Re: [Xen-devel] [PATCH 4/5] xen: use masking operation instead of test_bit for VPF bits

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 05.10.15 at 15:18, <george.dunlap@xxxxxxxxxx> wrote:
> On 02/10/15 05:40, Juergen Gross wrote:
>> Use a bit mask for testing of a set bit instead of test_bit in case no
>> atomic operation is needed, as this will lead to smaller and more
>> effective code.
>> 
>> Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> 
> I'm a bit confused here -- exactly when is an atomic operation needed or
> not needed?  Isn't it the case that we always need to do an atomic read
> if we ever do an atomic write without a lock held?

First of all - what is an atomic read from CPU perspective other than
just a read? Since we talk about individual bits here, we don't care
about the granularity the compiler may convert the read to, and even
if the compiler chose to do multiple reads the result would still be
correct, as only one of those reads can possibly have read the bit
in question.

And then, the old mechanism was in no way "atomic", all it added was
a kind of compiler barrier (due to the cast to volatile). Yet in none of
the cases changed I was able to spot a need for such a barrier.

Jan


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