Re: [RFC PATCH] perf: fix panic by mark recursion inside perf_log_throttle

[王贇]

On 2021/9/13 下午6:36, Peter Zijlstra wrote:
> On Mon, Sep 13, 2021 at 12:24:24PM +0200, Peter Zijlstra wrote:
> 
> FWIW:
> 
>> I'm confused tho; where does the #DF come from? Because taking a #PF
>> from NMI should be perfectly fine.
>>
>> AFAICT that callchain is something like:
>>
>> 	NMI
>> 	  perf_event_nmi_handler()
>> 	    (part of the chain is missing here)
>> 	      perf_log_throttle()
>> 	        perf_output_begin() /* events/ring_buffer.c */
>> 		  rcu_read_lock()
>> 		    rcu_lock_acquire()
>> 		      lock_acquire()
>> 		        trace_lock_acquire() --> perf_trace_foo
> 
> This function also calls perf_trace_buf_alloc(), and will have
> incremented the recursion count, such that:
> 
>>
>> 			  ...
>> 			    perf_callchain()
>> 			      perf_callchain_user()
>> 			        #PF (fully expected during a userspace callchain)
>> 				  (some stuff, until the first __fentry)
>> 				    perf_trace_function_call
>> 				      perf_trace_buf_alloc()
>> 				        perf_swevent_get_recursion_context()
>> 					  *BOOM*
> 
> this one, if it wouldn't mysteriously explode, would find recursion and
> terminate, except that seems to be going side-ways.

Yes, it supposed to avoid recursion in the same context, but it never got
chance to do that, the function and struct should all be fine, any idea
in such situation what can trigger this kind of double fault?

Regards,
Michael Wang

> 
>> Now, supposedly we then take another #PF from get_recursion_context() or
>> something, but that doesn't make sense. That should just work...
>>
>> Can you figure out what's going wrong there? going with the RIP, this
>> almost looks like 'swhash->recursion' goes splat, but again that makes
>> no sense, that's a per-cpu variable.
>>
>>