Re: [RFC PATCH 4/5] PCI: Introduce hotplug-safe pci bus searching interfaces

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On Sun, Mar 11, 2012 at 11:48 AM, Jiang Liu <liuj97@xxxxxxxxx> wrote:
> By design, pci_find_bus() and pci_find_next_bus() should be used at boot
> time only. But currently these two interfaces have been used at runtime
> by other components. With the introduction of pci root bus hotplug,
> the situation becomes more serious. So introduce several hotplug-safe pci
> bus searching interfaces to be used at runtime.  The new interfaces use
> rculist instead of the pci_bus_sem to protect themselves from dynamic changes.
> The proposed interfaces are straight-forward replacement of the old ones:
> pci_bus_get()/put(), pci_get_bus(), pci_get_next_bus() and pci_bus_present().
> And the old interface may be deprecated or marked as __init in future.

This looks like a lot of work to fix something that shouldn't need to
be fixed.  I don't think we should be doing any of this blind probing
at hotplug-time.  If we do blind probing at all, it should only be
done at boot-time.

> Signed-off-by: Jiang Liu <jiang.liu@xxxxxxxxxx>
> ---
>  drivers/pci/bus.c    |   20 ++++++-
>  drivers/pci/probe.c  |    5 +-
>  drivers/pci/remove.c |    5 +-
>  drivers/pci/search.c |  165 ++++++++++++++++++++++++++++++++++++++++++--------
>  include/linux/pci.h  |    8 +++
>  5 files changed, 173 insertions(+), 30 deletions(-)
>
> diff --git a/drivers/pci/bus.c b/drivers/pci/bus.c
> index 1eb7944..0543d47 100644
> --- a/drivers/pci/bus.c
> +++ b/drivers/pci/bus.c
> @@ -15,6 +15,7 @@
>  #include <linux/proc_fs.h>
>  #include <linux/init.h>
>  #include <linux/slab.h>
> +#include <linux/rculist.h>
>
>  #include "pci.h"
>
> @@ -238,7 +239,7 @@ void pci_bus_add_devices(const struct pci_bus *bus)
>                        continue;
>                if (list_empty(&child->node)) {
>                        down_write(&pci_bus_sem);
> -                       list_add_tail(&child->node, &dev->bus->children);
> +                       list_add_tail_rcu(&child->node, &dev->bus->children);
>                        up_write(&pci_bus_sem);
>                }
>                pci_bus_add_devices(child);
> @@ -277,7 +278,7 @@ void pci_bus_add_single_device(struct pci_dev *dev)
>        if (child) {
>                if (list_empty(&child->node)) {
>                        down_write(&pci_bus_sem);
> -                       list_add_tail(&child->node, &dev->bus->children);
> +                       list_add_tail_rcu(&child->node, &dev->bus->children);
>                        up_write(&pci_bus_sem);
>                }
>                pci_bus_add_devices(child);
> @@ -364,6 +365,21 @@ void pci_walk_bus(struct pci_bus *top, int (*cb)(struct pci_dev *, void *),
>  }
>  EXPORT_SYMBOL_GPL(pci_walk_bus);
>
> +struct pci_bus *pci_bus_get(struct pci_bus *bus)
> +{
> +       if (bus)
> +               get_device(&bus->dev);
> +       return bus;
> +}
> +EXPORT_SYMBOL(pci_bus_get);
> +
> +void pci_bus_put(struct pci_bus *bus)
> +{
> +       if (bus)
> +               put_device(&bus->dev);
> +}
> +EXPORT_SYMBOL(pci_bus_put);
> +
>  EXPORT_SYMBOL(pci_bus_alloc_resource);
>  EXPORT_SYMBOL_GPL(pci_bus_add_device);
>  EXPORT_SYMBOL(pci_bus_add_devices);
> diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> index 0ca213c..273c387 100644
> --- a/drivers/pci/probe.c
> +++ b/drivers/pci/probe.c
> @@ -10,6 +10,7 @@
>  #include <linux/module.h>
>  #include <linux/cpumask.h>
>  #include <linux/pci-aspm.h>
> +#include <linux/rculist.h>
>  #include "pci.h"
>
>  #define CARDBUS_LATENCY_TIMER  176     /* secondary latency timer */
> @@ -633,7 +634,7 @@ struct pci_bus *__ref pci_add_new_bus(struct pci_bus *parent, struct pci_dev *de
>        child = pci_alloc_child_bus(parent, dev, busnr);
>        if (child) {
>                down_write(&pci_bus_sem);
> -               list_add_tail(&child->node, &parent->children);
> +               list_add_tail_rcu(&child->node, &parent->children);
>                up_write(&pci_bus_sem);
>        }
>        return child;
> @@ -1889,7 +1890,7 @@ struct pci_bus *pci_create_root_bus(struct device *parent, int bus,
>        }
>
>        down_write(&pci_bus_sem);
> -       list_add_tail(&b->node, &pci_root_buses);
> +       list_add_tail_rcu(&b->node, &pci_root_buses);
>        up_write(&pci_bus_sem);
>
>        return b;
> diff --git a/drivers/pci/remove.c b/drivers/pci/remove.c
> index 25f368e..120bee9 100644
> --- a/drivers/pci/remove.c
> +++ b/drivers/pci/remove.c
> @@ -1,6 +1,7 @@
>  #include <linux/pci.h>
>  #include <linux/module.h>
>  #include <linux/pci-aspm.h>
> +#include <linux/rculist.h>
>  #include "pci.h"
>
>  static void pci_free_resources(struct pci_dev *dev)
> @@ -67,9 +68,11 @@ void pci_remove_bus(struct pci_bus *pci_bus)
>        pci_proc_detach_bus(pci_bus);
>
>        down_write(&pci_bus_sem);
> -       list_del(&pci_bus->node);
> +       list_del_rcu(&pci_bus->node);
>        pci_bus_release_busn_res(pci_bus);
>        up_write(&pci_bus_sem);
> +       synchronize_rcu();
> +
>        if (pci_bus->is_added || pci_is_root_bus(pci_bus)) {
>                pci_remove_legacy_files(pci_bus);
>                device_unregister(&pci_bus->dev);
> diff --git a/drivers/pci/search.c b/drivers/pci/search.c
> index b572730..de31957 100644
> --- a/drivers/pci/search.c
> +++ b/drivers/pci/search.c
> @@ -12,6 +12,7 @@
>  #include <linux/slab.h>
>  #include <linux/module.h>
>  #include <linux/interrupt.h>
> +#include <linux/rculist.h>
>  #include "pci.h"
>
>  DECLARE_RWSEM(pci_bus_sem);
> @@ -52,20 +53,63 @@ pci_find_upstream_pcie_bridge(struct pci_dev *pdev)
>
>  static struct pci_bus *pci_do_find_bus(struct pci_bus *bus, unsigned char busnr)
>  {
> -       struct pci_bus* child;
> -       struct list_head *tmp;
> +       struct pci_bus *child;
> +       struct pci_bus *tmp_bus;
>
>        if(bus->number == busnr)
>                return bus;
>
> -       list_for_each(tmp, &bus->children) {
> -               child = pci_do_find_bus(pci_bus_b(tmp), busnr);
> -               if(child)
> -                       return child;
> +       list_for_each_entry_rcu(child, &bus->children, node) {
> +               tmp_bus = pci_do_find_bus(child, busnr);
> +               if(tmp_bus)
> +                       return tmp_bus;
>        }
>        return NULL;
>  }
>
> +static struct pci_bus *__pci_find_bus(int domain, int busnr)
> +{
> +       struct pci_bus *bus;
> +       struct pci_bus *tmp_bus;
> +
> +       list_for_each_entry_rcu(bus, &pci_root_buses, node) {
> +               if (pci_domain_nr(bus) != domain)
> +                       continue;
> +               tmp_bus = pci_do_find_bus(bus, busnr);
> +               if (tmp_bus)
> +                       return tmp_bus;
> +       }
> +
> +       return NULL;
> +}
> +
> +static struct pci_bus *__pci_find_next_bus(const struct pci_bus *from)
> +{
> +       struct list_head *n;
> +       struct pci_bus *b = NULL;
> +
> +       /* First search, start from the pci root bus list */
> +       if (from == NULL) {
> +               n = rcu_dereference(list_next_rcu(&pci_root_buses));
> +               if (n != &pci_root_buses)
> +                       b = pci_bus_b(n);
> +       /* Continue on the pci root bus list */
> +       } else if (pci_is_root_bus((struct pci_bus *)from)) {
> +               n = rcu_dereference(list_next_rcu(&from->node));
> +               if (n != &pci_root_buses)
> +                       b = pci_bus_b(n);
> +       /* Continue on other non pci root bus list */
> +       } else {
> +               struct pci_bus *parent = from->self->bus;
> +
> +               n = rcu_dereference(list_next_rcu(&from->node));
> +               if (n != &parent->children)
> +                       b = pci_bus_b(n);
> +       }
> +
> +       return b;
> +}
> +
>  /**
>  * pci_find_bus - locate PCI bus from a given domain and bus number
>  * @domain: number of PCI domain to search
> @@ -74,20 +118,19 @@ static struct pci_bus *pci_do_find_bus(struct pci_bus *bus, unsigned char busnr)
>  * Given a PCI bus number and domain number, the desired PCI bus is located
>  * in the global list of PCI buses.  If the bus is found, a pointer to its
>  * data structure is returned.  If no bus is found, %NULL is returned.
> + *
> + * TODO: By design, this function should only be called at boot time.
> + * So either mark it as __init or deprecate it.
>  */
>  struct pci_bus * pci_find_bus(int domain, int busnr)
>  {
> -       struct pci_bus *bus = NULL;
> -       struct pci_bus *tmp_bus;
> +       struct pci_bus *bus;
>
> -       while ((bus = pci_find_next_bus(bus)) != NULL)  {
> -               if (pci_domain_nr(bus) != domain)
> -                       continue;
> -               tmp_bus = pci_do_find_bus(bus, busnr);
> -               if (tmp_bus)
> -                       return tmp_bus;
> -       }
> -       return NULL;
> +       rcu_read_lock();
> +       bus = __pci_find_bus(domain, busnr);
> +       rcu_read_unlock();
> +
> +       return bus;
>  }
>
>  /**
> @@ -98,21 +141,93 @@ struct pci_bus * pci_find_bus(int domain, int busnr)
>  * initiated by passing %NULL as the @from argument.  Otherwise if
>  * @from is not %NULL, searches continue from next device on the
>  * global list.
> + *
> + * TODO: By design, this function should only be called at boot time.
> + * So either mark it as __init or deprecate it.
>  */
>  struct pci_bus *
>  pci_find_next_bus(const struct pci_bus *from)
>  {
> -       struct list_head *n;
> -       struct pci_bus *b = NULL;
> +       struct pci_bus *bus;
>
> -       WARN_ON(in_interrupt());
> -       down_read(&pci_bus_sem);
> -       n = from ? from->node.next : pci_root_buses.next;
> -       if (n != &pci_root_buses)
> -               b = pci_bus_b(n);
> -       up_read(&pci_bus_sem);
> -       return b;
> +       rcu_read_lock();
> +       bus = __pci_find_next_bus(from);
> +       rcu_read_unlock();
> +
> +       return bus;
> +}
> +
> +/**
> + * pci_get_bus - locate PCI bus from a given domain and bus number
> + * @domain: number of PCI domain to search
> + * @busnr: number of desired PCI bus
> + *
> + * Given a PCI bus number and domain number, the desired PCI bus is located
> + * in the global list of PCI buses. If the bus is found, a pointer to its
> + * data structure is returned, and the reference count to the bus is increased.
> + * Otherwise, %NULL is returned.
> + */
> +struct pci_bus *
> +pci_get_bus(int domain, int busnr)
> +{
> +       struct pci_bus *bus;
> +
> +       rcu_read_lock();
> +       bus = pci_bus_get(__pci_find_bus(domain, busnr));
> +       rcu_read_unlock();
> +
> +       return bus;
> +}
> +EXPORT_SYMBOL(pci_get_bus);
> +
> +/**
> + * pci_get_next_bus - begin or continue searching for a PCI bus
> + * @from: Previous PCI bus found, or %NULL for new search.
> + *
> + * Iterates through the list of known PCI busses.  A new search is
> + * initiated by passing %NULL as the @from argument.  Otherwise if
> + * @from is not %NULL, searches continue from next device on the
> + * global list. If a bus is found, a pointer to its data structure
> + * is returned, and the reference count to the bus is increased.
> + * Otherwise, %NULL is returned.
> + * The reference count for @from is always decremented if it is not %NULL.
> + */
> +struct pci_bus *
> +pci_get_next_bus(struct pci_bus *from)
> +{
> +       struct pci_bus *bus;
> +
> +       rcu_read_lock();
> +       bus = pci_bus_get(__pci_find_next_bus(from));
> +       rcu_read_unlock();
> +
> +       return bus;
> +}
> +EXPORT_SYMBOL(pci_get_next_bus);
> +
> +/**
> + * pci_bus_present - Returns true if a bus with the (@domain, @busnr)
> + * is present, false if not.
> + * @domain: number of PCI domain to search
> + * @busnr: number of desired PCI bus
> + *
> + * Obvious fact: You do not have a reference to any bus that might be found
> + * by this function, so if that bus is removed from the system right after
> + * this function is finished, the value will be stale.  Use this function to
> + * find buses that are usually built into a system, or for a general hint as
> + * to if another device happens to be present at this specific moment in time.
> + */
> +bool pci_bus_present(int domain, int busnr)
> +{
> +       struct pci_bus *bus;
> +
> +       rcu_read_lock();
> +       bus = __pci_find_bus(domain, busnr);
> +       rcu_read_unlock();
> +
> +       return !!bus;
>  }
> +EXPORT_SYMBOL(pci_bus_present);
>
>  /**
>  * pci_get_slot - locate PCI device for a given PCI slot
> diff --git a/include/linux/pci.h b/include/linux/pci.h
> index ec8c4cf..222bc88 100644
> --- a/include/linux/pci.h
> +++ b/include/linux/pci.h
> @@ -731,6 +731,11 @@ int pci_find_ht_capability(struct pci_dev *dev, int ht_cap);
>  int pci_find_next_ht_capability(struct pci_dev *dev, int pos, int ht_cap);
>  struct pci_bus *pci_find_next_bus(const struct pci_bus *from);
>
> +struct pci_bus *pci_bus_get(struct pci_bus *bus);
> +void pci_bus_put(struct pci_bus *bus);
> +struct pci_bus *pci_get_bus(int domain, int busnr);
> +struct pci_bus *pci_get_next_bus(struct pci_bus *from);
> +bool pci_bus_present(int domain, int busnr);
>  struct pci_dev *pci_get_device(unsigned int vendor, unsigned int device,
>                                struct pci_dev *from);
>  struct pci_dev *pci_get_subsys(unsigned int vendor, unsigned int device,
> @@ -1321,6 +1326,9 @@ static inline void pci_unblock_cfg_access(struct pci_dev *dev)
>  static inline struct pci_bus *pci_find_next_bus(const struct pci_bus *from)
>  { return NULL; }
>
> +static inline struct pci_bus *pci_get_next_bus(const struct pci_bus *from)
> +{ return NULL; }
> +
>  static inline struct pci_dev *pci_get_slot(struct pci_bus *bus,
>                                                unsigned int devfn)
>  { return NULL; }
> --
> 1.7.5.4
>
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