linux-kernel - Re: [patch 05/24] perfmon: X86 generic code (x86)

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Message-ID: <7c86c4470811270530j4f3c5413j9e6e68cee08631a9@mail.gmail.com>
Date:	Thu, 27 Nov 2008 14:30:31 +0100
From:	"stephane eranian" <eranian@...glemail.com>
To:	"Andi Kleen" <andi@...stfloor.org>
Cc:	"Thomas Gleixner" <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org,
	mingo@...e.hu, x86@...nel.org, sfr@...b.auug.org.au
Subject: Re: [patch 05/24] perfmon: X86 generic code (x86)

On Thu, Nov 27, 2008 at 1:45 PM, Andi Kleen <andi@...stfloor.org> wrote:
>> What if a threads reprograms the counters while another is reading them?
>
> In the worst case you get an invalid event, which is then discarded.
> I think. I've never tried to understand it in all details, but
> at least it seems to work.
>
>> How is the buffer reset?
>
> drivers/oprofile/cpu_buffer.c:
>
> /* Resets the cpu buffer to a sane state. */
> void cpu_buffer_reset(struct oprofile_cpu_buffer *cpu_buf)
> {
>        /* reset these to invalid values; the next sample
>         * collected will populate the buffer with proper
>         * values to initialize the buffer
>         */
>        cpu_buf->last_is_kernel = -1;
>        cpu_buf->last_task = NULL;
> }
>
What about a thread doing this and another one in the middle of read the buffer?

Or what about a thread trying to reset the buffer while you're processing an PMU
interrupt on another CPU. I know each buffer is per-CPU, but that does not
prevent two threads for trying to operate on it at the same time from
different CPUs.
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