linux-kernel - RE: Why hold device_lock when calling callback in pci_walk

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Date:	Fri, 28 Sep 2012 08:29:20 +0000
From:	"Zhang, Yanmin" <yanmin.zhang@...el.com>
To:	"Huang, Ying" <ying.huang@...el.com>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	"linux-pci@...r.kernel.org" <linux-pci@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"rjw@...k.pl" <rjw@...k.pl>
Subject: RE: Why hold device_lock when calling callback in pci_walk_bus?

Some error handling functions call pci_walk_bus. For example, pci-e aer. Here we lock the device, so the driver wouldn't detach from the device, as the cb might call driver's callback function.

-----Original Message-----
From: Huang, Ying 
Sent: Friday, September 28, 2012 4:15 PM
To: bhelgaas@...gle.com
Cc: Greg Kroah-Hartman; Zhang, Yanmin; linux-pci@...r.kernel.org; linux-kernel@...r.kernel.org; rjw@...k.pl
Subject: Why hold device_lock when calling callback in pci_walk_bus?

Hi, All,

If my understanding were correct, device_lock is used to provide mutual exclusion between device probe/remove/suspend/resume etc.  Why hold device_lock when calling callback in pci_walk_bus.

This is introduced by the following commit.

commit d71374dafbba7ec3f67371d3b7e9f6310a588808
Author: Zhang Yanmin <yanmin.zhang@...el.com>
Date:   Fri Jun 2 12:35:43 2006 +0800

    [PATCH] PCI: fix race with pci_walk_bus and pci_destroy_dev

    pci_walk_bus has a race with pci_destroy_dev. When cb is called
    in pci_walk_bus, pci_destroy_dev might unlink the dev pointed by next.
    Later on in the next loop, pointer next becomes NULL and cause
    kernel panic.

    Below patch against 2.6.17-rc4 fixes it by changing pci_bus_lock (spin_lock)
    to pci_bus_sem (rw_semaphore).

    Signed-off-by: Zhang Yanmin <yanmin.zhang@...el.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@...e.de>

Corresponding email thread is: https://lkml.org/lkml/2006/5/26/38

But from the commit and email thread, I can not find why we need to do that.

I ask this question because I want to use pci_walk_bus in a function (in pci runtime resume path) which may be called with device_lock held.

Can anyone help me on that?

Best Regards,
Huang Ying