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

["Zhang, Yanmin" <yanmin dot zhang at intel dot com>]

>>-----Original Message-----
>>From: Prasanna S Panchamukhi [mailto:prasanna@in.ibm.com]
>>Sent: 2006年2月20日 12:52
>>To: Zhang, Yanmin
>>Cc: systemtap@sources.redhat.com
>>Subject: Re: [3/3] Userspace probes prototype-take2
>>
>>Yanmin,
>>
>>On Mon, Feb 20, 2006 at 11:15:31AM +0800, Zhang, Yanmin wrote:
>>> I lost an important comment. The patch is not aware of signal processing. After kernel prepares the single-step-inst on the stack,
>>if a signal is delivered to the thread, kernel will save some states into stack and switch to signal handler function, so single-step-inst
>>on the stack might be erased.
>>
>>AFAIK this problem can be addressed in the following ways.
>>
>>1. Leave the sufficient stack space for the kernel to deliver the
>>signals and then copy the instructions on the stack.
[YM] signal action handler itself could be nested again, so it looks not a good approach.


>>
>>2. Synchronize usage of stack between signal processing and user space probes.
[YM] This approach looks not good. Another issue if doing so is that the single-step-insn itself might change esp.


>>
>>3. Block the signal processing by disabling interrupts and preemption from
>>the time we copy the instruction on the stack untill we single step on the
>>original instruction. Or even wait for the signal processing to be
>>complete and then setup the stack for single stepping on the original
>>instructions and single step.
[YM] We could do not check signal when exiting from kernel if trying to single-step. It's easy to be implemented, but community might argue. I prefer this one. For example, add a new flag, TIF_UPROBING, at thread_info->flags. And check it at appropriate time.


>>
>>Your suggestion are welcome to provide better solutions to this problem.
>>
>>Thanks
>>Prasanna