On Wed, 12 Sep 2012 17:40:45 -0600
Bjorn Helgaas <bhelgaas@xxxxxxxxxx> wrote:
> On Mon, Sep 3, 2012 at 2:06 AM, Taku Izumi <izumi.taku@xxxxxxxxxxxxxx> wrote:
> > Use mutex and RCU to protect global acpi_pci_roots list against
> > PCI host bridge hotplug operations.
> >
> > RCU is used to avoid possible deadlock in function acpi_pci_find_root()
> > and acpi_get_pci_rootbridge_handle(). A possible call graph:
> > acpi_pci_register_driver()
> > mutex_lock(&acpi_pci_root_lock)
> > driver->add(root)
> > ......
> > acpi_pci_find_root()
>
> Where does this path occur? I didn't see in in the current tree
> (where the only users of acpi_pci_register_driver() are for
> acpi_pci_slot_driver and acpi_pci_hp_driver). Maybe it's in Yinghai's
> work, which adds more acpi_pci_register_driver() users.
First I protected acpi_pci_roots list by using mutex(acpi_pci_root_lock).
In that case I faced deadlock at the following path:
acpiphp_glue_init
+ acpi_pci_register_driver
...
+ add_bridge
+ acpi_pci_find_root
So I used RCU instead.
> RCU seems unnecessarily complicated for this list, but I haven't gone
> through Yinghai's work yet, so I don't know what it requires.
>
> In acpi_pci_root_start() and acpi_pci_root_remove(), we have the
> struct acpi_pci_root, which has all sorts of information that would be
> useful to the .add() and .remove() methods of sub-drivers. It seems
> sort of stupid that we only pass the acpi_handle to the sub-drivers,
> forcing them to use hacks like acpi_pci_find_root() to look up the
> information we just threw away. Can we just fix the .add() and
> .remove() interfaces to pass something more useful so we avoid the
> need for this deadlock path?
Maybe yes. Do you prefer imprementation without RCU ?
Best regards,
Taku Izumi
>
> > Signed-off-by: Jiang Liu <jiang.liu@xxxxxxxxxx>
> > Signed-off-by: Yinghai Lu <yinghai@xxxxxxxxxx>
> > Signed-off-by: Taku Izumi <izumi.taku@xxxxxxxxxxxxxx>
> > ---
> > drivers/acpi/pci_root.c | 50 +++++++++++++++++++++++++++++++-----------------
> > 1 file changed, 33 insertions(+), 17 deletions(-)
> >
> > Index: Bjorn-next-0903/drivers/acpi/pci_root.c
> > ===================================================================
> > --- Bjorn-next-0903.orig/drivers/acpi/pci_root.c
> > +++ Bjorn-next-0903/drivers/acpi/pci_root.c
> > @@ -28,6 +28,7 @@
> > #include <linux/init.h>
> > #include <linux/types.h>
> > #include <linux/mutex.h>
> > +#include <linux/rculist.h>
> > #include <linux/pm.h>
> > #include <linux/pm_runtime.h>
> > #include <linux/pci.h>
> > @@ -86,7 +87,7 @@ int acpi_pci_register_driver(struct acpi
> > mutex_lock(&acpi_pci_root_lock);
> > list_add_tail(&driver->node, &acpi_pci_drivers);
> > if (driver->add)
> > - list_for_each_entry(root, &acpi_pci_roots, node) {
> > + list_for_each_entry_rcu(root, &acpi_pci_roots, node) {
> > driver->add(root->device->handle);
> > n++;
> > }
> > @@ -103,7 +104,7 @@ void acpi_pci_unregister_driver(struct a
> > mutex_lock(&acpi_pci_root_lock);
> > list_del(&driver->node);
> > if (driver->remove)
> > - list_for_each_entry(root, &acpi_pci_roots, node)
> > + list_for_each_entry_rcu(root, &acpi_pci_roots, node)
> > driver->remove(root->device->handle);
> > mutex_unlock(&acpi_pci_root_lock);
> > }
> > @@ -112,14 +113,19 @@ EXPORT_SYMBOL(acpi_pci_unregister_driver
> > acpi_handle acpi_get_pci_rootbridge_handle(unsigned int seg, unsigned int bus)
> > {
> > struct acpi_pci_root *root;
> > + struct acpi_handle *handle = NULL;
> >
> > - list_for_each_entry(root, &acpi_pci_roots, node)
> > + rcu_read_lock();
> > + list_for_each_entry_rcu(root, &acpi_pci_roots, node)
> > if ((root->segment == (u16) seg) &&
> > - (root->secondary.start == (u16) bus))
> > - return root->device->handle;
> > - return NULL;
> > -}
> > + (root->secondary.start == (u16) bus)) {
> > + handle = root->device->handle;
> > + break;
> > + }
> > + rcu_read_unlock();
> >
> > + return handle;
> > +}
> > EXPORT_SYMBOL_GPL(acpi_get_pci_rootbridge_handle);
> >
> > /**
> > @@ -266,10 +272,14 @@ struct acpi_pci_root *acpi_pci_find_root
> > {
> > struct acpi_pci_root *root;
> >
> > - list_for_each_entry(root, &acpi_pci_roots, node) {
> > - if (root->device->handle == handle)
> > + rcu_read_lock();
> > + list_for_each_entry_rcu(root, &acpi_pci_roots, node)
> > + if (root->device->handle == handle) {
> > + rcu_read_unlock();
> > return root;
> > - }
> > + }
> > + rcu_read_unlock();
> > +
> > return NULL;
> > }
> > EXPORT_SYMBOL_GPL(acpi_pci_find_root);
> > @@ -506,8 +516,9 @@ static int __devinit acpi_pci_root_add(s
> > * TBD: Need PCI interface for enumeration/configuration of roots.
> > */
> >
> > - /* TBD: Locking */
> > - list_add_tail(&root->node, &acpi_pci_roots);
> > + mutex_lock(&acpi_pci_root_lock);
> > + list_add_tail_rcu(&root->node, &acpi_pci_roots);
> > + mutex_unlock(&acpi_pci_root_lock);
> >
> > printk(KERN_INFO PREFIX "%s [%s] (domain %04x %pR)\n",
> > acpi_device_name(device), acpi_device_bid(device),
> > @@ -526,7 +537,7 @@ static int __devinit acpi_pci_root_add(s
> > "Bus %04x:%02x not present in PCI namespace\n",
> > root->segment, (unsigned int)root->secondary.start);
> > result = -ENODEV;
> > - goto end;
> > + goto out_del_root;
> > }
> >
> > /*
> > @@ -536,7 +547,7 @@ static int __devinit acpi_pci_root_add(s
> > */
> > result = acpi_pci_bind_root(device);
> > if (result)
> > - goto end;
> > + goto out_del_root;
> >
> > /*
> > * PCI Routing Table
> > @@ -614,9 +625,12 @@ static int __devinit acpi_pci_root_add(s
> >
> > return 0;
> >
> > +out_del_root:
> > + mutex_lock(&acpi_pci_root_lock);
> > + list_del_rcu(&root->node);
> > + mutex_unlock(&acpi_pci_root_lock);
> > + synchronize_rcu();
> > end:
> > - if (!list_empty(&root->node))
> > - list_del(&root->node);
> > kfree(root);
> > return result;
> > }
> > @@ -646,11 +660,13 @@ static int acpi_pci_root_remove(struct a
> > list_for_each_entry(driver, &acpi_pci_drivers, node)
> > if (driver->remove)
> > driver->remove(root->device->handle);
> > - mutex_unlock(&acpi_pci_root_lock);
> >
> > device_set_run_wake(root->bus->bridge, false);
> > pci_acpi_remove_bus_pm_notifier(device);
> >
> > + list_del_rcu(&root->node);
> > + mutex_unlock(&acpi_pci_root_lock);
> > + synchronize_rcu();
> > kfree(root);
> > return 0;
> > }
> >
>
--
Taku Izumi <izumi.taku@xxxxxxxxxxxxxx>
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