Messages in this thread | | | Date | Fri, 23 Dec 2011 09:13:57 -0600 (CST) | From | Christoph Lameter <> | Subject | Re: irqsafe_cpu_...() 64-bit operations on 32-bit platform |
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On Fri, 23 Dec 2011, Jan Beulich wrote:
> you introduced these for 2.6.33, but I wonder about their completeness
> in the current state: On ix86, I can add, sub, xor, etc a 64-bit variable > this way (through the generic implementation), but there's no way to > read or write such a variable. While not documented explicitly, it seems > like the assumption is that this_cpu_{read,write}() can be used for this > purpose, but that's clearly wrong.
Hmmm... We could add those but then you would usually not assign the initial values in a situation where concurrency is an issue. Typically you would have a loop over all possible cpus and then simply do a 64 bit assign to the 64 bit values address determined via per_cpu_ptr().
And yes reading could be an issue if you have concurrent updates ongoing.
However, if you are using a loop to sum up all counters from every processor then you cannot use the this_cpu ops but would have to use something like get_64(). Reliably reading a 64 bit value on 32 bit may strangely require a write operation via cmpxchg8b.
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