On Wednesday 06 December 2006 06:10, Benjamin Herrenschmidt wrote:
>
> On UP configuration, there is no real problem: local_irq_disable/enable
> around the code tweaking those bits (read/modify/write) is enough and
> everybody is happy. That's what the current emac code does.
>
> However, on SMP, we need spinlocking (or a semaphore, but for simple
> register accesses like that, spinlocks are probably better).
>
It is actually simple.
You should use
spin_lock_irq_save()
include/linux/spinlock.h
#if defined(CONFIG_SMP) || defined(CONFIG_DEBUG_SPINLOCK)
#define spin_lock_irqsave(lock, flags) flags = _spin_lock_irqsave(lock)
- - -
#else
#define spin_lock_irqsave(lock, flags) _spin_lock_irqsave(lock, flags)
- - -
#endif
spinlock_api_smp.h:
unsigned long __lockfunc _spin_lock_irqsave(spinlock_t *lock)
spinlock_api_up.h:
/*
* In the UP-nondebug case there's no real locking going on, so the
* only thing we have to do is to keep the preempt counts and irq
* flags straight, to supress compiler warnings of unused lock
* variables, and to add the proper checker annotations:
*/
#define _spin_lock_irqsave(lock, flags) __LOCK_IRQSAVE(lock, flags)
#define __LOCK_IRQSAVE(lock, flags) \
do { local_irq_save(flags); __LOCK(lock); } while (0)
#define __LOCK(lock) \
do { preempt_disable(); __acquire(lock); (void)(lock); } while
(0)
-> disable preemption if compiled with preemption
-> static checker annotation
-> make compiler happy, use variable lock
So by using spin_lock_irqsave always you won't get anything more than
local_irq_save when compiling on an UP box with kernel preemption disabled.
You could of cause use spin_lock_irq(lock) if you do not have nested locks -
it is not the lock that is the problem but the unlock.
#define local_irq_disable() __asm__ __volatile__("cli": : :"memory")
#define local_irq_enable() __asm__ __volatile__("sti": : :"memory")
local_irq_disable()
call
local_irq_disable()
local_irq_enable()
do_something(); // Is IRQ enabled or disabled here? Was that intended?
local_irq_enable()
Documentation to read:
linux2.6/Documentation/spinlocks.txt
linux2.6/Documentation/io_ordering.txt
linux2.6/Documentation/preempt-locking.txt
/RogerL
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