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

From: Peter Zijlstra
Date: Mon Sep 13 2021 - 06:24:50 EST


On Mon, Sep 13, 2021 at 11:00:47AM +0800, 王贇 wrote:
>
>
> On 2021/9/10 下午11:38, Peter Zijlstra wrote:
> > On Thu, Sep 09, 2021 at 11:13:21AM +0800, 王贇 wrote:
> >> When running with ftrace function enabled, we observed panic
> >> as below:
> >>
> >> traps: PANIC: double fault, error_code: 0x0
> >> [snip]
> >> RIP: 0010:perf_swevent_get_recursion_context+0x0/0x70
> >> [snip]
> >> Call Trace:
> >> <NMI>
> >> perf_trace_buf_alloc+0x26/0xd0
> >> perf_ftrace_function_call+0x18f/0x2e0
> >> kernelmode_fixup_or_oops+0x5/0x120
> >> __bad_area_nosemaphore+0x1b8/0x280
> >> do_user_addr_fault+0x410/0x920
> >> exc_page_fault+0x92/0x300
> >> asm_exc_page_fault+0x1e/0x30
> >> RIP: 0010:__get_user_nocheck_8+0x6/0x13
> >> perf_callchain_user+0x266/0x2f0
> >> get_perf_callchain+0x194/0x210
> >> perf_callchain+0xa3/0xc0
> >> perf_prepare_sample+0xa5/0xa60
> >> perf_event_output_forward+0x7b/0x1b0
> >> __perf_event_overflow+0x67/0x120
> >> perf_swevent_overflow+0xcb/0x110
> >> perf_swevent_event+0xb0/0xf0
> >> perf_tp_event+0x292/0x410
> >> perf_trace_run_bpf_submit+0x87/0xc0
> >> perf_trace_lock_acquire+0x12b/0x170
> >> lock_acquire+0x1bf/0x2e0
> >> perf_output_begin+0x70/0x4b0
> >> perf_log_throttle+0xe2/0x1a0
> >> perf_event_nmi_handler+0x30/0x50
> >> nmi_handle+0xba/0x2a0
> >> default_do_nmi+0x45/0xf0
> >> exc_nmi+0x155/0x170
> >> end_repeat_nmi+0x16/0x55
> >
> > kernel/events/Makefile has:
> >
> > ifdef CONFIG_FUNCTION_TRACER
> > CFLAGS_REMOVE_core.o = $(CC_FLAGS_FTRACE)
> > endif
> >
> > Which, afaict, should avoid the above, no?
>
> I'm afraid it's not working for this case, the
> start point of tracing is at lock_acquire() which
> is not from 'kernel/events/core', the following PF
> related function are also not from 'core', prevent
> ftrace on 'core' can't prevent this from happen...

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

...
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*

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.