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Valentine Barshak
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Josh Boyer
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Aug 5, 2008
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| @@ -1,2 +1,2 @@ | ||
| --- | ||
| refs/heads/master: a7ef6a40f700496c60b8f7206fff74fecd67b3a2 | ||
| refs/heads/master: 1c5402ba55e809f0b685f07728794ea27b197f33 |
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| #### cli()/sti() removal guide, started by Ingo Molnar <mingo@redhat.com> | ||
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| as of 2.5.28, five popular macros have been removed on SMP, and | ||
| are being phased out on UP: | ||
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| cli(), sti(), save_flags(flags), save_flags_cli(flags), restore_flags(flags) | ||
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| 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. | ||
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| for example, driver code that used to do something like: | ||
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| struct driver_data; | ||
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| irq_handler (...) | ||
| { | ||
| .... | ||
| driver_data.finish = 1; | ||
| driver_data.new_work = 0; | ||
| .... | ||
| } | ||
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| ... | ||
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| ioctl_func (...) | ||
| { | ||
| ... | ||
| cli(); | ||
| ... | ||
| driver_data.finish = 0; | ||
| driver_data.new_work = 2; | ||
| ... | ||
| sti(); | ||
| ... | ||
| } | ||
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| 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. | ||
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| but from now on a more direct method of locking has to be used: | ||
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| DEFINE_SPINLOCK(driver_lock); | ||
| struct driver_data; | ||
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| 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); | ||
| .... | ||
| } | ||
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| ... | ||
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| ioctl_func (...) | ||
| { | ||
| ... | ||
| spin_lock_irq(&driver_lock); | ||
| ... | ||
| driver_data.finish = 0; | ||
| driver_data.new_work = 2; | ||
| ... | ||
| spin_unlock_irq(&driver_lock); | ||
| ... | ||
| } | ||
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| the above code has a number of advantages: | ||
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| - 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. | ||
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| - 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. | ||
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| 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. | ||
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| drivers that want to disable local interrupts (interrupts on the | ||
| current CPU), can use the following five macros: | ||
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| local_irq_disable(), local_irq_enable(), local_save_flags(flags), | ||
| local_irq_save(flags), local_irq_restore(flags) | ||
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| 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: | ||
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| local_irq_disable() => turn local IRQs off | ||
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| local_irq_enable() => turn local IRQs on | ||
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| 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.) | ||
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| local_irq_save(flags) => save the current IRQ state into flags and | ||
| disable interrupts. | ||
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| local_irq_restore(flags) => restore the IRQ state from flags. | ||
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| (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.) | ||
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| 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. | ||
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| 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 :-) | ||
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