Re: Cached IO Synchronization Question

[Peter Zijlstra]

On Mon, 2009-04-06 at 14:31 -0700, Matt Klein wrote:
> (Please CC me on any responses)
> 
> Hi,
> 
> I am studying 2.6.27.21 to try to understand the IO synchronization
> model used in the Linux kernel.
> 
> I have the following question:
> 
> How are cached reads/writes synchronized? I assume they are, but I can
> not determine how it is done.
> 
> 1) do_generic_file_read calls find_get_page to get the existing page
> cache page if it exists (it uses RCU to prevent the page from being
> blown away while reading takes place). Assuming it does, it performs
> various checks to make sure the page is up to date before performing an
> atomic copy to user mode. The read path does not take the page lock.
> 
> 2) The write path takes both the inode lock and the page lock to prevent
> multiple writers. It also does an atomic copy from user mode into the
> page cache page.
> 
> From my reading of the code, the atomic copies are atomic only on a
> single processor, not across processors. Right?

Not even that ;-)

> If so, what is to prevent one CPU from starting a cached read into a
> user mode buffer while another CPU has partially copied data into the
> page cache page?

Note how do_generic_file_read() checks PageUptodate() and does a
lock_page_killable() in case its not.



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