Re: Q: user_enable_single_step() && update_debugctlmsr()

From: Sebastian Andrzej Siewior
Date: Wed Aug 01 2012 - 11:12:15 EST


On 08/01/2012 05:01 PM, Oleg Nesterov wrote:
On 08/01, Sebastian Andrzej Siewior wrote:
So a patch like
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -173,8 +173,8 @@ static void enable_step(struct task_struct *child,
bool block)
unsigned long debugctl = get_debugctlmsr();

debugctl |= DEBUGCTLMSR_BTF;
- update_debugctlmsr(debugctl);
set_tsk_thread_flag(child, TIF_BLOCKSTEP);
+ update_debugctlmsr(debugctl);
} else if (test_tsk_thread_flag(child, TIF_BLOCKSTEP)) {
unsigned long debugctl = get_debugctlmsr();

should fix the race

No, I don't think it can fix something ;) or make any difference.

Why? You _first_ set the task flag followed by the CPU register. Now switch_to() would see the bit set and act.

and _yes_ I also would follow your suggestion to
remove this update_debugctlmsr() here since switch_to() should do this.

Agreed, but once again, uprobes needs it if child == current (but we should
move this code into the trivial helper). If we change (I hope) uprobes to
avoid user_enable_single_step() we will export the helper.

Okay. Looking at TIF_NOTSC I see that it does a preempt_disable() while
playing with the bit. So this would be probably more obvious than
switching the order :)


Oleg.

Sebastian
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