linux-kernel - Re: [PATCH] PCI: Using lockless config space accessors based on

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Message-ID: <20250507090407.2146324-1-kerayhuang@tencent.com>
Date: Wed,  7 May 2025 17:04:07 +0800
From: Zijiang Huang <huangzjsmile@...il.com>
To: lukas@...ner.de
Cc: bhelgaas@...gle.com,
	flyingpeng@...cent.com,
	huangzjsmile@...il.com,
	kerayhuang@...cent.com,
	linux-kernel@...r.kernel.org,
	linux-pci@...r.kernel.org,
	tglx@...utronix.de
Subject: Re: [PATCH] PCI: Using lockless config space accessors based on

Hi Lukas,

I think it's safe to make this change for user-space accessors as well,
since user-space only reads from proc files.

> Why is performance of the user space accessors important?
> Perhaps because of vfio?

During stability testing on large-scale machines (384+ CPUs), we always                                                                                                                                                                                           
observed that heavy concurrent user-space access to PCI config space triggers 
kernel softlockups.
 
Reproduction method: stress-ng --pci 384 

Thanks,

keray

> That's the information I'm missing in the commit message.
> 
> Thanks,
> 
> Lukas
> 
> > Signed-off-by: Zijiang Huang <kerayhuang@...cent.com>
> > Reviewed-by: Hao Peng <flyingpeng@...cent.com>
> > ---
> >  drivers/pci/access.c | 12 +++++++-----
> >  1 file changed, 7 insertions(+), 5 deletions(-)
> > 
> > diff --git a/drivers/pci/access.c b/drivers/pci/access.c
> > index 3c230ca3d..5200f7bbc 100644
> > --- a/drivers/pci/access.c
> > +++ b/drivers/pci/access.c
> > @@ -216,20 +216,21 @@ static noinline void pci_wait_cfg(struct pci_dev *dev)
> >  }
> >  
> >  /* Returns 0 on success, negative values indicate error. */
> > -#define PCI_USER_READ_CONFIG(size, type)					\
> > +#define PCI_USER_READ_CONFIG(size, type)				\
> >  int pci_user_read_config_##size						\
> >  	(struct pci_dev *dev, int pos, type *val)			\
> >  {									\
> >  	int ret = PCIBIOS_SUCCESSFUL;					\
> >  	u32 data = -1;							\
> > +	unsigned long flags;						\
> >  	if (PCI_##size##_BAD)						\
> >  		return -EINVAL;						\
> > -	raw_spin_lock_irq(&pci_lock);				\
> > +	pci_lock_config(flags);						\
> >  	if (unlikely(dev->block_cfg_access))				\
> >  		pci_wait_cfg(dev);					\
> >  	ret = dev->bus->ops->read(dev->bus, dev->devfn,			\
> >  					pos, sizeof(type), &data);	\
> > -	raw_spin_unlock_irq(&pci_lock);				\
> > +	pci_unlock_config(flags);					\
> >  	if (ret)							\
> >  		PCI_SET_ERROR_RESPONSE(val);				\
> >  	else								\
> > @@ -244,14 +245,15 @@ int pci_user_write_config_##size					\
> >  	(struct pci_dev *dev, int pos, type val)			\
> >  {									\
> >  	int ret = PCIBIOS_SUCCESSFUL;					\
> > +	unsigned long flags;						\
> >  	if (PCI_##size##_BAD)						\
> >  		return -EINVAL;						\
> > -	raw_spin_lock_irq(&pci_lock);				\
> > +	pci_lock_config(flags);						\
> >  	if (unlikely(dev->block_cfg_access))				\
> >  		pci_wait_cfg(dev);					\
> >  	ret = dev->bus->ops->write(dev->bus, dev->devfn,		\
> >  					pos, sizeof(type), val);	\
> > -	raw_spin_unlock_irq(&pci_lock);				\
> > +	pci_unlock_config(flags);					\
> >  	return pcibios_err_to_errno(ret);				\
> >  }									\
> >  EXPORT_SYMBOL_GPL(pci_user_write_config_##size);
> > -- 
> > 2.43.5