On Tue, Dec 10, 2019 at 11:15:27AM +0800, Xiang Zheng wrote:
> 7ea7e98fd8d0 ("PCI: Block on access to temporarily unavailable pci
> device") suggests that the "pci_lock" is sufficient, and all the
> callers of pci_wait_cfg() are wrapped with the "pci_lock".
>
> However, since the commit cdcb33f98244 ("PCI: Avoid possible deadlock on
> pci_lock and p->pi_lock") merged, the accesses to the pci_cfg_wait queue
> are not safe anymore. This would cause kernel panic in a very low chance
> (See more detailed information from the below link). A "pci_lock" is
> insufficient and we need to hold an additional queue lock while read/write
> the wait queue.
>
> So let's use the add_wait_queue()/remove_wait_queue() instead of
> __add_wait_queue()/__remove_wait_queue(). Also move the wait queue
> functionality around the "schedule()" function to avoid reintroducing
> the deadlock addressed by "cdcb33f98244".
I see that add_wait_queue() acquires the wq_head->lock, while
__add_wait_queue() does not.
But I don't understand why the existing pci_lock is insufficient.
pci_cfg_wait is only used in pci_wait_cfg() and
pci_cfg_access_unlock().
In pci_wait_cfg(), both __add_wait_queue() and __remove_wait_queue()
are called while holding pci_lock, so that doesn't seem like the
problem.
In pci_cfg_access_unlock(), we have:
pci_cfg_access_unlock
wake_up_all(&pci_cfg_wait)
__wake_up(&pci_cfg_wait, ...)
__wake_up_common_lock(&pci_cfg_wait, ...)
spin_lock(&pci_cfg_wait->lock)
__wake_up_common(&pci_cfg_wait, ...)
list_for_each_entry_safe_from(...)
list_add_tail(...) <-- problem?
spin_unlock(&pci_cfg_wait->lock)
Is the problem that the wake_up_all() modifies the pci_cfg_wait list
without holding pci_lock?
If so, I don't quite see how the patch below fixes it. Oh, wait,
maybe I do ... by using add_wait_queue(), we protect the list using
the *same* lock used by __wake_up_common_lock. Is that it?
> Signed-off-by: Xiang Zheng <zhengxiang9@xxxxxxxxxx>
> Cc: Heyi Guo <guoheyi@xxxxxxxxxx>
> Cc: Biaoxiang Ye <yebiaoxiang@xxxxxxxxxx>
> Link: https://lore.kernel.org/linux-pci/79827f2f-9b43-4411-1376-b9063b67aee3@xxxxxxxxxx/
> ---
>
> v3:
> Improve the commit subject and message.
>
> v2:
> Move the wait queue functionality around the "schedule()".
>
> ---
> drivers/pci/access.c | 4 ++--
> 1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/pci/access.c b/drivers/pci/access.c
> index 2fccb5762c76..09342a74e5ea 100644
> --- a/drivers/pci/access.c
> +++ b/drivers/pci/access.c
> @@ -207,14 +207,14 @@ static noinline void pci_wait_cfg(struct pci_dev *dev)
> {
> DECLARE_WAITQUEUE(wait, current);
>
> - __add_wait_queue(&pci_cfg_wait, &wait);
> do {
> set_current_state(TASK_UNINTERRUPTIBLE);
> raw_spin_unlock_irq(&pci_lock);
> + add_wait_queue(&pci_cfg_wait, &wait);
> schedule();
> + remove_wait_queue(&pci_cfg_wait, &wait);
> raw_spin_lock_irq(&pci_lock);
> } while (dev->block_cfg_access);
> - __remove_wait_queue(&pci_cfg_wait, &wait);
> }
>
> /* Returns 0 on success, negative values indicate error. */
> --
> 2.19.1
>
>
