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SubjectRe: [PATCH] staging: lustre: avoid going through unlock/lock overhead
On Thu, Sep 07, 2017 at 01:57:42PM +0300, Cihangir Akturk wrote:
> Unlocking a spin lock and then immediately locking without doing
> anything useful in between buys us nothing, except wasting CPU cycles.

Not always, it can be a "gate" for other users of the lock.

Are you sure that is not what is going on here? Did you test this out
on a lustre system? The locks here are anything but trivial...

>
> Also code size gets smaller.
>
> Before:
>
> text data bss dec hex filename
> 70415 2356 4108 76879 12c4f drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd.o
>
> After:
>
> text data bss dec hex filename
> 70095 2356 4108 76559 12b0f drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd.o
>
> Signed-off-by: Cihangir Akturk <cakturk@gmail.com>
> ---
> drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd.c | 18 ++++++++----------
> 1 file changed, 8 insertions(+), 10 deletions(-)
>
> diff --git a/drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd.c b/drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd.c
> index 64763aa..5d9cd33 100644
> --- a/drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd.c
> +++ b/drivers/staging/lustre/lnet/klnds/o2iblnd/o2iblnd.c
> @@ -1624,8 +1624,9 @@ int kiblnd_fmr_pool_map(struct kib_fmr_poolset *fps, struct kib_tx *tx,
> __u64 version;
> int rc;
>
> - again:
> +again:
> spin_lock(&fps->fps_lock);
> +again_locked:
> version = fps->fps_version;
> list_for_each_entry(fpo, &fps->fps_pool_list, fpo_list) {
> fpo->fpo_deadline = cfs_time_shift(IBLND_POOL_DEADLINE);
> @@ -1722,10 +1723,8 @@ int kiblnd_fmr_pool_map(struct kib_fmr_poolset *fps, struct kib_tx *tx,
> }
>
> /* EAGAIN and ... */
> - if (version != fps->fps_version) {
> - spin_unlock(&fps->fps_lock);
> - goto again;
> - }
> + if (version != fps->fps_version)
> + goto again_locked;
> }
>
> if (fps->fps_increasing) {
> @@ -1754,9 +1753,8 @@ int kiblnd_fmr_pool_map(struct kib_fmr_poolset *fps, struct kib_tx *tx,
> } else {
> fps->fps_next_retry = cfs_time_shift(IBLND_POOL_RETRY);
> }
> - spin_unlock(&fps->fps_lock);
>
> - goto again;
> + goto again_locked;

Really, gotos backwards? Ick, that's horrid as well, so maybe this is
better? I hate this whole codebase...

I'll let the Lustre maintainers decide about this one...

greg k-h

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