Re: [PATCHv2 1/2] PCI: pciehp: fix concurrent sub-tree removal deadlock

[Lukas Wunner <lukas@wunner.de>]

On Mon, Nov 11, 2024 at 08:38:03AM +0100, Lukas Wunner wrote:
> Thinking about this some more:
> 
> The problem is pci_lock_rescan_remove() is a single global lock.
> 
> What if we introduce a lock at each bridge or for each pci_bus.
> Before a portion of the hierarchy is removed, all locks in that
> sub-hierarchy are acquired bottom-up.
> 
> I think that should avoid the deadlock.  Thoughts?

I note that you attempted something similar back in July:

https://lore.kernel.org/all/20240722151936.1452299-9-kbusch@meta.com/

However I'd suggest to solve this differently:

Keep the pci_lock_rescan_remove() everywhere, don't add pci_lock_bus()
adjacent to it.

Instead, amend pci_lock_rescan_remove() to walk the sub-hierarchy
bottom-up and acquire all the bus locks.  Obviously you'll have to amend
pci_lock_rescan_remove() to accept a pci_dev which is the bridge atop
the sub-hierarchy.  (Or alternatively, the top-most pci_bus in the
sub-hierarchy.)

Thanks,

Lukas