Re: [3/3] Userspace probes prototype-take2

[Prasanna S Panchamukhi <prasanna at in dot ibm dot com>]

Yanmin,


Please see my comments inline below.

Thanks
Prasanna
> >>
> >>2. This patch works only with PREEMPT config option disabled, to work
> >>in PREEMPT enabled condition handlers must be re-written and must
> >>be seperated out from kernel probes allowing preemption.
> One of my old comments is an external device interrupt might happen when cpu is single-stepping the original instruction, then the task might be switched to another cpu. If we disable irq when exiting to user space to single step the instruction, kernel might switch the task off just on the exit kernel path. 1) uprobe_page; 2) kprobe_ctlblk, These 2 resources shouldn't be pre cpu, or we need get another approach. How could you resolve the task switch issue?

My new design does not use the kprobe handlers and per cpu kprobe data
structures itself  so that task switch issue will be resolved.
We register a separte set of uprobe handlers and use uprobe data structure.
Also now we will be handling uprobes serially and synchronize using some lock/mutex, but later on we have scale it up for better performance.

> >>+static int __kprobes copy_insn_on_new_page(struct uprobe *uprobe ,
> >>+			struct pt_regs *regs, struct vm_area_struct *vma)
> >>+{
> >>+	unsigned long addr, *vaddr, stack_addr = regs->esp;
> >>+	int size = MAX_INSN_SIZE * sizeof(kprobe_opcode_t);
> >>+	struct uprobe_page *upage;
> >>+	struct page *page;
> >>+	pte_t *pte;
> >>+
> >>+
> >>+	if (vma->vm_flags & VM_GROWSDOWN) {
> >>+		if (((stack_addr - sizeof(long long))) < (vma->vm_start + size))
> >>+			return -ENOMEM;
> >>+
> >>+		addr = vma->vm_start;
> >>+	} else if (vma->vm_flags & VM_GROWSUP) {
> >>+		if ((vma->vm_end - size) < (stack_addr + sizeof(long long)))
> >>+			return -ENOMEM;
> >>+
> >>+		addr = vma->vm_end - size;
> >>+	} else
> >>+		return -EFAULT;
> >>+
> The multi-thread case is not resolved here. One of typical multi-thread model is that the all threads share the same vma and every thread has 8-k stack. 

>If 2 threads trigger uprobe (although might be not the same uprobe) at the same time, one thread might erase single-step instruction of another.

Do these threads share the same stack pages?


-- 
Prasanna S Panchamukhi
Linux Technology Center
India Software Labs, IBM Bangalore
Email: prasanna@in.ibm.com
Ph: 91-80-51776329