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Date: Thu, 22 Oct 2015 15:57:45 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Alexei Starovoitov <ast@plumgrid.com>
Cc: "David S. Miller" <davem@davemloft.net>, Ingo Molnar <mingo@kernel.org>,
        Wang Nan <wangnan0@huawei.com>, He Kuang <hekuang@huawei.com>,
        Kaixu Xia <xiakaixu@huawei.com>,
        Daniel Borkmann <daniel@iogearbox.net>, netdev@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 net-next] bpf: fix bpf_perf_event_read() helper
Message-ID: <20151022135745.GM3604@twins.programming.kicks-ass.net>
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On Wed, Oct 21, 2015 at 03:58:03PM -0700, Alexei Starovoitov wrote:
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 64754bfecd70..0b6333265872 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -3258,7 +3258,7 @@ static inline u64 perf_event_count(struct perf_event *event)
>  u64 perf_event_read_local(struct perf_event *event)
>  {
>  	unsigned long flags;
> -	u64 val;
> +	u64 val = -EINVAL;

No, you cannot do this, -EINVAL is a valid count value. You simply must
not call this function on !local events, ever.