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Date:   Tue, 12 Dec 2023 00:28:05 -0800
From:   Saurabh Singh Sengar <ssengar@linux.microsoft.com>
To:     Alexander Stein <alexander.stein@ew.tq-group.com>
Cc:     bhelgaas@google.com, linux-pci@vger.kernel.org,
        linux-kernel@vger.kernel.org, decui@microsoft.com
Subject: Re: [PATCH] PCI/sysfs: Fix race in pci sysfs creation
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On Tue, Dec 12, 2023 at 08:19:11AM +0100, Alexander Stein wrote:
> Hi Saurabh,
> 
> thanks for the patch.
> 
> Am Samstag, 9. Dezember 2023, 04:46:16 CET schrieb Saurabh Sengar:
> > Currently there is a race in calling pci_create_resource_files function
> > from two different therads, first therad is triggered by pci_sysfs_init
> > from the late initcall where as the second thread is initiated by
> > pci_bus_add_devices from the respective PCI drivers probe.
> > 
> > The synchronization between these threads relies on the sysfs_initialized
> > flag. However, in pci_sysfs_init, sysfs_initialized is set right before
> > calling pci_create_resource_files which is wrong as it can create race
> > condition with pci_bus_add_devices threads. Fix this by setting
> > sysfs_initialized flag at the end of pci_sysfs_init and direecly call the
> 
> Small typo here: direecly -> directly
> 
> > pci_create_resource_files function from it.
> > 
> > There can be an additional case where driver probe is so delayed that
> > pci_bus_add_devices is called after the sysfs is created by pci_sysfs_init.
> > In such cases, attempting to access already existing sysfs resources is
> > unnecessary. Fix this by adding a check for sysfs attributes and return
> > if they are already allocated.
> > 
> > In both cases, the consequence will be the removal of sysfs resources that
> > were appropriately allocated by pci_sysfs_init following the warning below.
> 
> I'm not sure if this is the way to go. Unfortunately I can't trigger this 
> error on my imx6 platform at the moment (apparently timing is off).
> But reading [1] again, the most expressive way is that pci_bus_add_devices() 
> needs to wait until pci_sysfs_init() has passed.

(I correct my self a bit in my earlier reply)
The problem with waiting is that sysfs entries will be created by pci_sysfs_init
already and when pci_bus_add_devices try to create it will observe that the
entries are already existing and in such case PCI code will remove the sysfs
entries created by pci_sysfs_init. Resulting system will be having no sysfs
entries.

- Saurabh