Re: [PATCH 0/2] cpustat: use atomic operations to read/update stats

From: Ingo Molnar
Date: Fri Feb 22 2013 - 08:54:22 EST

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Fri, 2013-02-22 at 13:50 +0100, Frederic Weisbecker wrote:

> > atomic64_read() and atomic64_set() are supposed to take care
> > of that, without even the need for _inc() or _add() parts
> > that use LOCK.
> Are you sure? Generally atomic*_set() is not actually an
> atomic operation.

as per Documentation/atomic_ops.h:

#define atomic_read(v) ((v)->counter)

which simply reads the counter value currently visible to the
calling thread. The read is atomic in that the return value is
guaranteed to be one of the values initialized or modified with
the interface operations if a proper implicit or explicit
memory barrier is used after possible runtime initialization by
any other thread and the value is modified only with the
interface operations.


Properly aligned pointers, longs, ints, and chars (and unsigned
equivalents) may be atomically loaded from and stored to in the
same sense as described for atomic_read() and atomic_set().

The ACCESS_ONCE() macro should be used to prevent the compiler
from using optimizations that might otherwise optimize accesses
out of existence on the one hand, or that might create
unsolicited accesses on the other.

This is usually a side effect of M[O]ESI cache coherency
protocols - you can only get a 'split' word access if the word
crosses cache line boundaries (multiples of 32 bytes,


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