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Subject: Re: [PATCH v2] perf/core: Fix data race between perf_event_set_output
 and perf_mmap_close
From:   Yang Jihong <yangjihong1@huawei.com>
To:     Peter Zijlstra <peterz@infradead.org>
CC:     <mingo@redhat.com>, <acme@kernel.org>, <mark.rutland@arm.com>,
        <alexander.shishkin@linux.intel.com>, <jolsa@kernel.org>,
        <namhyung@kernel.org>, <linux-perf-users@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
References: <20220704120006.98141-1-yangjihong1@huawei.com>
 <YsMGixSL4CDPTTZs@worktop.programming.kicks-ass.net>
 <YsQ3jm2GR38SW7uD@worktop.programming.kicks-ass.net>
 <1e28533a-33ed-cae3-0389-c68e7c52cead@huawei.com>
Message-ID: <3ba262b0-a92f-e57d-af31-baccca765ac3@huawei.com>
Date:   Sat, 9 Jul 2022 10:00:29 +0800
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Hello,

On 2022/7/6 20:29, Yang Jihong wrote:
> Hello,
> 
> On 2022/7/5 21:07, Peter Zijlstra wrote:
>> On Mon, Jul 04, 2022 at 05:26:04PM +0200, Peter Zijlstra wrote:
>>> On Mon, Jul 04, 2022 at 08:00:06PM +0800, Yang Jihong wrote:
>>>> Data race exists between perf_event_set_output and perf_mmap_close.
>>>> The scenario is as follows:
>>>>
>>>>                    
>>>> CPU1                                                       CPU2
>>>>                                                                      
>>>> perf_mmap_close(event2)
>>>>                                                                        
>>>> if (atomic_dec_and_test(&event2->rb->mmap_count)  // mmap_count 1 -> 0
>>>>                                                                          
>>>> detach_rest = true;
>>>> ioctl(event1, PERF_EVENT_IOC_SET_OUTPUT, event2)
>>>>    perf_event_set_output(event1, event2)
>>>>                                                                        
>>>> if (!detach_rest)
>>>>                                                                          
>>>> goto out_put;
>>>>                                                                        
>>>> list_for_each_entry_rcu(event, &event2->rb->event_list, rb_entry)
>>>>                                                                          
>>>> ring_buffer_attach(event, NULL)
>>>>                                                                        
>>>> // because event1 has not been added to event2->rb->event_list,
>>>>                                                                        
>>>> // event1->rb is not set to NULL in these loops
>>>>
>>>>      ring_buffer_attach(event1, event2->rb)
>>>>        list_add_rcu(&event1->rb_entry, &event2->rb->event_list)
>>>>
>>>> The above data race causes a problem, that is, event1->rb is not 
>>>> NULL, but event1->rb->mmap_count is 0.
>>>> If the perf_mmap interface is invoked for the fd of event1, the 
>>>> kernel keeps in the perf_mmap infinite loop:
>>>>
>>>> again:
>>>>          mutex_lock(&event->mmap_mutex);
>>>>          if (event->rb) {
>>>> <SNIP>
>>>>                  if (!atomic_inc_not_zero(&event->rb->mmap_count)) {
>>>>                          /*
>>>>                           * Raced against perf_mmap_close() through
>>>>                           * perf_event_set_output(). Try again, hope 
>>>> for better
>>>>                           * luck.
>>>>                           */
>>>>                          mutex_unlock(&event->mmap_mutex);
>>>>                          goto again;
>>>>                  }
>>>> <SNIP>
>>>
>>> Too tired, must look again tomorrow, little feeback below.
>>
>> With brain more awake I ended up with the below. Does that work?
I have verified that this patch can solve the problem.

Do I submit this patch? Or do you submit it?

Thanks,
Yang

> 
> Yes, I apply the patch on kernel versions 5.10 and mainline,
> and it could fixed the problem.
> 
> Tested-by: Yang Jihong <yangjihong1@huawei.com>
> 
> Thanks,
> Yang
> .