On Thu, Jul 16, 2015 at 8:14 PM, Yijing Wang <wangyijing@xxxxxxxxxx> wrote:
>>>> If I'm mistaken, please correct me and explain why this patch is safe.
>>>
>>> Hi Bjorn, I think pci_bus_sem here was introduced to protect the bus->slots list, because it
>>> use down_write() here, for bus->devices list, we only traverse it, won't add/remove it, for the latter, down_read() is enough.
>>> When I posted this patch, I thought we should protect the bus when we start to register a slot,
>>> something like a big lock at outermost routine to tell others not to touch its children devices, use pci_bus_sem to protect hotplug
>>> cases is not a good idea, and actually in PCI code, we have found several deadlock caused by the pci_bus_sem.
>>>
>>> But for this patch, I know what you worried, what about add a down_read(&pci_bus_sem) to avoid to introduce a regression ?
>>>
>>>
>>> diff --git a/drivers/pci/slot.c b/drivers/pci/slot.c
>>> index 396c200..a9079d9 100644
>>> --- a/drivers/pci/slot.c
>>> +++ b/drivers/pci/slot.c
>>> @@ -14,6 +14,7 @@
>>>
>>> struct kset *pci_slots_kset;
>>> EXPORT_SYMBOL_GPL(pci_slots_kset);
>>> +static DEFINE_MUTEX(pci_slot_mutex);
>>>
>>> static ssize_t pci_slot_attr_show(struct kobject *kobj,
>>> struct attribute *attr, char *buf)
>>> @@ -106,9 +107,11 @@ static void pci_slot_release(struct kobject *kobj)
>>> dev_dbg(&slot->bus->dev, "dev %02x, released physical slot %s\n",
>>> slot->number, pci_slot_name(slot));
>>>
>>> + down_read(&pci_bus_sem);
>>> list_for_each_entry(dev, &slot->bus->devices, bus_list)
>>> if (PCI_SLOT(dev->devfn) == slot->number)
>>> dev->slot = NULL;
>>> + up_read(&pci_bus_sem);
>>>
>>> list_del(&slot->list);
>>
>> This list_del() updates the bus->slots list.
>
> It's safe here, because we have locked the pci_slot_mutex in pci_destroy_slot(), which is the only caller of pci_slot_release().
That doesn't protect anybody else who might be traversing the
bus->slots list while we're deleting this entry.
>>> @@ -195,7 +198,7 @@ static struct pci_slot *get_slot(struct pci_bus *parent, int slot_nr)
>>> {
>>> struct pci_slot *slot;
>>> /*
>>> - * We already hold pci_bus_sem so don't worry
>>> + * We already hold pci_slot_mutex so don't worry
>>> */
>>> list_for_each_entry(slot, &parent->slots, list)
>>> if (slot->number == slot_nr) {
>>> @@ -253,7 +256,7 @@ struct pci_slot *pci_create_slot(struct pci_bus *parent, int slot_nr,
>>> int err = 0;
>>> char *slot_name = NULL;
>>>
>>> - down_write(&pci_bus_sem);
>>> + mutex_lock(&pci_slot_mutex);
>>>
>>> if (slot_nr == -1)
>>> goto placeholder;
>>> @@ -301,16 +304,18 @@ placeholder:
>>> INIT_LIST_HEAD(&slot->list);
>>> list_add(&slot->list, &parent->slots);
>>>
>>> + down_read(&pci_bus_sem);
>>> list_for_each_entry(dev, &parent->devices, bus_list)
>>> if (PCI_SLOT(dev->devfn) == slot_nr)
>>> dev->slot = slot;
>>> + up_read(&pci_bus_sem);
>>>
>>> dev_dbg(&parent->dev, "dev %02x, created physical slot %s\n",
>>> slot_nr, pci_slot_name(slot));
>>>
>>> out:
>>> kfree(slot_name);
>>> - up_write(&pci_bus_sem);
>>> + mutex_unlock(&pci_slot_mutex);
>>> return slot;
>>> err:
>>> kfree(slot);
>>> @@ -332,9 +337,9 @@ void pci_destroy_slot(struct pci_slot *slot)
>>> dev_dbg(&slot->bus->dev, "dev %02x, dec refcount to %d\n",
>>> slot->number, atomic_read(&slot->kobj.kref.refcount) - 1);
>>>
>>> - down_write(&pci_bus_sem);
>>> + mutex_lock(&pci_slot_mutex);
>>> kobject_put(&slot->kobj);
>>> - up_write(&pci_bus_sem);
>>> + mutex_unlock(&pci_slot_mutex);
>>> }
>>> EXPORT_SYMBOL_GPL(pci_destroy_slot);
>>
>> .
>>
>
>
> --
> Thanks!
> Yijing
>
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