Re: [PATCH -next] perf/core: fix unlock balance in perf_init_event

[Peter Zijlstra]

On Wed, Nov 06, 2019 at 12:29:35AM -0500, Qian Cai wrote:
> The -next commit "perf/core: Optimize perf_init_event()" [1] introduced
> an unlock imbalance in perf_init_event() where it calls "goto again" and
> then only repeat rcu_read_unlock().
> 
>   WARNING: bad unlock balance detected!
>   perf_event_open/6185 is trying to release lock (rcu_read_lock) at:
>   [<ffffffffb5eb4039>] perf_event_alloc+0xbb9/0x17f0
>   but there are no more locks to release!
>   other info that might help us debug this:
>   2 locks held by perf_event_open/6185:
>   #0: ffff888526780b50 (&sig->cred_guard_mutex){+.+.}, at: __do_sys_perf_event_open+0x6ee/0x1460
>   #1: ffffffffb866b4e8 (&pmus_srcu){....}, at: perf_event_alloc+0xab8/0x17f0
>   Call Trace:
>    dump_stack+0xa0/0xea
>    print_unlock_imbalance_bug.cold.40+0xb1/0xb6
>    lock_release+0x349/0x4b0
>    perf_event_alloc+0xbcf/0x17f0
>    __do_sys_perf_event_open+0x1e2/0x1460
>    __x64_sys_perf_event_open+0x62/0x70
>    do_syscall_64+0xcc/0xaec
>    entry_SYSCALL_64_after_hwframe+0x49/0xbe
> 
> [1] https://lore.kernel.org/lkml/20191022092307.425783389@xxxxxxxxxxxxx/

You wanted to write:

Fixes: 66d258c5b048 ("perf/core: Optimize perf_init_event()")

instead, right? Fixed that for you.

> Signed-off-by: Qian Cai <cai@xxxxxx>
> ---
>  kernel/events/core.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index cfd89b4a02d8..8226d6ecdb86 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -10307,7 +10307,6 @@ static struct pmu *perf_init_event(struct perf_event *event)
>  			goto unlock;
>  	}
>  
> -	rcu_read_lock();
>  	/*
>  	 * PERF_TYPE_HARDWARE and PERF_TYPE_HW_CACHE
>  	 * are often aliases for PERF_TYPE_RAW.
> @@ -10317,6 +10316,7 @@ static struct pmu *perf_init_event(struct perf_event *event)
>  		type = PERF_TYPE_RAW;
>  
>  again:
> +	rcu_read_lock();
>  	pmu = idr_find(&pmu_idr, type);
>  	rcu_read_unlock();
>  	if (pmu) {
> -- 
> 2.21.0 (Apple Git-122)
>