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Subject: Re: [Threads-devel] shared_mutex documentation request
From: Glenn L. Austin (glenn_at_[hidden])
Date: 2012-08-07 10:13:20

On Aug 7, 2012, at 6:23 AM, Fredrik Orderud <forderud_at_[hidden]> wrote:

> I've recently struggled with a deadlock caused by shared_mutex being
> unwilling to allow more readers to enter when a writer is already
> waiting (running on Windows 7/x64). Code to reproduce is pasted below.
> Prioritization of writers might be a perfectly good strategy, but I
> could find no mentioning of it neither in the shared_mutex.hpp headers
> nor in the boost::thread documentation webpage.
> Would it be possible to extend the shared_mutex documentation to also
> cover some implementation details about the reader vs. writer
> prioritization scheme (or similar)?

Much of this "writer blocks readers" comes from issues around multiprocessor design.

I can simply describe it by giving an example of how multi-byte values are usually managed from the hardware level.

Writing a misaligned value (one that isn't on a cache-line or long-aligned boundary) often requires multiple bus operations, which can be interrupted by an intervening read. So, if you have 0x12345678 in a misaligned long (split at the word boundary between 0x1234 and 0x5678) and write 0xFEDCBA98 to that long, an intervening read could actually end up with 0x1234BA98 or 0xFEDC5678 -- neither of which are values that are expected by your code at that time.

So, if your readers don't block on a writer, you may get unexpected results -- or not. Like most threading issues, you might never see this on any machines you test against, but there is a customer out there somewhere who WILL see this issue arise with your code.

Glenn L. Austin, Computer Wizard and Race Car Driver         <><
"Where there's breath, there's hope!"

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