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Subject: Re: [PATCH v2] perf/core: Fix data race between perf_event_set_output
 and perf_mmap_close
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>
From:   Yang Jihong <yangjihong1@huawei.com>
Message-ID: <1e28533a-33ed-cae3-0389-c68e7c52cead@huawei.com>
Date:   Wed, 6 Jul 2022 20:29:32 +0800
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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?

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