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Subject: Re: [PATCH] powerpc/32bit,PREEMPT:Load TI_FLAGS to check NEED_RESCHED
From: Benjamin Herrenschmidt <benh@kernel.crashing.org>
To: Priyanka Jain <Priyanka.Jain@freescale.com>
Date: Mon, 27 May 2013 17:12:55 +1000
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On Mon, 2013-05-27 at 11:57 +0530, Priyanka Jain wrote:
> Add instruction to load TI_FLAGS in r8
>     
> While returning from exception handling in case of PREEMPT enabled,
> _TIF_NEED_RESCHED bit is checked in TI_FLAGS (thread_info flag) of
> current
> task. Only if this bit is set, it should continue with the process of
> calling preempt_schedule_irq() to schedule highest priority task if
> available.
>
> Current code assumes that r8 contains TI_FLAGS and check this for
> _TIF_NEED_RESCHED, but as r8 is modified in the code which executes
> before
> this check, r8 no longer contains the expected TI_FLAGS information.

Hrm, the code was supposed to still have TI_FLAGS in r8 and mostly does
until the 

	andis.	r8,r8,_TIF_EMULATE_STACK_STORE@h

Was added which clobbers it.

Can't we just fix the above to use a different destination register ?
r0 looks fair game at this point...
 
> As a result check for comparison with _TIF_NEED_RESCHED was failing
> even if
> NEED_RESCHED bit is set in the current thread_info flag. Due to this,
> preempt_schedule_irq() and in turn scheduler was not getting called
> even if
> highest priority task is ready for execution.

Ben.