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    SubjectRe: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    Herbert Xu wrote:
    > On Thu, Aug 16, 2007 at 04:56:21PM +1000, Paul Mackerras wrote:
    >> Note that I said these are the cases _where one might want to allow
    >> caching_, so of course adding volatile doesn't help _these_ cases.
    >> There are of course other cases where one definitely doesn't want to
    >> allow the compiler to cache the value, such as when polling an atomic
    >> variable waiting for another CPU to change it, and from my inspection
    >> so far these cases seem to be the majority.
    > We've been through that already. If it's a busy-wait it
    > should use cpu_relax. If it's scheduling away that already
    > forces the compiler to reread anyway.
    > Do you have an actual example where volatile is needed?
    >> - It matches the normal expectation based on the name "atomic_read"
    >> - It matches the behaviour of the other atomic_* primitives
    > Can't argue since you left out what those expectations
    > or properties are.

    We use atomic_t for data that is concurrently locklessly written and
    read at arbitrary times. My naive expectation as driver author (driver
    maintainer) is that all atomic_t accessors, including atomic_read, (and
    atomic bitops) work with the then current value of the atomic data.

    >> - It avoids bugs in the cases where "volatile" behaviour is required
    > Do you (or anyone else for that matter) have an example of this?

    The only code I somewhat know, the ieee1394 subsystem, was perhaps
    authored and is currently maintained with the expectation that each
    occurrence of atomic_read actually results in a load operation, i.e. is
    not optimized away. This means all atomic_t (bus generation, packet and
    buffer refcounts, and some other state variables)* and likewise all
    atomic bitops in that subsystem.

    If that assumption is wrong, then what is the API or language primitive
    to force a load operation to occur?

    *) Interesting what a quick LXR session in search for all atomic_t
    usages in 'my' subsystem brings to light. I now noticed an apparently
    unused struct member in the bitrotting pcilynx driver, and more
    importantly, a pairing of two atomic_t variables in raw1394 that should
    be audited for race conditions and for possible replacement by plain int.
    Stefan Richter
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      Last update: 2007-08-16 10:09    [W:0.027 / U:2.520 seconds]
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