[webkit-dev] Atomic read/write operations and thread safety

Alexey Proskuryakov ap at webkit.org
Wed Dec 15 13:34:45 PST 2010

15.12.2010, в 10:36, Stephan Aßmus написал(а):

> The only thing one needs to watch out for is to declare such a boolean volatile (which I believe this code does, if memory serves). Otherwise the thread which polls the condition may read from a cached location and miss the change. Worst that can happen on a hypothecial architecture where writing a byte is not atomic is that the changed condition takes affect one loop cycle later.

The behavior of volatile depends on the compiler - there is no guarantee that a memory barrier will be emitted. Historically, volatile existed not for multiprocessor systems, but for special addresses in address space (like memory mapped I/O), where it is important to actually access the address each time, and not than keep a local variable in a boolean.

There is code in WebKit that doesn't care about the other thread immediately seeing the change of a boolean value, and just hopes that it will propagate soon enough in practice. I remember implementing worker thread termination in that way. This code is of course formally wrong.

- WBR, Alexey Proskuryakov

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