atomic_set & gcc. atomicity question

From: Q?=Kirill KorotaevQ=20?= (kksx_at_mail.ru)
Date: 07/31/03

  • Next message: Tomas Szepe: "Re: module-init-tools don't support gzipped modules." 
    To: linux-kernel@vger.kernel.org
Date:	Thu, 31 Jul 2003 14:00:06 +0400

    Hi!

    Thinking about atomicity of some operations in kernel I've got the following question about atomic_XXX operations.
    atomic_set and atomic_read (on i386+ and some others) are simple write to and read from memory, i.e. they are defined as:
    #define atomic_set(v,i) (((v)->counter) = (i))
    #define atomic_read(v) ((v)->counter)

    If we call atomic_set() with constant 2nd argument it's ok - it's a simple write to var. But what if we do atomic_set(var, var1+var2)?
    Probably, it can happen that compiler will do "var=var1; var+=var2", can't it? If so, atomic_read() can return intermediate value and write won't seem atomic at all. Who guarentees that compiler won't compile it this way? Optimization? gcc developers?

    Kirill

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