On 2/11/2016 2:41 AM, James Morse wrote:
Hi!
On 10/02/16 18:12, Shi, Yang wrote:
On 2/10/2016 4:10 AM, Will Deacon wrote:
On Wed, Feb 10, 2016 at 11:52:31AM +0000, James Morse wrote:
On 10/02/16 10:29, Will Deacon wrote:
On Tue, Feb 09, 2016 at 01:26:22PM -0800, Yang Shi wrote:
dump_backtrace may be called in kthread context, which is not bound to a single cpu, i.e. khungtaskd, then calling smp_processor_id may trigger the below bug report:
If we're preemptible here, it means that our irq_stack_ptr is potentially bogus. Whilst this isn't an issue for kthreads, it does feel like we could make this slightly more robust in the face of potential frame corruption. Maybe just zero the IRQ stack pointer if we're in preemptible context?
Switching between stacks is only valid if we are tracing ourselves while on the irq_stack, we should probably prevent it for other tasks too.
Something like (untested):
if (tsk == current && in_atomic()) irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id());
One follow up question, is it possible to have both tsk != current and on_irq_stack is true at the same time?
No. If you are tracing an irq stack, it must be your own stack.
If this weren't the case, it would be the stack of a running task on a remote CPU, and you would be racing with the remote CPU changing the values you are reading. Fortunately nothing tries to do this.
(The third case would be tracing a sleeping irq stack - this doesn't happen either, as we switch back to the original stack before calling schedule()).
If it is possible, this may be a problem in unwind_frame called by profile_pc which has tsk being NULL.
Ah, well spotted. I guess there should also be a != NULL comparison thrown into the mix. I don't think it will be a problem for profile_pc() as it should always find a !in_lock_functions() frame before it needs to switch stack, (which we are preventing it from doing). If this ever did happen, it will return 0.
Thanks for the elaboration. I changed the logic a little bit to:
if (tsk == current && !preemptible()) irq_stack_ptr = IRQ_STACK_PTR(smp_processor_id()); else irq_stack_ptr = 0;
In this way, the NULL pointer will be covered by "else" too.
v2 patch will be sent out soon once I'm done some smoke testing.
Yang
Thanks,
James