();
> }
> - return sd;
> }
I don't quite like v and t, but don't have a better naming suggestion
either.
-
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