Re: Preemption-safe kprobe-booster(Re: [PATCH]kprobe booster for IA64)

[Masami Hiramatsu <masami dot hiramatsu dot pt at hitachi dot com>]

Hi, Anil
Thank you for precise advice.

Keshavamurthy Anil S wrote:
> On Wed, Jul 12, 2006 at 03:22:37PM +0900, Masami Hiramatsu wrote:
>> As far as we know, this function waits until all processes
>> are expired. And any preempted processes can't be expired, only
>> the processes who are scheduled by itself are expired.
>> This means these processes already left from kprobe's slots.
>> So, after that, we can release/reuse these slots safely.
> I am not sure about the behavior of the synchronized_sched() for
> preemptable kernels. When you say this function waits until
> all processes are expired, can this mean that process'es alloted time
> slot expired and hence it is scheduled out. If this is true,
> then it could very well happen that when we finish executing 
> the original instruction in the booster buffer and just before
> jumping back to original code sequence, this process time slot
> can get expired and can be scheduled out and synchronize_sched() 
> returns and you try to reuse the buffer :(
> Please confirm the behavior of synchroniz_sched() before going
> ahead and implementing the code.

I confirmed it and found that synchronize_sched() is an interface
to wait the RCU quiescent state. And the RCU quiescent state means;
- switch processes
- switch to user space
- idle state
As you say, it is not enough to ensure safety of kprobe-booster.

Now, we are searching for other good solutions against this problem.

Anyway, I'd like to disable booster when CONFIG_PREEMPT is defined
until this problem is solved. I attached the patch to this mail.
Please review it.

Thanks,

-- 
Masami HIRAMATSU
2nd Research Dept.
Hitachi, Ltd., Systems Development Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com

 kprobes.c |    9 +++------
 1 file changed, 3 insertions(+), 6 deletions(-)
diff --exclude=CVS -Narup a/arch/i386/kernel/kprobes.c b/arch/i386/kernel/kprobes.c
--- a/arch/i386/kernel/kprobes.c	2006-07-10 12:11:43.000000000 +0900
+++ b/arch/i386/kernel/kprobes.c	2006-07-10 15:41:17.000000000 +0900
@@ -256,11 +256,6 @@ static int __kprobes kprobe_handler(stru
 	int ret = 0;
 	kprobe_opcode_t *addr;
 	struct kprobe_ctlblk *kcb;
-#ifdef CONFIG_PREEMPT
-	unsigned pre_preempt_count = preempt_count();
-#else
-	unsigned pre_preempt_count = 1;
-#endif

 	addr = (kprobe_opcode_t *)(regs->eip - sizeof(kprobe_opcode_t));

@@ -338,13 +333,15 @@ static int __kprobes kprobe_handler(stru
 		return 1;

 ss_probe:
-	if (pre_preempt_count && p->ainsn.boostable == 1 && !p->post_handler){
+#ifdef CONFIG_PREEMPT
+	if (p->ainsn.boostable == 1 && !p->post_handler){
 		/* Boost up -- we can execute copied instructions directly */
 		reset_current_kprobe();
 		regs->eip = (unsigned long)p->ainsn.insn;
 		preempt_enable_no_resched();
 		return 1;
 	}
+#endif
 	prepare_singlestep(p, regs);
 	kcb->kprobe_status = KPROBE_HIT_SS;
 	return 1;