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    SubjectRe: [PATCH 0/24] make atomic_read() behave consistently across all architectures
    Herbert Xu writes:

    > If you're referring to the code in sk_stream_mem_schedule
    > then it's working as intended. The atomicity guarantees

    You mean it's intended that *sk->sk_prot->memory_pressure can end up
    as 1 when sk->sk_prot->memory_allocated is small (less than
    ->sysctl_mem[0]), or as 0 when ->memory_allocated is large (greater
    than ->sysctl_mem[2])? Because that's the effect of the current code.
    If so I wonder why you bother computing it.

    > that the atomic_add/atomic_sub won't be seen in parts by
    > other readers.
    >
    > We certainly do not need to see other atomic_add/atomic_sub
    > operations immediately.
    >
    > If you're referring to another code snippet please cite.
    >
    > > I'd go so far as to say that anywhere where you want a non-"volatile"
    > > atomic_read, either your code is buggy, or else an int would work just
    > > as well.
    >
    > An int won't work here because += and -= do not have the
    > atomicity guarantees that atomic_add/atomic_sub do. In
    > particular, this may cause an atomic_read on another CPU
    > to give a bogus reading.

    The point is that guaranteeing the atomicity of the increment or
    decrement does not suffice to make the code race-free. In this case
    the race arises from the fact that reading ->memory_allocated and
    setting *->memory_pressure are separate operations. To make that code
    work properly you need a lock. And once you have the lock an ordinary
    int would suffice for ->memory_allocated.

    Paul.
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