mirror of
https://github.com/torvalds/linux.git
synced 2024-11-27 06:31:52 +00:00
1da177e4c3
Initial git repository build. I'm not bothering with the full history, even though we have it. We can create a separate "historical" git archive of that later if we want to, and in the meantime it's about 3.2GB when imported into git - space that would just make the early git days unnecessarily complicated, when we don't have a lot of good infrastructure for it. Let it rip!
134 lines
3.9 KiB
Plaintext
134 lines
3.9 KiB
Plaintext
|
|
#### cli()/sti() removal guide, started by Ingo Molnar <mingo@redhat.com>
|
|
|
|
|
|
as of 2.5.28, five popular macros have been removed on SMP, and
|
|
are being phased out on UP:
|
|
|
|
cli(), sti(), save_flags(flags), save_flags_cli(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(), save_flags_cli() and restore_flags() macros defined
|
|
on UP systems - but their usage will be phased out until 2.6 is
|
|
released.
|
|
|
|
drivers that want to disable local interrupts (interrupts on the
|
|
current CPU), can use the following five macros:
|
|
|
|
local_irq_disable(), local_irq_enable(), local_save_flags(flags),
|
|
local_irq_save(flags), local_irq_restore(flags)
|
|
|
|
but beware, their meaning and semantics are much simpler, far from
|
|
that of the old cli(), sti(), save_flags(flags) and restore_flags(flags)
|
|
SMP meaning:
|
|
|
|
local_irq_disable() => turn local IRQs off
|
|
|
|
local_irq_enable() => turn local IRQs on
|
|
|
|
local_save_flags(flags) => save the current IRQ state into flags. The
|
|
state can be on or off. (on some
|
|
architectures there's even more bits in it.)
|
|
|
|
local_irq_save(flags) => save the current IRQ state into flags and
|
|
disable interrupts.
|
|
|
|
local_irq_restore(flags) => restore the IRQ state from flags.
|
|
|
|
(local_irq_save can save both irqs on and irqs off state, and
|
|
local_irq_restore can restore into both irqs on and irqs off state.)
|
|
|
|
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 :-)
|
|
|