[PATCH] PCI: Avoid possible deadlock on pci_lock and p->pi_lock

[PATCH] PCI: Avoid possible deadlock on pci_lock and p->pi_lock

[Bjorn Helgaas]

pci_lock is an IRQ-safe spinlock that protects all accesses to PCI
configuration space (see PCI_OP_READ() and PCI_OP_WRITE() in pci/access.c).

The pci_cfg_access_unlock() path acquires pci_lock, then p->pi_lock (inside
wake_up_all()).  According to lockdep, there is a possible path involving
snbep_uncore_pci_read_counter() that could acquire them in the reverse
order: acquiring p->pi_lock, then pci_lock, which could result in a
deadlock.  Lockdep details are in the bugzilla below.

Avoid the possible deadlock by dropping pci_lock before waking up any
config access waiters.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=192901
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
---
 drivers/pci/access.c |    3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/pci/access.c b/drivers/pci/access.c
index db239547fefd..68bd7201d8bc 100644
--- a/drivers/pci/access.c
+++ b/drivers/pci/access.c
@@ -684,8 +684,9 @@ void pci_cfg_access_unlock(struct pci_dev *dev)
 	WARN_ON(!dev->block_cfg_access);
 
 	dev->block_cfg_access = 0;
-	wake_up_all(&pci_cfg_wait);
 	raw_spin_unlock_irqrestore(&pci_lock, flags);
+
+	wake_up_all(&pci_cfg_wait);
 }
 EXPORT_SYMBOL_GPL(pci_cfg_access_unlock);
 

Re: [PATCH] PCI: Avoid possible deadlock on pci_lock and p->pi_lock

[Bjorn Helgaas]

On Wed, Jan 18, 2017 at 05:46:12PM -0600, Bjorn Helgaas wrote:
> pci_lock is an IRQ-safe spinlock that protects all accesses to PCI
> configuration space (see PCI_OP_READ() and PCI_OP_WRITE() in pci/access.c).
> 
> The pci_cfg_access_unlock() path acquires pci_lock, then p->pi_lock (inside
> wake_up_all()).  According to lockdep, there is a possible path involving
> snbep_uncore_pci_read_counter() that could acquire them in the reverse
> order: acquiring p->pi_lock, then pci_lock, which could result in a
> deadlock.  Lockdep details are in the bugzilla below.
> 
> Avoid the possible deadlock by dropping pci_lock before waking up any
> config access waiters.
> 
> Link: https://bugzilla.kernel.org/show_bug.cgi?id=192901
> Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>

Applied to pci/enumeration for v4.11.

> ---
>  drivers/pci/access.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/pci/access.c b/drivers/pci/access.c
> index db239547fefd..68bd7201d8bc 100644
> --- a/drivers/pci/access.c
> +++ b/drivers/pci/access.c
> @@ -684,8 +684,9 @@ void pci_cfg_access_unlock(struct pci_dev *dev)
>  	WARN_ON(!dev->block_cfg_access);
>  
>  	dev->block_cfg_access = 0;
> -	wake_up_all(&pci_cfg_wait);
>  	raw_spin_unlock_irqrestore(&pci_lock, flags);
> +
> +	wake_up_all(&pci_cfg_wait);
>  }
>  EXPORT_SYMBOL_GPL(pci_cfg_access_unlock);
>  
> 
> --
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