Re: PCI service interrupt handlers & access to PCI config space

[Lukas Wunner <lukas@wunner.de>]

On Sat, Apr 10, 2021 at 02:28:45PM +0200, Pali Rohár wrote:
> I see that more PCI service drivers in their interrupt handlers are
> accessing PCI config space.
> 
> E.g. in PCIe Hot Plug interrupt handler pciehp_isr handler is called
> pcie_capability_read_word() function.
> 
> It is correct? Because these capability functions are protected by
> pci_lock_config() / pci_unlock_config() calls.

Looks fine to me.  That's a raw_spin_lock, which is allowed to be taken
in interrupt context.

> And what would happen when during execution of reading or writing to
> PCIe config space (e.g. via pcie_capability_read_word()) is triggered
> PCIe HP interrupt? Would not enter interrupt handler function in
> deadlock (waiting for unlocking config space)?

The interrupt handler would spin until the lock is released.
raw_spin_locks are not supposed to be held for a prolonged
period of time.

The interrupt is masked when it triggers and until it's been handled,
so the interrupt handler never runs multiple times concurrently.
See e.g. handle_level_irq() in kernel/irq/chip.c.

Thanks,

Lukas