include/linux/percpu.h

   1 #ifndef __LINUX_PERCPU_H
   2 #define __LINUX_PERCPU_H
   3 #include <linux/spinlock.h> /* For preempt_disable() */
   4 #include <linux/slab.h> /* For kmalloc() */
   5 #include <linux/smp.h>
   6 #include <linux/string.h> /* For memset() */
   7 #include <asm/percpu.h>
   8
   9 /* Enough to cover all DEFINE_PER_CPUs in kernel, including modules. */
  10 #ifndef PERCPU_ENOUGH_ROOM
  11 #define PERCPU_ENOUGH_ROOM 32768
  12 #endif
  13
  14 /* Must be an lvalue. */
  15 #define get_cpu_var(var) (*({ preempt_disable(); &__get_cpu_var(var); }))
  16 #define put_cpu_var(var) preempt_enable()
  17
  18 #ifdef CONFIG_SMP
  19
  20 struct percpu_data {
  21         void *ptrs[NR_CPUS];
  22         void *blkp;
  23 };
  24
  25 /*
  26  * Use this to get to a cpu's version of the per-cpu object allocated using
  27  * alloc_percpu.  If you want to get "this cpu's version", maybe you want
  28  * to use get_cpu_ptr...
  29  */
  30 #define per_cpu_ptr(ptr, cpu)                   \
  31 ({                                              \
  32         struct percpu_data *__p = (struct percpu_data *)~(unsigned long)(ptr); \
  33         (__typeof__(ptr))__p->ptrs[(cpu)];      \
  34 })
  35
  36 extern void *__alloc_percpu(size_t size, size_t align);
  37 extern void free_percpu(const void *);
  38
  39 #else /* CONFIG_SMP */
  40
  41 #define per_cpu_ptr(ptr, cpu) (ptr)
  42
  43 static inline void *__alloc_percpu(size_t size, size_t align)
  44 {
  45         void *ret = kmalloc(size, GFP_KERNEL);
  46         if (ret)
  47                 memset(ret, 0, size);
  48         return ret;
  49 }
  50 static inline void free_percpu(const void *ptr)
  51 {
  52         kfree(ptr);
  53 }
  54
  55 #endif /* CONFIG_SMP */
  56
  57 /* Simple wrapper for the common case: zeros memory. */
  58 #define alloc_percpu(type) \
  59         ((type *)(__alloc_percpu(sizeof(type), __alignof__(type))))
  60
  61 /*
  62  * Use these with alloc_percpu. If
  63  * 1. You want to operate on memory allocated by alloc_percpu (dereference
  64  *    and read/modify/write)  AND
  65  * 2. You want "this cpu's version" of the object AND
  66  * 3. You want to do this safely since:
  67  *    a. On multiprocessors, you don't want to switch between cpus after
  68  *    you've read the current processor id due to preemption -- this would
  69  *    take away the implicit  advantage to not have any kind of traditional
  70  *    serialization for per-cpu data
  71  *    b. On uniprocessors, you don't want another kernel thread messing
  72  *    up with the same per-cpu data due to preemption
  73  *
  74  * So, Use get_cpu_ptr to disable preemption and get pointer to the
  75  * local cpu version of the per-cpu object. Use put_cpu_ptr to enable
  76  * preemption.  Operations on per-cpu data between get_ and put_ is
  77  * then considered to be safe. And ofcourse, "Thou shalt not sleep between
  78  * get_cpu_ptr and put_cpu_ptr"
  79  */
  80 #define get_cpu_ptr(ptr) per_cpu_ptr(ptr, get_cpu())
  81 #define put_cpu_ptr(ptr) put_cpu()
  82
  83 #endif /* __LINUX_PERCPU_H */