linux-kernel - Re: [RFC PATCH 1/6] Revert "PCI: Fix is_added/is

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Date:   Mon, 20 Aug 2018 09:17:03 +0200
From:   Lukas Wunner <lukas@...ner.de>
To:     Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:     Bjorn Helgaas <helgaas@...nel.org>, linux-pci@...r.kernel.org,
        Hari Vyas <hari.vyas@...adcom.com>,
        Ray Jui <ray.jui@...adcom.com>,
        Srinath Mannam <srinath.mannam@...adcom.com>,
        Guenter Roeck <linux@...ck-us.net>,
        Jens Axboe <axboe@...nel.dk>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        Marta Rybczynska <mrybczyn@...ray.eu>,
        Pierre-Yves Kerbrat <pkerbrat@...ray.eu>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 1/6] Revert "PCI: Fix is_added/is_busmaster race
 condition"

On Mon, Aug 20, 2018 at 12:10:59PM +1000, Benjamin Herrenschmidt wrote:
> I chose to create a new mutex which we should be able to address other
> similar races if we find them. The other solutions that I dismissed
> were:
> 
>  - Using the device_lock. A explained previously, this is tricky, I
> prefer not using this for anything other than locking against
> concurrent add/remove. The main issue is that drivers will be sometimes
> called in context where that's already held, so we can't take it inside
> pci_enable_device() and I'd rather not add new constraints such as
> "pci_enable_device() must be only called from probe() unless you also
> take the device lock". It would be tricky to audit everybody...
> 
>  - Using a global mutex. We could move the bridge lock from AER to core
> code for example, and use that. But it doesn't buy us much, and
> slightly redecuces parallelism. It also makes it a little bit more
> messy to walk up the bridge chain, we'd have to do a
> pci_enable_device_unlocked or something, messy.

+1 from my side for adding a struct mutex to struct pci_dev to protect
state changes.

The device_lock() primarily protects binding / unbinding of the device
and pci_dev state may have to be changed while binding / unbinding.

A global lock invites deadlocks if multiple devices are added / removed
concurrently where one is a parent of the other.  (Think hot-removal of
multiple devices on a Thunderbolt daisy-chain.)

As said I'd also welcome folding PCI_DEV_DISCONNECTED into enum
pci_channel_state, either as an additional state or by using
pci_channel_io_perm_failure.

Thanks,

Lukas