linux-kernel - Re: pci: kernel crash in bus_find

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Date:	Tue, 20 May 2014 16:38:12 -0700
From:	Guenter Roeck <linux@...ck-us.net>
To:	Francesco Ruggeri <fruggeri@...sta.com>
Cc:	linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
	Francesco Ruggeri <fruggeri@...stanetworks.com>
Subject: Re: pci: kernel crash in bus_find_device

On Tue, May 20, 2014 at 03:35:15PM -0700, Francesco Ruggeri wrote:
> Hi Guenter,
> thank you for your reply. I will check out the changes that you pointed to.
> The problem we are seeing is a race condition between for_each_pci_dev
> (or similar) and device_unregisters. I am not sure if use of the new
> lock should be extended to all code using for_each_pci_dev as well.
> 
> pci_scan is a kernel thread that I used for testing purposes, to
> mimick the dynamics that we saw in our crashes in
> edac_pci_clear_parity_errors:
> 
>         for (;;) {
>                 pci_dev = NULL;
>                 while ((pci_dev = pci_get_device(PCI_ANY_ID,
> PCI_ANY_ID, pci_dev)) != NULL)
>                         ;
>         }
> 
> It keeps traversing klist_devices in pci_bus_type using
> bus_find_device, costantly resuming its search for the next element
> starting from the one it got in the previous round.
> There are several loops of this kind in linux. In case of this thread
> no action is taken on the elements as they are "found".
> 
> The race condition occurs when bus_find_device resumes its search from
> a device that has been unregistered. Because device_unregister resets
> klist_bus in the device, bus_find device cannot resume from where it
> left off in the klist.
> The sequence is device_unregister, device_del, bus_remove_device,
> klist_del(&dev->p->knode_bus.).
> 
Hmmm ... sounds more like a generic problem, not specifically related to pci.
Essentially everything calling bus_find_device() with a starting device which
has been removed (though only pci and scsi seem to be doing that in practice).

Can you reproduce the problem with the latest kernel ? Also, can you
send me the entire file with the kernel thread you mentioned above ?
Maybe I can reproduce the problem here.

Thanks,
Guenter
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