linux-kernel - Re: [PATCH] ring_buffer: reset write when reserve buffer fail

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Message-ID: <alpine.DEB.1.10.0901141259510.15549@gandalf.stny.rr.com>
Date:	Wed, 14 Jan 2009 13:19:40 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Lai Jiangshan <laijs@...fujitsu.com>
cc:	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] ring_buffer: reset write when reserve buffer fail


Hi Lai,

Sorry for the late response, I've got a million things going on at once, 
and this needed more thought than a quick review.

On Mon, 12 Jan 2009, Lai Jiangshan wrote:

> 
> Impact: reset struct buffer_page.write when interrupt storm
> 
> if struct buffer_page.write is not reset, any succedent committing
> will corrupted ring_buffer:
> 
> static inline void
> rb_set_commit_to_write(struct ring_buffer_per_cpu *cpu_buffer)
> {
> 	......
> 		cpu_buffer->commit_page->commit =
> 			cpu_buffer->commit_page->write;
> 	......
> }
> 
> 
> when "if (RB_WARN_ON(cpu_buffer, next_page == reader_page))", ring_buffer
> is disabled, but some reserved buffers may haven't been committed.
> we need reset struct buffer_page.write.

So what you are saying is, when we hit this anomaly, another cpu might be 
writing to the ring buffer, and will soon do a commit.

> 
> when "if (unlikely(next_page == cpu_buffer->commit_page))", ring_buffer
> is still available, we should not corrupt it.

OK, makes sense. I'll queue it up.

Thanks!

-- Steve
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