Documentation/cli-sti-removal.txt

#### cli()/sti() removal guide, started by Ingo Molnar <mingo@redhat.com>

as of 2.5.28, four popular macros have been removed on SMP, and
are being phased out on UP:

 cli(), sti(), save_flags(flags), restore_flags(flags)

until now it was possible to protect driver code against interrupt
handlers via a cli(), but from now on other, more lightweight methods
have to be used for synchronization, such as spinlocks or semaphores.

for example, driver code that used to do something like:

	struct driver_data;

	irq_handler (...)
	{
		....
		driver_data.finish = 1;
		driver_data.new_work = 0;
		....
	}

	...

	ioctl_func (...)
	{
		...
		cli();
		...
		driver_data.finish = 0;
		driver_data.new_work = 2;
		...
		sti();
		...
	}

was SMP-correct because the cli() function ensured that no
interrupt handler (amongst them the above irq_handler()) function
would execute while the cli()-ed section is executing.

but from now on a more direct method of locking has to be used:

	spinlock_t driver_lock = SPIN_LOCK_UNLOCKED;
	struct driver_data;

	irq_handler (...)
	{
		unsigned long flags;
		....
		spin_lock_irqsave(&driver_lock, flags);
		....
		driver_data.finish = 1;
		driver_data.new_work = 0;
		....
		spin_unlock_irqrestore(&driver_lock, flags);
		....
	}

	...

	ioctl_func (...)
	{
		...
		spin_lock_irq(&driver_lock);
		...
		driver_data.finish = 0;
		driver_data.new_work = 2;
		...
		spin_unlock_irq(&driver_lock);
		...
	}

the above code has a number of advantages:

- the locking relation is easier to understand - actual lock usage
  pinpoints the critical sections. cli() usage is too opaque.
  Easier to understand means it's easier to debug.

- it's faster, because spinlocks are faster to acquire than the
  potentially heavily-used IRQ lock. Furthermore, your driver does
  not have to wait eg. for a big heavy SCSI interrupt to finish,
  because the driver_lock spinlock is only used by your driver.
  cli() on the other hand was used by many drivers, and extended
  the critical section to the whole IRQ handler function - creating
  serious lock contention.
 

to make the transition easier, we've still kept the cli(), sti(),
save_flags() and restore_flags() macros defined on UP systems - but
their usage will be phased out until the 2.6 kernel is released.

drivers that want to disable local interrupts (interrupts on the
current CPU), can use the following four macros:

  __cli(), __sti(), __save_flags(flags), __restore_flags(flags)

but beware, their meaning and semantics are much simpler, far from
that of cli(), sti(), save_flags(flags) and restore_flags(flags).

another related change is that synchronize_irq() now takes a parameter:
synchronize_irq(irq). This change too has the purpose of making SMP
synchronization more lightweight - this way you can wait for your own
interrupt handler to finish, no need to wait for other IRQ sources.

why were these changes done? The main reason was the architectural burden
of maintaining the cli()/sti() interface - it became a real problem. The
new interrupt system is much more streamlined, easier to understand, debug,
and it's also a bit faster - the same happened to it that will happen to
cli()/sti() using drivers once they convert to spinlocks :-)