Re: [PATCH v4] platform/x86: p2sb: Allow p2sb_bar() calls during PCI device probe

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On Dec 28, 2023 / 17:47, Ilpo Järvinen wrote:
> On Wed, 27 Dec 2023, Ilpo Järvinen wrote:
> 
> > On Wed, 27 Dec 2023, Shin'ichiro Kawasaki wrote:
> > 
> > > p2sb_bar() unhides P2SB device to get resources from the device. It
> > > guards the operation by locking pci_rescan_remove_lock so that parallel
> > > rescans do not find the P2SB device. However, this lock causes deadlock
> > > when PCI bus rescan is triggered by /sys/bus/pci/rescan. The rescan
> > > locks pci_rescan_remove_lock and probes PCI devices. When PCI devices
> > > call p2sb_bar() during probe, it locks pci_rescan_remove_lock again.
> > > Hence the deadlock.
> > > 
> > > To avoid the deadlock, do not lock pci_rescan_remove_lock in p2sb_bar().
> > > Instead, do the lock at fs_initcall. Introduce p2sb_cache_resources()
> > > for fs_initcall which gets and caches the P2SB resources. At p2sb_bar(),
> > > refer the cache and return to the caller.
> > > 
> > > Link: https://lore.kernel.org/linux-pci/6xb24fjmptxxn5js2fjrrddjae6twex5bjaftwqsuawuqqqydx@7cl3uik5ef6j/
> > > Suggested-by: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
> > > Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@xxxxxxx>
> > > ---
> > > This patch reflects discussions held at the Link tag. I confirmed this patch
> > > fixes the problem using a system with i2c_i801 device, building i2c_i801
> > > module as both built-in and loadable. Reviews will be appreicated.
> > 
> > Why no Fixes tag?

I should have added that. Will add it. I think the commit for the Fixes tag is:

    9745fb07474f ("platform/x86/intel: Add Primary to Sideband (P2SB) bridge support")

> > 
> > > Changes from v3:
> > > * Modified p2sb_valid_resource() to return boolean
> > > 
> > > Changes from v2:
> > > * Improved p2sb_scan_and_cache() and p2sb_scan_and_cache_devfn()
> > > * Reflected other review comments by Andy
> > > 
> > > Changes from v1:
> > > * Reflected review comments by Andy
> > > * Removed RFC prefix
> > > 
> > > Changes from RFC v2:
> > > * Reflected review comments on the list
> > > 
> > > Changes from RFC v1:
> > > * Fixed a build warning poitned out in llvm list by kernel test robot
> > > 
> > >  drivers/platform/x86/p2sb.c | 171 +++++++++++++++++++++++++++---------
> > >  1 file changed, 129 insertions(+), 42 deletions(-)
> > > 
> > > diff --git a/drivers/platform/x86/p2sb.c b/drivers/platform/x86/p2sb.c
> > > index 1cf2471d54dd..ccebfd885718 100644
> > > --- a/drivers/platform/x86/p2sb.c
> > > +++ b/drivers/platform/x86/p2sb.c
> > > @@ -26,6 +26,19 @@ static const struct x86_cpu_id p2sb_cpu_ids[] = {
> > >  	{}
> > >  };
> > >  
> > > +/*
> > > + * Cache BAR0 of P2SB device functions 0 to 7.
> > > + * TODO: Move this definition to pci.h together with same other definitions.
> > 
> > "with same other definitions" sounds very odd and I don't understand what 
> > you're trying to say here.

I see, I will elaborate it as follows in the next post.

* TODO: The constant 8 is the number of functions that PCI specification
*       defines. Same definitions exist tree-wide. Unify this definition and
*       the other definitions then move to include/uapi/linux/pci.h.

> > 
> > > + */
> > > +#define NR_P2SB_RES_CACHE 8
> > > +
> > > +struct p2sb_res_cache {
> > > +	u32 bus_dev_id;
> > > +	struct resource res;
> > > +};
> > > +
> > > +static struct p2sb_res_cache p2sb_resources[NR_P2SB_RES_CACHE];
> > > +
> > >  static int p2sb_get_devfn(unsigned int *devfn)
> > >  {
> > >  	unsigned int fn = P2SB_DEVFN_DEFAULT;
> > > @@ -39,10 +52,18 @@ static int p2sb_get_devfn(unsigned int *devfn)
> > >  	return 0;
> > >  }
> > >  
> > > +static bool p2sb_valid_resource(struct resource *res)
> > > +{
> > > +	if (res->flags)
> > > +		return true;
> > > +
> > > +	return false;
> > > +}
> > > +
> > >  /* Copy resource from the first BAR of the device in question */
> > > -static int p2sb_read_bar0(struct pci_dev *pdev, struct resource *mem)
> > > +static void p2sb_read_bar0(struct pci_dev *pdev, struct resource *mem)
> > >  {
> > > -	struct resource *bar0 = &pdev->resource[0];
> > > +	struct resource *bar0 = pci_resource_n(pdev, 0);
> > 
> > This change is just cleanup and belongs IMO to own patch. (It's a good 
> > change still, just doesn't belong to this fix.)

I see. I will drop the hunk above from this patch, and will add a following
patch dedicated for it.

> > 
> > >  	/* Make sure we have no dangling pointers in the output */
> > >  	memset(mem, 0, sizeof(*mem));
> > > @@ -56,47 +77,64 @@ static int p2sb_read_bar0(struct pci_dev *pdev, struct resource *mem)
> > >  	mem->end = bar0->end;
> > >  	mem->flags = bar0->flags;
> > >  	mem->desc = bar0->desc;
> > > -
> > > -	return 0;
> > >  }
> > >  
> > > -static int p2sb_scan_and_read(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
> > > +static void p2sb_scan_and_cache_devfn(struct pci_bus *bus, unsigned int devfn)
> > >  {
> > > +	struct p2sb_res_cache *cache = &p2sb_resources[PCI_FUNC(devfn)];
> > >  	struct pci_dev *pdev;
> > > -	int ret;
> > >  
> > >  	pdev = pci_scan_single_device(bus, devfn);
> > >  	if (!pdev)
> > > -		return -ENODEV;
> > > +		return;
> > >  
> > > -	ret = p2sb_read_bar0(pdev, mem);
> > > +	p2sb_read_bar0(pdev, &cache->res);
> > > +	cache->bus_dev_id = bus->dev.id;
> > >  
> > >  	pci_stop_and_remove_bus_device(pdev);
> > > -	return ret;
> > > +	return;
> > >  }
> > >  
> > > -/**
> > > - * p2sb_bar - Get Primary to Sideband (P2SB) bridge device BAR
> > > - * @bus: PCI bus to communicate with
> > > - * @devfn: PCI slot and function to communicate with
> > > - * @mem: memory resource to be filled in
> > > - *
> > > - * The BIOS prevents the P2SB device from being enumerated by the PCI
> > > - * subsystem, so we need to unhide and hide it back to lookup the BAR.
> > > - *
> > > - * if @bus is NULL, the bus 0 in domain 0 will be used.
> > > - * If @devfn is 0, it will be replaced by devfn of the P2SB device.
> > > - *
> > > - * Caller must provide a valid pointer to @mem.
> > > - *
> > > - * Locking is handled by pci_rescan_remove_lock mutex.
> > > - *
> > > - * Return:
> > > - * 0 on success or appropriate errno value on error.
> > > - */
> > > -int p2sb_bar(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
> > > +static int p2sb_scan_and_cache(struct pci_bus *bus, unsigned int devfn)
> > > +{
> > > +	unsigned int slot, fn;
> > > +
> > > +	if (PCI_FUNC(devfn) == 0) {
> > > +		/*
> > > +		 * When function number of the P2SB device is zero, scan it and
> > > +		 * other function numbers, and if devices are available, cache
> > > +		 * their BAR0s.
> > > +		 */
> > > +		slot = PCI_SLOT(devfn);
> > > +		for (fn = 0; fn < NR_P2SB_RES_CACHE; fn++)
> > > +			p2sb_scan_and_cache_devfn(bus, PCI_DEVFN(slot, fn));
> > > +	} else {
> > > +		/* Scan the P2SB device and cache its BAR0 */
> > > +		p2sb_scan_and_cache_devfn(bus, devfn);
> > > +	}
> > > +
> > > +	if (!p2sb_valid_resource(&p2sb_resources[PCI_FUNC(devfn)].res))
> > > +		return -ENOENT;
> > > +
> > > +	return 0;
> > > +}
> > > +
> > > +static struct pci_bus *p2sb_get_bus(struct pci_bus *bus)
> > > +{
> > > +	static struct pci_bus *p2sb_bus;
> > > +
> > > +	bus = bus ?: p2sb_bus;
> > > +	if (bus)
> > > +		return bus;
> > > +
> > > +	/* Assume P2SB is on the bus 0 in domain 0 */
> > > +	p2sb_bus = pci_find_bus(0, 0);
> > > +	return p2sb_bus;
> > > +}
> > > +
> > > +static int p2sb_cache_resources(void)
> > >  {
> > > -	struct pci_dev *pdev_p2sb;
> > > +	struct pci_bus *bus;
> > >  	unsigned int devfn_p2sb;
> > >  	u32 value = P2SBC_HIDE;
> > >  	int ret;
> > > @@ -106,8 +144,9 @@ int p2sb_bar(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
> > >  	if (ret)
> > >  		return ret;
> > >  
> > > -	/* if @bus is NULL, use bus 0 in domain 0 */
> > > -	bus = bus ?: pci_find_bus(0, 0);
> > > +	bus = p2sb_get_bus(NULL);
> > > +	if (!bus)
> > > +		return -ENODEV;
> > >  
> > >  	/*
> > >  	 * Prevent concurrent PCI bus scan from seeing the P2SB device and
> > > @@ -115,17 +154,16 @@ int p2sb_bar(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
> > >  	 */
> > >  	pci_lock_rescan_remove();
> > >  
> > > -	/* Unhide the P2SB device, if needed */
> > > +	/*
> > > +	 * The BIOS prevents the P2SB device from being enumerated by the PCI
> > > +	 * subsystem, so we need to unhide and hide it back to lookup the BAR.
> > > +	 * Unhide the P2SB device here, if needed.
> > > +	 */
> > >  	pci_bus_read_config_dword(bus, devfn_p2sb, P2SBC, &value);
> > >  	if (value & P2SBC_HIDE)
> > >  		pci_bus_write_config_dword(bus, devfn_p2sb, P2SBC, 0);
> > >  
> > > -	pdev_p2sb = pci_scan_single_device(bus, devfn_p2sb);
> > > -	if (devfn)
> > > -		ret = p2sb_scan_and_read(bus, devfn, mem);
> > > -	else
> > > -		ret = p2sb_read_bar0(pdev_p2sb, mem);
> > > -	pci_stop_and_remove_bus_device(pdev_p2sb);
> > > +	ret = p2sb_scan_and_cache(bus, devfn_p2sb);
> > >  
> > >  	/* Hide the P2SB device, if it was hidden */
> > >  	if (value & P2SBC_HIDE)
> > > @@ -133,12 +171,61 @@ int p2sb_bar(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
> > >  
> > >  	pci_unlock_rescan_remove();
> > >  
> > > -	if (ret)
> > > -		return ret;
> > > +	return ret;
> > > +}
> > > +
> > > +/**
> > > + * p2sb_bar - Get Primary to Sideband (P2SB) bridge device BAR
> > > + * @bus: PCI bus to communicate with
> > > + * @devfn: PCI slot and function to communicate with
> > > + * @mem: memory resource to be filled in
> > > + *
> > > + * If @bus is NULL, the bus 0 in domain 0 will be used.
> > > + * If @devfn is 0, it will be replaced by devfn of the P2SB device.
> > > + *
> > > + * Caller must provide a valid pointer to @mem.
> > > + *
> > > + * Return:
> > > + * 0 on success or appropriate errno value on error.
> > > + */
> > > +int p2sb_bar(struct pci_bus *bus, unsigned int devfn, struct resource *mem)
> > > +{
> > > +	struct p2sb_res_cache *cache;
> > > +	int ret;
> > >  
> > > -	if (mem->flags == 0)
> > > +	bus = p2sb_get_bus(bus);
> > > +	if (!bus)
> > >  		return -ENODEV;
> > >  
> > > +	if (!devfn) {
> > > +		ret = p2sb_get_devfn(&devfn);
> > > +		if (ret)
> > > +			return ret;
> > > +	}
> > > +
> > > +	cache = &p2sb_resources[PCI_FUNC(devfn)];
> > > +	if (cache->bus_dev_id != bus->dev.id)
> > > +		return -ENODEV;
> > > +
> > > +	if (!p2sb_valid_resource(&cache->res))
> > > +		return -ENOENT;
> > > +
> > > +	memcpy(mem, &cache->res, sizeof(*mem));
> > >  	return 0;
> > >  }
> > >  EXPORT_SYMBOL_GPL(p2sb_bar);
> > > +
> > > +static int __init p2sb_fs_init(void)
> > > +{
> > > +	p2sb_cache_resources();
> > > +	return 0;
> > > +}
> > > +
> > > +/*
> > > + * pci_rescan_remove_lock avoids access to unhidden P2SB devices, but it causes
> > > + * deadlock with sysfs pci bus rescan.
> > 
> > Does this actually tell how situation was before this change? If so, it 
> > should be either removed or reworded to something that says "x cannot be 
> > used to do y because of z".

I will rephrase the sentence as follows.

 * pci_rescan_remove_lock to avoid access to unhidden P2SB devices can
 * not be locked in sysfs pci bus rescan path because of deadlock.

> > 
> > > To avoid the deadlock, access to P2SB
> > > + * devices at an early step in kernel initialization and cache required
> > > + * resources. This should happen after subsys_initcall which initializes PCI
> > > + * subsystem and before device_initcall which requires P2SB resources.
> > > + */
> > > +fs_initcall(p2sb_fs_init);
> > > 
> > 
> > 
> 
> I've applied this into review-ilpo branch with a Fixes tag but without 
> any changes to the code. Please do consider my feedback though to see if 
> they comments would need to be improved.

Thanks for the comments. Will reflect them to v5.





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