At Wed, 07 Mar 2007 11:10:59 +0100,
Benjamin Herrenschmidt wrote:
quoted text
> 
> On Wed, 2007-03-07 at 10:16 +0100, Ingo Molnar wrote:
> > * Tsutomu OWA <tsutomu.owa@toshiba.co.jp> wrote:
> > 
> > > @@ -342,6 +342,7 @@ static int xmon_core(struct pt_regs *reg
> > >  
> > >  	msr = mfmsr();
> > >  	mtmsr(msr & ~MSR_EE);	/* disable interrupts */
> > > +	preempt_disable();
> > 
> > i'm not an xmon expert, but maybe it might make more sense to first 
> > disable preemption, then interrupts - otherwise you could be preempted 
> > right after having disabled these interrupts (and be scheduled to 
> > another CPU, etc.). What is the difference between local_irq_save() and 
> > the above 'disable interrupts' sequence? If it's not the same and 
> > xmon_core() relied on having hardirqs disabled then it might make sense 
> > to do a local_irq_save() there, instead of a preempt_disable().
> 
> powerpc 64 bits nowadays does lazy HW masking, so local_irq_disable()
> will not actually switch MSR_EE off. However, xmon needs that to happen
> (though we have a nicer accessor to do it, I suspect some bitrot need
> fixing in there, possibly already fixed in .21)
> 
> I agree that preempt_disable() should be put before the MSR tweaking
> though.

  As all of you said, I'm resending the patch here.  

  To fix the following runtime warnings when entering xmon.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 
Entering xmon
BUG: using smp_processor_id() in preemptible [00000000] code: khvcd/280
caller is .xmon_core+0xb8/0x8ec
Call Trace:
[C00000000FD737C0] [C00000000000FAAC] .show_stack+0x68/0x1b0 (unreliable)
[C00000000FD73860] [C0000000001F71F0] .debug_smp_processor_id+0xc8/0xf8
[C00000000FD738F0] [C00000000004AF30] .xmon_core+0xb8/0x8ec
[C00000000FD73A80] [C00000000004B918] .xmon+0x38/0x4c
[C00000000FD73C60] [C00000000004BA8C] .sysrq_handle_xmon+0x48/0x5c
[C00000000FD73CD0] [C000000000243A68] .__handle_sysrq+0xe0/0x1b0
[C00000000FD73D70] [C000000000244974] .hvc_poll+0x18c/0x2b4
[C00000000FD73E50] [C000000000244E80] .khvcd+0x88/0x164
[C00000000FD73EE0] [C000000000075014] .kthread+0x124/0x174
[C00000000FD73F90] [C000000000023D48] .kernel_thread+0x4c/0x68
BUG: khvcd:280 task might have lost a preemption check!
Call Trace:
[C00000000FD73740] [C00000000000FAAC] .show_stack+0x68/0x1b0 (unreliable)
[C00000000FD737E0] [C000000000054920] .preempt_enable_no_resched+0x64/0x7c
[C00000000FD73860] [C0000000001F71F8] .debug_smp_processor_id+0xd0/0xf8
[C00000000FD738F0] [C00000000004AF30] .xmon_core+0xb8/0x8ec
[C00000000FD73A80] [C00000000004B918] .xmon+0x38/0x4c
[C00000000FD73C60] [C00000000004BA8C] .sysrq_handle_xmon+0x48/0x5c
[C00000000FD73CD0] [C000000000243A68] .__handle_sysrq+0xe0/0x1b0
[C00000000FD73D70] [C000000000244974] .hvc_poll+0x18c/0x2b4
[C00000000FD73E50] [C000000000244E80] .khvcd+0x88/0x164
[C00000000FD73EE0] [C000000000075014] .kthread+0x124/0x174
[C00000000FD73F90] [C000000000023D48] .kernel_thread+0x4c/0x68
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 

thanks a lot!

Signed-off-by: Tsutomu Owa <tsutomu.owa@toshiba.co.jp>
-- owa

--- linux-rt8/arch/powerpc/xmon/xmon.c	2007-02-20 09:38:52.000000000 +0900
+++ rt/arch/powerpc/xmon/xmon.c	2007-03-07 19:49:38.000000000 +0900
@@ -340,6 +340,7 @@ static int xmon_core(struct pt_regs *reg
 	unsigned long timeout;
 #endif
 
+	preempt_disable();
 	msr = mfmsr();
 	mtmsr(msr & ~MSR_EE);	/* disable interrupts */
 
@@ -517,6 +518,7 @@ static int xmon_core(struct pt_regs *reg
 	insert_cpu_bpts();
 
 	mtmsr(msr);		/* restore interrupt enable */
+	preempt_enable();
 
 	return cmd != 'X' && cmd != EOF;
 }
-
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