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Date: Tue, 29 Mar 2016 14:55:31 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Wang Nan <wangnan0@huawei.com>
Cc: Alexei Starovoitov <ast@kernel.org>,
        Arnaldo Carvalho de Melo <acme@redhat.com>,
        linux-kernel@vger.kernel.org,
        Brendan Gregg <brendan.d.gregg@gmail.com>,
        He Kuang <hekuang@huawei.com>, Jiri Olsa <jolsa@kernel.org>,
        Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>,
        Namhyung Kim <namhyung@kernel.org>, pi3orama@163.com,
        Zefan Li <lizefan@huawei.com>
Subject: Re: [PATCH 1/4] perf core: Introduce new ioctl options to pause and
 resume ring buffer
Message-ID: <20160329125531.GA11035@twins.programming.kicks-ass.net>
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On Tue, Mar 29, 2016 at 02:54:23PM +0200, Peter Zijlstra wrote:
> +++ b/kernel/events/core.c
> @@ -4346,7 +4346,7 @@ static long _perf_ioctl(struct perf_even
>  
>  		rcu_read_lock();
>  		rb = rcu_dereference(event->rb);
> -		if (!event->rb) {
> +		if (!event->rb || !event->nr_pages) {
>  			rcu_read_unlock();
>  			return -EINVAL;
>  		}

Clearly that should've been:

--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -4346,7 +4346,7 @@ static long _perf_ioctl(struct perf_even
 
 		rcu_read_lock();
 		rb = rcu_dereference(event->rb);
-		if (!event->rb || !event->nr_pages) {
+		if (!rb || !rb->nr_pages) {
 			rcu_read_unlock();
 			return -EINVAL;
 		}