On Tue, 16 Nov 2010 08:53:10 +0800
Huang Ying <ying.huang@xxxxxxxxx> wrote:
> This version of the gen_pool memory allocator supports lockless
> operation.
>
> This makes it safe to use in NMI handlers and other special
> unblockable contexts that could otherwise deadlock on locks. This is
> implemented by using atomic operations and retries on any conflicts.
> The disadvantage is that there may be livelocks in extreme cases. For
> better scalability, one gen_pool allocator can be used for each CPU.
>
> The lockless operation only works if there is enough memory available.
> If new memory is added to the pool a lock has to be still taken. So
> any user relying on locklessness has to ensure that sufficient memory
> is preallocated.
>
> The basic atomic operation of this allocator is cmpxchg on long. On
> architectures that don't support cmpxchg natively a fallback is used.
> If the fallback uses locks it may not be safe to use it in NMI
> contexts on these architectures.
The code assumes that cmpxchg is atomic wrt NMI. That would be news to
me - at present an architecture can legitimately implement cmpxchg()
with, say, spin_lock_irqsave() on a hashed spinlock. I don't know
whether any architectures _do_ do anything like that. If so then
that's a problem. If not, it's an additional requirement on future
architecture ports.
> Signed-off-by: Huang Ying <ying.huang@xxxxxxxxx>
> Reviewed-by: Andi Kleen <ak@xxxxxxxxxxxxxxx>
> ---
> include/linux/bitmap.h | 1
> include/linux/genalloc.h | 35 +++++--
> lib/bitmap.c | 2
> lib/genalloc.c | 228 ++++++++++++++++++++++++++++++++++++++---------
> 4 files changed, 215 insertions(+), 51 deletions(-)
>
> --- a/include/linux/bitmap.h
> +++ b/include/linux/bitmap.h
> @@ -142,6 +142,7 @@ extern void bitmap_release_region(unsign
> extern int bitmap_allocate_region(unsigned long *bitmap, int pos, int order);
> extern void bitmap_copy_le(void *dst, const unsigned long *src, int nbits);
>
> +#define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) % BITS_PER_LONG))
> #define BITMAP_LAST_WORD_MASK(nbits) \
> ( \
> ((nbits) % BITS_PER_LONG) ? \
> --- a/include/linux/genalloc.h
> +++ b/include/linux/genalloc.h
> @@ -1,19 +1,26 @@
> +#ifndef GENALLOC_H
> +#define GENALLOC_H
> /*
> - * Basic general purpose allocator for managing special purpose memory
> - * not managed by the regular kmalloc/kfree interface.
> - * Uses for this includes on-device special memory, uncached memory
> - * etc.
> + * Basic general purpose allocator for managing special purpose
> + * memory, for example, memory not managed by the regular
> + * kmalloc/kfree interface. Uses for this includes on-device special
> + * memory, uncached memory etc.
> + *
> + * The gen_pool_alloc, gen_pool_free, gen_pool_avail and gen_pool_size
> + * implementation is lockless, that is, multiple users can
> + * allocate/free memory in the pool simultaneously without lock. This
> + * also makes the gen_pool memory allocator can be used to
That sentence needs a fixup.
>
> +static inline int set_bits_ll(unsigned long *addr, unsigned long mask_to_set)
> +{
> + unsigned long val, nval;
> +
> + nval = *addr;
> + do {
> + val = nval;
> + if (val & mask_to_set)
> + return -EBUSY;
> + } while ((nval = cmpxchg(addr, val, val | mask_to_set)) != val);
> +
> + return 0;
> +}
> +
> +static inline int clear_bits_ll(unsigned long *addr,
> + unsigned long mask_to_clear)
> +{
> + unsigned long val, nval;
> +
> + nval = *addr;
> + do {
> + val = nval;
> + if ((val & mask_to_clear) != mask_to_clear)
> + return -EBUSY;
> + } while ((nval = cmpxchg(addr, val, val & ~mask_to_clear)) != val);
> +
> + return 0;
> +}
These are waaaay too big to be inlined. Let the compiler decide.
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