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Date: Wed, 7 Feb 2024 08:30:27 -0800
From: Saurabh Singh Sengar <ssengar@linux.microsoft.com>
To: Bjorn Helgaas <helgaas@kernel.org>
Cc: bhelgaas@google.com, linux-pci@vger.kernel.org,
	linux-kernel@vger.kernel.org, alexander.stein@ew.tq-group.com,
	decui@microsoft.com,
	Krzysztof =?utf-8?Q?Wilczy=C5=84ski?= <kwilczynski@kernel.org>
Subject: Re: [PATCH] PCI/sysfs: Fix race in pci sysfs creation
Message-ID: <20240207163027.GA13964@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
References: <1702093576-30405-1-git-send-email-ssengar@linux.microsoft.com>
 <20240206220715.GA884075@bhelgaas>
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On Tue, Feb 06, 2024 at 04:07:15PM -0600, Bjorn Helgaas wrote:
> [+cc Krzysztof]
> 
> On Fri, Dec 08, 2023 at 07:46:16PM -0800, Saurabh Sengar wrote:
> > 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
> > 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.
> > 
> > [    3.376688] sysfs: cannot create duplicate filename '/devices/LNXSYSTM:00/LNXSYBUS:00/PNP0A03:00/device:07/VMBUS:01/47505500-0001-0000-3130-444531454238/pci0001:00/0001:00:00.0/resource0'
> > [    3.385103] CPU: 3 PID: 9 Comm: kworker/u8:0 Not tainted 5.15.0-1046-azure #53~20.04.1-Ubuntu
> > [    3.389585] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS 090008  12/07/2018
> > [    3.394663] Workqueue: events_unbound async_run_entry_fn
> > [    3.397687] Call Trace:
> > [    3.399312]  <TASK>
> > [    3.400780]  dump_stack_lvl+0x38/0x4d
> > [    3.402998]  dump_stack+0x10/0x16
> > [    3.406050]  sysfs_warn_dup.cold+0x17/0x2b
> > [    3.408476]  sysfs_add_file_mode_ns+0x17b/0x190
> > [    3.411072]  sysfs_create_bin_file+0x64/0x90
> > [    3.413514]  pci_create_attr+0xc7/0x260
> > [    3.415827]  pci_create_resource_files+0x6f/0x150
> > [    3.418455]  pci_create_sysfs_dev_files+0x18/0x30
> > [    3.421136]  pci_bus_add_device+0x30/0x70
> > [    3.423512]  pci_bus_add_devices+0x31/0x70
> > [    3.425958]  hv_pci_probe+0x4ce/0x640
> > [    3.428106]  vmbus_probe+0x67/0x90
> > [    3.430121]  really_probe.part.0+0xcb/0x380
> > [    3.432516]  really_probe+0x40/0x80
> > [    3.434581]  __driver_probe_device+0xe8/0x140
> > [    3.437119]  driver_probe_device+0x23/0xb0
> > [    3.439504]  __driver_attach_async_helper+0x31/0x90
> > [    3.442296]  async_run_entry_fn+0x33/0x120
> > [    3.444666]  process_one_work+0x225/0x3d0
> > [    3.447043]  worker_thread+0x4d/0x3e0
> > [    3.449233]  ? process_one_work+0x3d0/0x3d0
> > [    3.451632]  kthread+0x12a/0x150
> > [    3.453583]  ? set_kthread_struct+0x50/0x50
> > [    3.456103]  ret_from_fork+0x22/0x30
> > 
> > Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
> > ---
> > There has been earlier attempts to fix this problem, below are the patches
> > for reference of these attempts.
> > 1. https://lore.kernel.org/linux-pci/20230316103036.1837869-1-alexander.stein@ew.tq-group.com/T/#u
> > 2. https://lwn.net/ml/linux-kernel/20230316091540.494366-1-alexander.stein@ew.tq-group.com/
> > 
> > Bug details: https://bugzilla.kernel.org/show_bug.cgi?id=215515
> > 
> >  drivers/pci/pci-sysfs.c | 9 +++++++--
> >  1 file changed, 7 insertions(+), 2 deletions(-)
> > 
> > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c
> > index f2909ae93f2f..a31f6f2cf309 100644
> > --- a/drivers/pci/pci-sysfs.c
> > +++ b/drivers/pci/pci-sysfs.c
> > @@ -1230,6 +1230,10 @@ static int pci_create_resource_files(struct pci_dev *pdev)
> >  		if (!pci_resource_len(pdev, i))
> >  			continue;
> >  
> > +		/* Check if resource already allocated and proceed no further */
> > +		if (pdev->res_attr[i] || pdev->res_attr_wc[i])
> > +			return 0;
> > +
> >  		retval = pci_create_attr(pdev, i, 0);
> >  		/* for prefetchable resources, create a WC mappable file */
> >  		if (!retval && arch_can_pci_mmap_wc() &&
> > @@ -1411,9 +1415,8 @@ static int __init pci_sysfs_init(void)
> >  	struct pci_bus *pbus = NULL;
> >  	int retval;
> >  
> > -	sysfs_initialized = 1;
> >  	for_each_pci_dev(pdev) {
> > -		retval = pci_create_sysfs_dev_files(pdev);
> > +		retval = pci_create_resource_files(pdev);
> >  		if (retval) {
> >  			pci_dev_put(pdev);
> >  			return retval;
> > @@ -1423,6 +1426,8 @@ static int __init pci_sysfs_init(void)
> >  	while ((pbus = pci_find_next_bus(pbus)))
> >  		pci_create_legacy_files(pbus);
> >  
> > +	sysfs_initialized = 1;
> > +
> >  	return 0;
> >  }
> >  late_initcall(pci_sysfs_init);
> 
> Sorry for the delay in looking at this.  Consider the following
> sequence where thread A is executing pci_sysfs_init() at the same time
> as thread B enumerates and adds device X:
> 
>   Thread A:
> 
>     pci_sysfs_init
>       for_each_pci_dev(pdev) {                  # device X not included
>         pci_create_resource_files(pdev);
>       }
> 
>   Thread B:
> 
>     pci_bus_add_device                          # add device X
>       pci_create_sysfs_dev_files
>         if (!sysfs_initialized)                 # sysfs_initialized still zero
>           return -EACCES;
>         pci_create_resource_files(pdev);        # not executed
> 
>   Thread A:
> 
>     while ((pbus = pci_find_next_bus(pbus)))
>       pci_create_legacy_files(pbus);
> 
>     sysfs_initialized = 1;
> 
> Doesn't this have a similar race where instead of the duplicate
> filename from having two threads try to create the resource files,
> neither thread creates them and device X ends up with no resource
> files at all?
> 
> Krzysztof has done a ton of work to convert these files to static
> attributes, where the device model prevents most of these races:
> 
>   506140f9c06b ("PCI/sysfs: Convert "index", "acpi_index", "label" to static attributes")
>   d93f8399053d ("PCI/sysfs: Convert "vpd" to static attribute")
>   f42c35ea3b13 ("PCI/sysfs: Convert "reset" to static attribute")
>   527139d738d7 ("PCI/sysfs: Convert "rom" to static attribute")
>   e1d3f3268b0e ("PCI/sysfs: Convert "config" to static attribute")
> 
> and he even posted a series to do the same for the resource files:
> 
>   https://lore.kernel.org/linux-pci/20210910202623.2293708-1-kw@linux.com/
> 
> I can't remember why we didn't apply that at the time, and it no
> longer applies cleanly, but I think that's the direction we should go.
> 
> Bjorn


Thanks for you review.

Krzysztof,

Please inform me if there's existing feedback explaining why this
series hasn't been merged yet. I am willing to further improve it
if necessary.

- Saurabh