How does this work?

Are there any x86 programming geeks that now how this down() function works? A colleague and I were puzzling over the "jmp 1b" code below (bolded). How does this function ever break out of this? This is from the file include/asm-i386/semaphore.h in the Linux kernel source.

static inline void down(struct semaphore * sem)
 {
         might_sleep();
         __asm__ __volatile__(
                 "# atomic down operation\n\t"
                 LOCK "decl %0\n\t"     /* --sem->count */
                 "js 2f\n"
                 "1:\n"
                 LOCK_SECTION_START("")
                 "2:\tlea %0,%%eax\n\t"
                 "call __down_failed\n\t"
                 "jmp 1b\n"
                 LOCK_SECTION_END
                 :"=m" (sem->count)
                 :
                 :"memory","ax");
 }

Tags: x86, linux, kernel, asm, assembly+language, semaphore