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From: Ingo Molnar <>
To: Thomas Gleixner <>
Cc: Russell King <>,
	LKML <>,
	Andrew Morton <>
Subject: Re: [PATCH 2.6.10-mm2] Use the new preemption code [2/3] Resend
Date: Mon, 10 Jan 2005 12:02:52 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

* Thomas Gleixner <> 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

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.


  reply	other threads:[~2005-01-10 11:03 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2005-01-10  0:35 [PATCH 2.6.10-mm2] Fix preemption race [1/3] (Core) tglx
2005-01-10  0:53 ` [PATCH 2.6.10-mm2] Use the new preemption code [2/3] Thomas Gleixner
2005-01-10  1:06   ` Russell King
2005-01-10  1:18     ` [PATCH 2.6.10-mm2] Use the new preemption code [2/3] Resend Thomas Gleixner
2005-01-10  7:32       ` Andrew Morton
2005-01-10 10:57         ` Thomas Gleixner
2005-01-10  9:46       ` Russell King
2005-01-10 10:12         ` Thomas Gleixner
2005-01-10 11:02           ` Ingo Molnar [this message]
2005-01-10 13:46             ` Thomas Gleixner
2005-01-10  1:00 ` [PATCH 2.6.10-mm2] Use the new preemption code [3/3] Thomas Gleixner
2005-01-10 14:56   ` Tom Rini
2005-01-10 15:49     ` tglx
2005-01-10  9:15 ` [PATCH 2.6.10-mm2] Fix preemption race [1/3] (Core) Ingo Molnar
2005-01-10 10:14   ` Thomas Gleixner

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