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SubjectRe: [PATCH 2.6.10-mm2] Use the new preemption code [2/3] Resend

* Thomas Gleixner <tglx@linutronix.de> wrote:

> > Are you sure ARM suffers from this race condition? It sets preempt count
> > before enabling IRQs and doesn't use preempt_schedule().
>
> There is no race for arm, but using the preempt_schedule() interface
> is the approach which Ingo suggested for common usage, but his x86
> implementation was racy, so I fixed this first before modifying arm to
> use the interface. Ingo pointed out that he will change it to
> preempt_schedule_irq, but I'm not religious about the name.

i wouldnt raise this issue if it was the name only, but there's more to
preempt_schedule_irq() than its name: it gets called with irqs off and
the scheduler returns with irqs off and with a guarantee that there is
no (irq-generated) pending preemption request for this task right now.
I.e. the checks for need_resched can be skipped, and interrupts dont
have to be disabled to do a safe return-to-usermode (as done on some
architectures).

as far as i can see do_preempt_schedule() doesnt have these properties:
what it guarantees is that it avoids the preemption recursion via the
lowlevel code doing the PREEMPT_ACTIVE setting.

lets agree upon a single, common approach. I went for splitting up
preempt_schedule() into two variants: the 'synchronous' one (called
preempt_schedule()) is only called from syscall level and has no
repeat-preemption and hence stack-recursion worries. The 'asynchronous'
one (called preempt_schedule_irq()) is called from asynchronous contexts
(hardirq events) and is fully ready to deal with all the reentrancy
situations that may occur. It's careful about not re-enabling
interrupts, etc.

Ingo
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