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    SubjectRe: [PATCH] make atomic_t volatile on all architectures
    Linus Torvalds wrote:
    > On Wed, 8 Aug 2007, Chris Snook wrote:
    >> Some architectures currently do not declare the contents of an atomic_t to be
    >> volatile. This causes confusion since atomic_read() might not actually read
    >> anything if an optimizing compiler re-uses a value stored in a register, which
    >> can break code that loops until something external changes the value of an
    >> atomic_t.
    > I'd be *much* happier with "atomic_read()" doing the "volatile" instead.
    > The fact is, volatile on data structures is a bug. It's a wart in the C
    > language. It shouldn't be used.
    > Volatile accesses in *code* can be ok, and if we have "atomic_read()"
    > expand to a "*(volatile int *)&(x)->value", then I'd be ok with that.
    > But marking data structures volatile just makes the compiler screw up
    > totally, and makes code for initialization sequences etc much worse.
    > Linus

    Fair enough. Casting to (volatile int *) will give us the behavior people
    expect when using atomic_t without needing to use inefficient barriers.

    While we have the hood up, should we convert all the atomic_t's to non-volatile
    and put volatile casts in all the atomic_reads? I don't know enough about the
    various arches to say with confidence that those changes alone will preserve
    existing behavior. We might need some arch-specific tweaking of the atomic

    -- Chris
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      Last update: 2007-08-09 09:41    [W:0.021 / U:3.708 seconds]
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