[PATCH] Don't disable preemption in exception handlers without IST

[Andi Kleen]

Some of the exception handlers that run on an IST in a normal kernel
still disable preemption. This causes might_sleep warning when sending signals
for debugging in PREEMPT-RT because sending signals can take a lock.
Since the ISTs are disabled now for those don't disable the preemption.

This completes the remove IST patch I sent some time ago and fixes
another case where using gdb caused warnings.

Also it will likely improve latency a little bit.

Signed-off-by: Andi Kleen <ak@xxxxxxx>


Thomas, I think this patch is worth adding, unless you have a better
work around.


-- Steve



> 
> Forward-ported from v2.6.33.9-rt31 to v3.2-rc2-rt3
> Signed-off-by: John Kacur <jkacur@xxxxxxxxxx>
> ---
>  arch/x86/kernel/traps.c |   24 +++++++++++++-----------
>  1 files changed, 13 insertions(+), 11 deletions(-)
> 
> diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
> index a8e3eb8..3654390 100644
> --- a/arch/x86/kernel/traps.c
> +++ b/arch/x86/kernel/traps.c
> @@ -87,9 +87,10 @@ static inline void conditional_sti(struct pt_regs *regs)
>  		local_irq_enable();
>  }
>  
> -static inline void preempt_conditional_sti(struct pt_regs *regs)
> +static inline void preempt_conditional_sti(struct pt_regs *regs, int stack)
>  {
> -	inc_preempt_count();
> +	if (stack)
> +		inc_preempt_count();
>  	if (regs->flags & X86_EFLAGS_IF)
>  		local_irq_enable();
>  }
> @@ -100,11 +101,12 @@ static inline void conditional_cli(struct pt_regs *regs)
>  		local_irq_disable();
>  }
>  
> -static inline void preempt_conditional_cli(struct pt_regs *regs)
> +static inline void preempt_conditional_cli(struct pt_regs *regs, int stack)
>  {
>  	if (regs->flags & X86_EFLAGS_IF)
>  		local_irq_disable();
> -	dec_preempt_count();
> +	if (stack)
> +		dec_preempt_count();
>  }
>  
>  static void __kprobes
> @@ -222,9 +224,9 @@ dotraplinkage void do_stack_segment(struct pt_regs *regs, long error_code)
>  	if (notify_die(DIE_TRAP, "stack segment", regs, error_code,
>  			12, SIGBUS) == NOTIFY_STOP)
>  		return;
> -	preempt_conditional_sti(regs);
> +	preempt_conditional_sti(regs, STACKFAULT_STACK);
>  	do_trap(12, SIGBUS, "stack segment", regs, error_code, NULL);
> -	preempt_conditional_cli(regs);
> +	preempt_conditional_cli(regs, STACKFAULT_STACK);
>  }
>  
>  dotraplinkage void do_double_fault(struct pt_regs *regs, long error_code)
> @@ -316,9 +318,9 @@ dotraplinkage void __kprobes do_int3(struct pt_regs *regs, long error_code)
>  		return;
>  #endif
>  
> -	preempt_conditional_sti(regs);
> +	preempt_conditional_sti(regs, DEBUG_STACK);
>  	do_trap(3, SIGTRAP, "int3", regs, error_code, NULL);
> -	preempt_conditional_cli(regs);
> +	preempt_conditional_cli(regs, DEBUG_STACK);
>  }
>  
>  #ifdef CONFIG_X86_64
> @@ -412,12 +414,12 @@ dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code)
>  		return;
>  
>  	/* It's safe to allow irq's after DR6 has been saved */
> -	preempt_conditional_sti(regs);
> +	preempt_conditional_sti(regs, DEBUG_STACK);
>  
>  	if (regs->flags & X86_VM_MASK) {
>  		handle_vm86_trap((struct kernel_vm86_regs *) regs,
>  				error_code, 1);
> -		preempt_conditional_cli(regs);
> +		preempt_conditional_cli(regs, DEBUG_STACK);
>  		return;
>  	}
>  
> @@ -436,7 +438,7 @@ dotraplinkage void __kprobes do_debug(struct pt_regs *regs, long error_code)
>  	si_code = get_si_code(tsk->thread.debugreg6);
>  	if (tsk->thread.debugreg6 & (DR_STEP | DR_TRAP_BITS) || user_icebp)
>  		send_sigtrap(tsk, regs, error_code, si_code);
> -	preempt_conditional_cli(regs);
> +	preempt_conditional_cli(regs, DEBUG_STACK);
>  
>  	return;
>  }


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