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Date: Mon, 29 Apr 2024 12:03:55 -0500
From: Bjorn Helgaas <helgaas@kernel.org>
To: Dexuan Cui <decui@microsoft.com>
Cc: bhelgaas@google.com, wei.liu@kernel.org, kys@microsoft.com,
	haiyangz@microsoft.com, lpieralisi@kernel.org,
	linux-pci@vger.kernel.org, linux-hyperv@vger.kernel.org,
	linux-kernel@vger.kernel.org, Boqun.Feng@microsoft.com,
	sunilmut@microsoft.com, ssengar@microsoft.com
Subject: Re: [PATCH] PCI: Add a mutex to protect the global list
 pci_domain_busn_res_list
Message-ID: <20240429170355.GA685838@bhelgaas>
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In-Reply-To: <20240425225136.GA541002@bhelgaas>

On Thu, Apr 25, 2024 at 05:51:38PM -0500, Bjorn Helgaas wrote:
> On Thu, Apr 18, 2024 at 06:53:02PM -0700, Dexuan Cui wrote:
> > There has been an effort to make the pci-hyperv driver support
> > async-probing to reduce the boot time. With async-probing, multiple
> > kernel threads can be running hv_pci_probe() -> create_root_hv_pci_bus() ->
> > pci_scan_root_bus_bridge() -> pci_bus_insert_busn_res() at the same time to
> > update the global list, causing list corruption.
> > 
> > Add a mutex to protect the list.
> 
> I think it's a good idea to support probing multiple PCI root buses in
> parallel.
> 
> The problem in get_pci_domain_busn_res() is the global
> pci_domain_busn_res_list.  I'm not even sure what that list contains,
> since it's a lookup by "domain_nr".  In the hv case, you probably have
> one host bridge per domain, but in general there may be multiple root
> buses in the same domain, e.g.,
> 
>   ACPI: PCI Root Bridge [PC00] (domain 0000 [bus 00-16])
>   ACPI: PCI Root Bridge [PC01] (domain 0000 [bus 17-39])
>   ACPI: PCI Root Bridge [PC02] (domain 0000 [bus 3a-5c])
>   ...
> 
> We only use get_pci_domain_busn_res() for root buses, and we should
> know the bus number range for root buses when we set up the struct
> pci_host_bridge, so it seems like we should keep the bus number
> resource there instead of allocating it in this sort of random place.
> 
> Then we shouldn't need this weird pci_domain_busn_res_list at all.

Oops, sorry, I totally missed the point here.  The point is that for
each domain, we get a new 00-ff range of possible bus numbers.  This
is independent of the host bridges for that domain that may exist.
Then each host bridge will allocate a piece of the 00-ff range.

But I do still think get_pci_domain_busn_res() isn't really the best
place for this.  It seems like it should be at a higher level,
connected somehow to domain number allocation, e.g., somewhere related
to bridge->domain_nr like the pci_bus_find_domain_nr() path.

> > Signed-off-by: Dexuan Cui <decui@microsoft.com>
> > ---
> >  drivers/pci/probe.c | 25 ++++++++++++++++++-------
> >  1 file changed, 18 insertions(+), 7 deletions(-)
> > 
> > diff --git a/drivers/pci/probe.c b/drivers/pci/probe.c
> > index e19b79821dd6..1327fd820b24 100644
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -37,6 +37,7 @@ LIST_HEAD(pci_root_buses);
> >  EXPORT_SYMBOL(pci_root_buses);
> >  
> >  static LIST_HEAD(pci_domain_busn_res_list);
> > +static DEFINE_MUTEX(pci_domain_busn_res_list_lock);
> >  
> >  struct pci_domain_busn_res {
> >  	struct list_head list;
> > @@ -47,14 +48,22 @@ struct pci_domain_busn_res {
> >  static struct resource *get_pci_domain_busn_res(int domain_nr)
> >  {
> >  	struct pci_domain_busn_res *r;
> > +	struct resource *ret;
> >  
> > -	list_for_each_entry(r, &pci_domain_busn_res_list, list)
> > -		if (r->domain_nr == domain_nr)
> > -			return &r->res;
> > +	mutex_lock(&pci_domain_busn_res_list_lock);
> > +
> > +	list_for_each_entry(r, &pci_domain_busn_res_list, list) {
> > +		if (r->domain_nr == domain_nr) {
> > +			ret = &r->res;
> > +			goto out;
> > +		}
> > +	}
> >  
> >  	r = kzalloc(sizeof(*r), GFP_KERNEL);
> > -	if (!r)
> > -		return NULL;
> > +	if (!r) {
> > +		ret = NULL;
> > +		goto out;
> > +	}
> >  
> >  	r->domain_nr = domain_nr;
> >  	r->res.start = 0;
> > @@ -62,8 +71,10 @@ static struct resource *get_pci_domain_busn_res(int domain_nr)
> >  	r->res.flags = IORESOURCE_BUS | IORESOURCE_PCI_FIXED;
> >  
> >  	list_add_tail(&r->list, &pci_domain_busn_res_list);
> > -
> > -	return &r->res;
> > +	ret = &r->res;
> > +out:
> > +	mutex_unlock(&pci_domain_busn_res_list_lock);
> > +	return ret;
> >  }
> >  
> >  /*
> > -- 
> > 2.25.1
> >