Messages in this thread | | | Subject | Re: [PATCH v2] perf/core: Fix data race between perf_event_set_output and perf_mmap_close | From | Yang Jihong <> | 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 > .
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