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Diffstat (limited to 'Documentation/cli-sti-removal.txt')
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diff --git a/Documentation/cli-sti-removal.txt b/Documentation/cli-sti-removal.txt deleted file mode 100644 index 60932b02fcb3..000000000000 --- a/Documentation/cli-sti-removal.txt +++ /dev/null @@ -1,133 +0,0 @@ - -#### 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: - - DEFINE_SPINLOCK(driver_lock); - 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 :-) - |