Re: [PATCH V2 3/6] PCI, x86: use RCU list to protect mmconfig list

[Jiang Liu <jiang.liu@xxxxxxxxxx>]

Hi Bjorn,
	Thanks for your comments!
	I have thought about using pci_config_lock to serialize
access to the pci_mmcfg_list, but that may hurt MMCFG on x86_64
systems because mmconfig_64.c supports concurrent access to
the MMCFG space without holding the pci_config_lock. So I use
RCU to avoid possible performance penalty for x86_64 systems.

	There are two mechanisms to protect list_add_sorted()
from concurrent updates. The first case is, list_add_sorted()
may be called without holding any lock for serialization at
early booting stages because the system is still in single-threaded
mode. The second case is, pci_config_lock is used to serialize
concurrent modification to the pci_mmcfg_list if list_add_sorted()
is called at runtime.
	The first case is cover by this patch, and the second case
is covered by "[PATCH V2 5/6] PCI, x86: introduce 
pci_mmconfig_insert()/delete() for PCI root bridge hotplug".
	Thanks!

On 2012-5-3 6:31, Bjorn Helgaas wrote:
> On Mon, Apr 9, 2012 at 10:22 AM, Jiang Liu<liuj97@xxxxxxxxx>  wrote:
> > Use RCU list to protect mmconfig list from dynamic change
> > when supporting PCI host bridge hotplug.
>
> I'm not convinced this is safe.  But I'm not an RCU expert, so maybe
> you can convince me :)
>
> Documentation/RCU/checklist.txt says the update code still requires
> mutual exclusion, and I don't see any in list_add_sorted() or its
> callers.
>
> MMCONFIG accesses are already protected by the pci_config_lock
> spinlock, so I don't think the performance advantages of RCU really
> gain us anything in this situation.
>
> It would definitely be simpler to just hold pci_config_lock while
> updating and searching the pci_mmcfg_list, but if there are issues
> with that, you can educate me about what they are and why this RCU
> code is correct.
>
> Bjorn
>
> > Signed-off-by: Jiang Liu<jiang.liu@xxxxxxxxxx>
> > ---
> >   arch/x86/pci/mmconfig-shared.c |   11 ++++++-----
> >   arch/x86/pci/mmconfig_32.c     |   13 +++++++++++--
> >   arch/x86/pci/mmconfig_64.c     |   13 +++++++++++--
> >   3 files changed, 28 insertions(+), 9 deletions(-)
> >
> > diff --git a/arch/x86/pci/mmconfig-shared.c b/arch/x86/pci/mmconfig-shared.c
> > index 5e2cd2a..3bcc361 100644
> > --- a/arch/x86/pci/mmconfig-shared.c
> > +++ b/arch/x86/pci/mmconfig-shared.c
> > @@ -17,6 +17,7 @@
> >   #include<linux/bitmap.h>
> >   #include<linux/dmi.h>
> >   #include<linux/slab.h>
> > +#include<linux/rculist.h>
> >   #include<asm/e820.h>
> >   #include<asm/pci_x86.h>
> >   #include<asm/acpi.h>
> > @@ -45,20 +46,20 @@ static __init void free_all_mmcfg(void)
> >                 pci_mmconfig_remove(cfg);
> >   }
> >
> > -static __init void list_add_sorted(struct pci_mmcfg_region *new)
> > +static __devinit void list_add_sorted(struct pci_mmcfg_region *new)
> >   {
> >         struct pci_mmcfg_region *cfg;
> >
> >         /* keep list sorted by segment and starting bus number */
> > -       list_for_each_entry(cfg,&pci_mmcfg_list, list) {
> > +       list_for_each_entry_rcu(cfg,&pci_mmcfg_list, list) {
> >                 if (cfg->segment>  new->segment ||
> >                     (cfg->segment == new->segment&&
> >                      cfg->start_bus>= new->start_bus)) {
> > -                       list_add_tail(&new->list,&cfg->list);
> > +                       list_add_tail_rcu(&new->list,&cfg->list);
> >                         return;
> >                 }
> >         }
> > -       list_add_tail(&new->list,&pci_mmcfg_list);
> > +       list_add_tail_rcu(&new->list,&pci_mmcfg_list);
> >   }
> >
> >   static __devinit struct pci_mmcfg_region *pci_mmconfig_alloc(int segment,
> > @@ -111,7 +112,7 @@ struct pci_mmcfg_region *pci_mmconfig_lookup(int segment, int bus)
> >   {
> >         struct pci_mmcfg_region *cfg;
> >
> > -       list_for_each_entry(cfg,&pci_mmcfg_list, list)
> > +       list_for_each_entry_rcu(cfg,&pci_mmcfg_list, list)
> >                 if (cfg->segment == segment&&
> >                     cfg->start_bus<= bus&&  bus<= cfg->end_bus)
> >                         return cfg;
> > diff --git a/arch/x86/pci/mmconfig_32.c b/arch/x86/pci/mmconfig_32.c
> > index 5372e86..5dad04a 100644
> > --- a/arch/x86/pci/mmconfig_32.c
> > +++ b/arch/x86/pci/mmconfig_32.c
> > @@ -11,6 +11,7 @@
> >
> >   #include<linux/pci.h>
> >   #include<linux/init.h>
> > +#include<linux/rcupdate.h>
> >   #include<asm/e820.h>
> >   #include<asm/pci_x86.h>
> >   #include<acpi/acpi.h>
> > @@ -60,9 +61,12 @@ err:         *value = -1;
> >                 return -EINVAL;
> >         }
> >
> > +       rcu_read_lock();
> >         base = get_base_addr(seg, bus, devfn);
> > -       if (!base)
> > +       if (!base) {
> > +               rcu_read_unlock();
> >                 goto err;
> > +       }
> >
> >         raw_spin_lock_irqsave(&pci_config_lock, flags);
> >
> > @@ -80,6 +84,7 @@ err:          *value = -1;
> >                 break;
> >         }
> >         raw_spin_unlock_irqrestore(&pci_config_lock, flags);
> > +       rcu_read_unlock();
> >
> >         return 0;
> >   }
> > @@ -93,9 +98,12 @@ static int pci_mmcfg_write(unsigned int seg, unsigned int bus,
> >         if ((bus>  255) || (devfn>  255) || (reg>  4095))
> >                 return -EINVAL;
> >
> > +       rcu_read_lock();
> >         base = get_base_addr(seg, bus, devfn);
> > -       if (!base)
> > +       if (!base) {
> > +               rcu_read_unlock();
> >                 return -EINVAL;
> > +       }
> >
> >         raw_spin_lock_irqsave(&pci_config_lock, flags);
> >
> > @@ -113,6 +121,7 @@ static int pci_mmcfg_write(unsigned int seg, unsigned int bus,
> >                 break;
> >         }
> >         raw_spin_unlock_irqrestore(&pci_config_lock, flags);
> > +       rcu_read_unlock();
> >
> >         return 0;
> >   }
> > diff --git a/arch/x86/pci/mmconfig_64.c b/arch/x86/pci/mmconfig_64.c
> > index 915a493..acc48c5 100644
> > --- a/arch/x86/pci/mmconfig_64.c
> > +++ b/arch/x86/pci/mmconfig_64.c
> > @@ -9,6 +9,7 @@
> >   #include<linux/init.h>
> >   #include<linux/acpi.h>
> >   #include<linux/bitmap.h>
> > +#include<linux/rcupdate.h>
> >   #include<asm/e820.h>
> >   #include<asm/pci_x86.h>
> >
> > @@ -34,9 +35,12 @@ err:         *value = -1;
> >                 return -EINVAL;
> >         }
> >
> > +       rcu_read_lock();
> >         addr = pci_dev_base(seg, bus, devfn);
> > -       if (!addr)
> > +       if (!addr) {
> > +               rcu_read_unlock();
> >                 goto err;
> > +       }
> >
> >         switch (len) {
> >         case 1:
> > @@ -49,6 +53,7 @@ err:          *value = -1;
> >                 *value = mmio_config_readl(addr + reg);
> >                 break;
> >         }
> > +       rcu_read_unlock();
> >
> >         return 0;
> >   }
> > @@ -62,9 +67,12 @@ static int pci_mmcfg_write(unsigned int seg, unsigned int bus,
> >         if (unlikely((bus>  255) || (devfn>  255) || (reg>  4095)))
> >                 return -EINVAL;
> >
> > +       rcu_read_lock();
> >         addr = pci_dev_base(seg, bus, devfn);
> > -       if (!addr)
> > +       if (!addr) {
> > +               rcu_read_unlock();
> >                 return -EINVAL;
> > +       }
> >
> >         switch (len) {
> >         case 1:
> > @@ -77,6 +85,7 @@ static int pci_mmcfg_write(unsigned int seg, unsigned int bus,
> >                 mmio_config_writel(addr + reg, value);
> >                 break;
> >         }
> > +       rcu_read_unlock();
> >
> >         return 0;
> >   }
> > --
> > 1.7.5.4
>
> .


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