[Fwd: Re: [PATCH] Return probe]

[Prasanna S Panchamukhi]

Hi Jim, Hien,

Please see my comments below.

>+/*
>+ * This function is called from do_exit or do_execv when task tk's stack is
>+ * about to be recycled. Recycle any function-return probe instances 
>+ * associated with this task. These represent probed functions that have 
>+ * been called but may never return.
>+ */
>+void kprobe_flush_task(struct task_struct *tk)
>+{
>+	unsigned long flags = 0;
>+	struct kretprobe_instance *ri;
>+	struct task_struct *tsk;
>+	struct hlist_head *head;
>+	struct hlist_node *node;
>+
>+	if (!arch_supports_kretprobes) {
>+		return;
>+	}
>+	spin_lock_irqsave(&kprobe_lock, flags);
>+	head = &kretprobe_inst_table[hash_ptr(tk, RPROBE_HASH_BITS)];
>+	hlist_for_each_entry(ri, node, head, hlist) {
>+		tsk = arch_get_kprobe_task(ri->stack_addr);
>+		if (tsk == tk) {
>+			/* Put the original return address back into stack */
>+			*((unsigned long *)(ri->stack_addr)) = (unsigned long) ri->ret_addr;
>+			hlist_del_rcu(&ri->hlist);
>+			recycle_kretprobe_instance(ri);
>+		}
>+	}
>+	spin_unlock_irqrestore(&kprobe_lock, flags);	
>+}
>+

The current implementation modifies the return address on the stack, hence the 
above routine called for every do_exit, do_execv. How much performance impact
will this cause? 

>+
>+	rp_tmp = kmalloc(sizeof(struct kretprobe), GFP_KERNEL);
>+	BUG_ON(rp_tmp == NULL);
>+	
>+	spin_lock_irqsave(&kprobe_lock, flags);
>+	old_p = get_kprobe(rp->kp.addr);
>+	if (old_p && (old_p->pre_handler == aggr_pre_handler)) {
>+		list_del(&rp->kp.list);
>+		if (list_empty(&old_p->list)) {
>+			remove_kprobe(old_p, flags);
>+			kfree(old_p);
>+		} 
>+	} else if (old_p == &rp->kp) {
>+		remove_kprobe(&rp->kp, flags);
>+	}

The patch by Ananth provides multiple probes feature at a given address and 
handles all the above cases. Can you pls check if the above checks can be
removed and use multiple probes interface.

Thanks
Prasanna

-- 

Prasanna S Panchamukhi
Linux Technology Center
India Software Labs, IBM Bangalore
Ph: 91-80-25044636
<prasanna@in.ibm.com>