linux-kernel - Re: [PATCH 1/3] perf events: fix WARN_ON

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Date:	Fri, 18 Feb 2011 07:33:28 -0700
From:	David Ahern <daahern@...co.com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	mingo@...e.hu, acme@...stprotocols.net, paulus@...ba.org
Subject: Re: [PATCH 1/3] perf events: fix WARN_ON_ONCE for 64-bit raw data,
 SW events



On 02/18/11 04:00, Peter Zijlstra wrote:
> On Thu, 2011-02-17 at 22:53 -0700, David Ahern wrote:
>> This check does not work out for 64-bit counter:
>>
>>     sizeof(u32) + sizeof(u64) = 4 + 8 = 12
>>
>> which does not pass the WARN_ON_ONCE test. I'm guessing (hoping
>> really) that the intent is that the size of the raw data is a
>> multiple of 4 bytes, not 8.
> 
> No the data must come in 8bytes strides.
> 

Then the sizeof the size field needs to be moved to u64; right now it is
u32:

struct perf_raw_record {
    u32             size;
    void                *data;
};

So size should be changed to u64 rather than modifying the WARN_ON_ONCE?


Another gotcha is in perf_output_sample there is:
            struct {
                u32 size;
                u32 data;
            } raw = {
                .size = sizeof(u32),
                .data = 0,
            };

While that meets the 8-byte stride I was concerned that changing the
size field to u64 breaks ABI.

David
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