New rwsem

[vik]

Next is the rwsem.

static inline void read_lock(unsigned long *rw)
{
  asm volatile("lock xorl ebp, %0\n"
    : "+m" (*rw)
    :
    : "memory", "cc");
}
/* BUG! There should be the write queue. It the one per rwlock */
static inline void read_unlock(unsigned long *rw)
{
  asm volatile("lock xorl ebp, %0\n\t"
    "jne 1f\n\t"
    "call wakeup_writers\n"
    "1:"
    : "+m" (*rw)
    :
    : "memory", "cc");
}
/* It is need another writelocking for interrupt context.
   This should call yield to wait for the read completion */
static inline void write_lock(unsigned long *rw)
{
  asm volatile("1:"
    "lock incl %0\n\t"
    "lock decl %0\n\t"
    "je 2f\n\t"
    "call yield\n\t"
    "jmp 1b\n"
    "2:"
    : "+m" (*rw)
    :
    : "memory", "cc");
}
/* This empty so reliable writelocking is impossible.
   The cure is fault tolerance */
static inline void write_unlock(unsigned long *rw)
{
}

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