Re: [PATCH v2] kernel/resource: Fix locking in request_free_mem_region

[Alistair Popple]

On Friday, 26 March 2021 4:15:36 PM AEDT Balbir Singh wrote:
> On Fri, Mar 26, 2021 at 12:20:35PM +1100, Alistair Popple wrote:
> > +static int __region_intersects(resource_size_t start, size_t size,
> > +                            unsigned long flags, unsigned long desc)
> > +{
> > +     struct resource res;
> > +     int type = 0; int other = 0;
> > +     struct resource *p;
> > +
> > +     res.start = start;
> > +     res.end = start + size - 1;
> > +
> > +     for (p = iomem_resource.child; p ; p = p->sibling) {
> > +             bool is_type = (((p->flags & flags) == flags) &&
> > +                             ((desc == IORES_DESC_NONE) ||
> > +                              (desc == p->desc)));
> 
> is_type is a bad name, are we saying "is" as in boolean question?
> Or is it short for something like intersection_type? I know you've
> just moved the code over :)

Yeah, I'm not a fan of that name either but I was just moving code over and 
couldn't come up with anything better :)

It is a boolean question though - it is checking to see if resource *p is the 
same type (flags+desc) of region as what is being checked for intersection.
 
> > +
> > +             if (resource_overlaps(p, &res))
> > +                     is_type ? type++ : other++;
> > +     }
> > +
> > +     if (type == 0)
> > +             return REGION_DISJOINT;
> > +
> > +     if (other == 0)
> > +             return REGION_INTERSECTS;
> > +
> > +     return REGION_MIXED;
> > +}
> > +
> >  /**
> >   * region_intersects() - determine intersection of region with known 
resources
> >   * @start: region start address
> > @@ -546,31 +574,12 @@ EXPORT_SYMBOL_GPL(page_is_ram);
> >  int region_intersects(resource_size_t start, size_t size, unsigned long 
flags,
> >                     unsigned long desc)
> >  {
> > -     struct resource res;
> > -     int type = 0; int other = 0;
> > -     struct resource *p;
> > -
> > -     res.start = start;
> > -     res.end = start + size - 1;
> > +     int rc;
> >
> >       read_lock(&resource_lock);
> > -     for (p = iomem_resource.child; p ; p = p->sibling) {
> > -             bool is_type = (((p->flags & flags) == flags) &&
> > -                             ((desc == IORES_DESC_NONE) ||
> > -                              (desc == p->desc)));
> > -
> > -             if (resource_overlaps(p, &res))
> > -                     is_type ? type++ : other++;
> > -     }
> > +     rc = __region_intersects(start, size, flags, desc);
> >       read_unlock(&resource_lock);
> > -
> > -     if (type == 0)
> > -             return REGION_DISJOINT;
> > -
> > -     if (other == 0)
> > -             return REGION_INTERSECTS;
> > -
> > -     return REGION_MIXED;
> > +     return rc;
> >  }
> >  EXPORT_SYMBOL_GPL(region_intersects);
> >
> > @@ -1171,31 +1180,17 @@ struct address_space *iomem_get_mapping(void)
> >       return smp_load_acquire(&iomem_inode)->i_mapping;
> >  }
> >
> > -/**
> > - * __request_region - create a new busy resource region
> > - * @parent: parent resource descriptor
> > - * @start: resource start address
> > - * @n: resource region size
> > - * @name: reserving caller's ID string
> > - * @flags: IO resource flags
> > - */
> > -struct resource * __request_region(struct resource *parent,
> > -                                resource_size_t start, resource_size_t n,
> > -                                const char *name, int flags)
> > +static bool request_region_locked(struct resource *parent,
> > +                                 struct resource *res, resource_size_t 
start,
> > +                                 resource_size_t n, const char *name, int 
flags)
> >  {
> > -     DECLARE_WAITQUEUE(wait, current);
> > -     struct resource *res = alloc_resource(GFP_KERNEL);
> >       struct resource *orig_parent = parent;
> > -
> > -     if (!res)
> > -             return NULL;
> > +     DECLARE_WAITQUEUE(wait, current);
> 
> This part of the diff looks confusing, do we have a waitqueue and we call
> schedule() within a function called with the lock held?

Good point. schedule() does get called but the lock is dropped first:

		if (conflict->flags & flags & IORESOURCE_MUXED) {
			add_wait_queue(&muxed_resource_wait, &wait);
			write_unlock(&resource_lock);
			set_current_state(TASK_UNINTERRUPTIBLE);
			schedule();
			remove_wait_queue(&muxed_resource_wait, &wait);
			write_lock(&resource_lock);
			continue;
		}

This isn't an issue though as it's only used for request_muxed_region() which 
isn't used for the ZONE_DEVICE allocation and by design doesn't search for 
free space. Ie. IORESOURCE_MUXED will never be set for 
request_free_mem_region().

> >
> >       res->name = name;
> >       res->start = start;
> >       res->end = start + n - 1;
> >
> > -     write_lock(&resource_lock);
> > -
> >       for (;;) {
> >               struct resource *conflict;
> >
> > @@ -1230,16 +1225,39 @@ struct resource * __request_region(struct resource 
*parent,
> >                       write_lock(&resource_lock);
> >                       continue;
> >               }
> > -             /* Uhhuh, that didn't work out.. */
> > -             free_resource(res);
> > -             res = NULL;
> > -             break;
> > +             return false;
> >       }
> > -     write_unlock(&resource_lock);
> >
> >       if (res && orig_parent == &iomem_resource)
> >               revoke_iomem(res);
> >
> > +     return true;
> > +}
> > +
> > +/**
> > + * __request_region - create a new busy resource region
> > + * @parent: parent resource descriptor
> > + * @start: resource start address
> > + * @n: resource region size
> > + * @name: reserving caller's ID string
> > + * @flags: IO resource flags
> > + */
> > +struct resource *__request_region(struct resource *parent,
> > +                               resource_size_t start, resource_size_t n,
> > +                               const char *name, int flags)
> > +{
> > +     struct resource *res = alloc_resource(GFP_KERNEL);
> > +
> > +     if (!res)
> > +             return NULL;
> > +
> > +     write_lock(&resource_lock);
> > +     if (!request_region_locked(parent, res, start, n, name, flags)) {
> > +             /* Uhhuh, that didn't work out.. */
> > +             free_resource(res);
> > +             res = NULL;
> > +     }
> > +     write_unlock(&resource_lock);
> 
> Should the function be called __request_region_locked?

This is the name of original function, so this is just maintaining the 
original behaviour by taking the lock and calling the inner function 
(request_region_locked) rather than having it coded directly there.

__request_region() is rarely called directly and is mostly called via macros:

include/linux/ioport.h:#define request_region(start,n,name)             
__request_region(&ioport_resource, (start), (n), (name), 0)
include/linux/ioport.h:#define request_muxed_region(start,n,name)       
__request_region(&ioport_resource, (start), (n), (name), IORESOURCE_MUXED)
include/linux/ioport.h:#define __request_mem_region(start,n,name, excl) 
__request_region(&iomem_resource, (start), (n), (name), excl)
include/linux/ioport.h:#define request_mem_region(start,n,name) 
__request_region(&iomem_resource, (start), (n), (name), 0)

Thanks,

 - Alistair

> >       return res;
> >  }
> >  EXPORT_SYMBOL(__request_region);
> > @@ -1779,26 +1797,50 @@ static struct resource 
*__request_free_mem_region(struct device *dev,
> >  {
> >       resource_size_t end, addr;
> >       struct resource *res;
> > +     struct region_devres *dr = NULL;
> > +
> > +     res = alloc_resource(GFP_KERNEL);
> > +     if (!res)
> > +             return ERR_PTR(-ENOMEM);
> > +
> > +     if (dev) {
> > +             dr = devres_alloc(devm_region_release, sizeof(struct 
region_devres),
> > +                               GFP_KERNEL);
> > +             if (!dr) {
> > +                     free_resource(res);
> > +                     return ERR_PTR(-ENOMEM);
> > +             }
> > +     }
> >
> >       size = ALIGN(size, 1UL << PA_SECTION_SHIFT);
> >       end = min_t(unsigned long, base->end, (1UL << MAX_PHYSMEM_BITS) - 
1);
> >       addr = end - size + 1UL;
> >
> > +     write_lock(&resource_lock);
> >       for (; addr > size && addr >= base->start; addr -= size) {
> > -             if (region_intersects(addr, size, 0, IORES_DESC_NONE) !=
> > +             if (__region_intersects(addr, size, 0, IORES_DESC_NONE) !=
> >                               REGION_DISJOINT)
> >                       continue;
> >
> > -             if (dev)
> > -                     res = devm_request_mem_region(dev, addr, size, 
name);
> > -             else
> > -                     res = request_mem_region(addr, size, name);
> > -             if (!res)
> > -                     return ERR_PTR(-ENOMEM);
> > +             if (!request_region_locked(&iomem_resource, res, addr,
> > +                                                size, name, 0))
> > +                     break;
> > +
> >               res->desc = IORES_DESC_DEVICE_PRIVATE_MEMORY;
> > +             if (dev) {
> > +                     dr->parent = &iomem_resource;
> > +                     dr->start = addr;
> > +                     dr->n = size;
> > +                     devres_add(dev, dr);
> > +             }
> > +
> > +             write_unlock(&resource_lock);
> >               return res;
> >       }
> >
> > +     write_unlock(&resource_lock);
> > +     free_resource(res);
> > +
> >       return ERR_PTR(-ERANGE);
> >  }
> >
> 
> Balbir Singh.