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Date: Tue, 6 Feb 2024 16:07:15 -0600
From: Bjorn Helgaas <helgaas@kernel.org>
To: Saurabh Sengar <ssengar@linux.microsoft.com>
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: <20240206220715.GA884075@bhelgaas>
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[+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