Fundu wrote:
>
>> that book explicitly covers your question. read chapter 2
>> where it
>> covers these irq functions.
> as i said i'm reading the book and actully i did read that chapter.
>
> for me, here's the exact line that needs clarification,
> In chapter 2, page 42 last paragraph(starts with "However, if ..." )
>
> here's the code snippet he's talking about.
> Point A:
> local_irq_disable();
> /* critical section ...*/
> local_irq_enable();
>
>
> Author say, if irg are already disabled at Point A (see snippet above) then local_irq_enable() creates an unpleasant side effect of re-enabling interrupts rather than restoring interrupt state.
>
> 1) first what's the difference between re-enabling and restoring interrupt state.
> 2) so is disable interrupts twice a problem, or just enabling them when after they are diabled (which sounds like how it should be ) a problem.
>
> hope i have made myself clear enough for you to respond.
I think you are confused by the term 'flags'. In this case 'flags' is
not the interrupts that are pending, there are the interrupts that where
enabled before calling local_irq_save().
Does an example help? Consider driver A that calls 'library' code B.
void driver_A()
{
local_irq_disable();
/* do something critical */
library_code_B();
/* do something else critical */
local_irq_enable();
/* NO BUG - if interrupts are not locked when local_irq_disable()
* was called.
*/
}
void library_code_B(void)
{
#if BUGGY_CODE_COMES_FIRST
local_irq_disable();
/* do something critical */
local_irq_enable();
/* BUG HERE - driver A thinks interrupts are still disabled but
* they are not
*/
#else
flags = local_irq_save();
/* do something critical */
local_irq_restore(flags);
/* NO BUG - interrupts are still locked (flags is used to remember
* that interrupts were locked when we called local_irq_save().
*/
#endif
}
In fewer words: use local_irq_save/restore() if you don't know whether
interrupts where locked or not when your function was called.
PS There are only a small number of drivers that should use the
local_irq_... family of functions anyway. Normally you should use
create a spin lock and use spin_lock_irq() and spin_lock_irqsave()
instead.
--
Daniel Thompson (STMicroelectronics) <daniel.thompson@xxxxxx>
1000 Aztec West, Almondsbury, Bristol, BS32 4SQ. 01454 462659
If a car is a horseless carriage then is a motorcycle a horseless horse?
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