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

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Message-ID: <1348820118.4530.41.camel@yhuang-mobile.sh.intel.com>
Date:	Fri, 28 Sep 2012 16:15:18 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	bhelgaas@...gle.com
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	yanmin.zhang@...el.com, 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


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