The code you are looking into is under the #else section of
#ifdef CONFIG_SMP. The actual implementation of
raw_spin_lock is under asm/spinlock.h
For e.g in MIPS asm-mips/spinlock.h has the following code
static inline void _raw_spin_lock(spinlock_t *lock)
{
unsigned int tmp;
if (R10000_LLSC_WAR) {
__asm__ __volatile__(
" .set noreorder # _raw_spin_lock \n"
"1: ll %1, %2 \n"
" bnez %1, 1b \n"
" li %1, 1 \n"
" sc %1, %0 \n"
" beqzl %1, 1b \n"
" nop \n"
" sync \n"
" .set reorder \n"
: "=m" (lock->lock), "=&r" (tmp)
: "m" (lock->lock)
: "memory");
} else {
__asm__ __volatile__(
" .set noreorder # _raw_spin_lock \n"
"1: ll %1, %2 \n"
" bnez %1, 1b \n"
" li %1, 1 \n"
" sc %1, %0 \n"
" beqz %1, 1b \n"
" sync \n"
" .set reorder \n"
: "=m" (lock->lock), "=&r" (tmp)
: "m" (lock->lock)
: "memory");
}
}
The ll/sc instructions are used for atomic operations.
Sadik.
On Thu, 03 Mar 2005 16:29:03 -0800, Om <omanakuttan@xxxxxxx> wrote:
> I am looking into spin lock structure and locking mechamism as defined
> in inclide/linux/spinlock.h and asm/spinlock.h
> typedef struct {
> unsigned long magic;
> volatile unsigned long lock;
> volatile unsigned int babble;
> const char *module;
> char *owner;
> int oline;
> } spinlock_t;
>
> I find all locking/unlocking mechanism is nothing but just decrementing
> / incrementing operations. Of course they are done with local interrupts
> disabled.
>
> #define _spin_lock_irqsave(lock, flags) \
> do { \
> local_irq_save(flags); \
> preempt_disable(); \
> _raw_spin_lock(lock); \
> } while (0)
>
> _raw_spin_lock() is,
>
> #define _raw_spin_lock(x) \
> do { \
> CHECK_LOCK(x); \
> if ((x)->lock&&(x)->babble) { \
> (x)->babble--; \
> printk("%s:%d: spin_lock(%s:%p) already locked by %s/%d\n", \
> __FILE__,__LINE__, (x)->module, \
> (x), (x)->owner, (x)->oline); \
> } \
> (x)->lock = 1; \
> (x)->owner = __FILE__; \
> (x)->oline = __LINE__; \
> } while (0)
>
> In an smp environment, what prevents 2 different cpu's from aquiring
> same spin lock at exactly the same time? I was expecting `lock` to be
> atomic, and the inc and dec operations were also atomic operations.
>
> Regards,
> Om.
>
> --
> Kernelnewbies: Help each other learn about the Linux kernel.
> Archive: http://mail.nl.linux.org/kernelnewbies/
> FAQ: http://kernelnewbies.org/faq/
>
>
--
Kernelnewbies: Help each other learn about the Linux kernel.
Archive: http://mail.nl.linux.org/kernelnewbies/
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