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SubjectRe: [PATCH] tcm: Convert to cpu_relax() for atomic_read() in tight loops
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On Fri, 2010-09-10 at 08:32 +0200, Jiri Slaby wrote:
> On 09/09/2010 11:29 PM, Nicholas A. Bellinger wrote:
> > From: Nicholas Bellinger <nab@linux-iscsi.org>
> >
> > Greetings all,
> >
> > This patch converts the following functions from within shutdown and release
> > path code to use cpu_relax() instead of msleep() while doing atomic_read() in
> > a tight loop waiting for various PR and MIB reference count(s) to return to zero:
> >
> > *) core_update_device_list_for_node() + struct se_dev_entry->pr_ref_count
> > *) core_release_port() + struct se_port->sep_tg_pt_ref_cnt
> > *) se_dev_stop() + struct se_hba->dev_mib_access_count
> > *) __core_scsi3_free_registration() + struct t10_pr_registration->pr_res_holders
> > *) core_tpg_wait_for_nacl_pr_ref() + struct se_node_acl->acl_pr_ref_count
> > *) core_tpg_wait_for_mib_ref() + struct se_node_acl->mib_ref_count
> > *) core_tpg_deregister() + struct se_portal_group->tpg_pr_ref_count
> > *) transport_deregister_session() + struct se_session->mib_ref_count
> >
> > Thanks to Konrad Rzeszutek Wilk for mentioning cpu_relax() usage!
> >
> > Signed-off-by: Nicholas A. Bellinger <nab@linux-iscsi.org>
> > ---
> > drivers/target/target_core_device.c | 6 +++---
> > drivers/target/target_core_pr.c | 2 +-
> > drivers/target/target_core_tpg.c | 6 +++---
> > drivers/target/target_core_transport.c | 2 +-
> > 4 files changed, 8 insertions(+), 8 deletions(-)
> >
> > diff --git a/drivers/target/target_core_device.c b/drivers/target/target_core_device.c
> > index 665b8e2..459f0f8 100644
> > --- a/drivers/target/target_core_device.c
> > +++ b/drivers/target/target_core_device.c
> > @@ -532,7 +532,7 @@ int core_update_device_list_for_node(
> > */
> > spin_unlock_irq(&nacl->device_list_lock);
> > while (atomic_read(&deve->pr_ref_count) != 0)
> > - msleep(100);
> > + cpu_relax();
> > spin_lock_irq(&nacl->device_list_lock);
> > /*
> > * Disable struct se_dev_entry LUN ACL mapping
>
> Why would you want to convert those to busy-waiting? This should rather
> be converted to wait_event/wake_up, right?

Hi Jiri,

I agree that this is the one scenario within the msleep() -> cpu_relax()
conversion patch that involves something besides a simple shortly held
'non CDB I/O emulation path context ref/deref', and might probably be
better served by using wait_event/wake_up.

I will take a look at a conversion to use wait_queue_head_t in the near
future.

Thanks for your comments Jiri!

--nab




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