Re: Preemption in do_cpu (Re: [PATCH]Preemption patch for 2.6)

[Manish Lachwani <mlachwani@mvista.com>]

Jun Sun wrote:

>On Mon, Apr 18, 2005 at 01:32:46PM +0400, Pavel Kiryukhin wrote:
>  
>
>>Hi,
>>the preempt_disable/preempt_enable sequence in do_cpu() [traps.c]
>>exists quite long (patch submitted in Oct. 2004), so it should be nothing
>>wrong there.
>>
>>Can somebody please comment why use of preempt_disable/enable in do_cpu
>>will not result in "scheduling while atomic" for fpu-less cpu (with enabled
>>preemption).
>>
>>The sequence looks like
>>
>>do_cpu()
>>| preempt_disable()
>>| fpu_emulator_cop1Handler()
>>| | cond_reshed()
>>| | | schedule()  <------ scheduling while atomic
>>
>>
>>The proposed patch was tested for Sibyte, but it has fpu (AFAIK) and has no
>>fpu_emulator_cop1Handler called.
>>
>>    
>>
>
>fpu_emulator maintains global variables and in general is dangerous
>to be preempted in the middle of processing.
>
>The quick fix for this problem is probably to move preemption disabling/
>enabling inside fpu_emulator_cop1Handler().
>
>Better fix is probably to modify fpu emulator so that it is preemption
>safe overall.
>
>Jun
>  
>
Missed this one ! I had a patch that enables preemption before the 
cond_resched and disables right after it. I forgot to send it to 
linux-mips though. But, I needed it to work on fpu-less CPU. My bad.

Thanks
Manish Lachwani