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

[Peter Zijlstra <peterz@infradead.org>]

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.

> 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.
> 
>