Re: What does atomic_read actually do?

From: Brian Gerst
Date: Sat Dec 18 2004 - 15:49:04 EST

Joseph Seigh wrote:
It doesn't do anything that would actually guarantee that the fetch from
memory would be atomic as far as I can see, at least in the x86 version.
The C standard has nothing to say about atomicity w.r.t. multithreading or
multiprocessing. Is this a gcc compiler thing? If so, does gcc guarantee
that it will fetch aligned ints with a single instruction on all platforms
or just x86? And what's with volatile since if the C standard implies
nothing about multithreading then it follows that volatile has no meaning
with respect to multithreading either? Also a gcc thing? Are volatile
semantics well defined enough that you can use it to make the compiler
synchronize memory state as far as it is concerned?

Joe Seigh

For x86, the processor guarantees atomicity for simple aligned reads or writes. Read-modify-write instructions need a lock prefix in order to become atomic. The volatile is there so gcc doesn't miss the value changing from within an interrupt.

Brian Gerst
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