On 12/17/2013 02:27 PM, Linus Torvalds wrote:
On Tue, Dec 17, 2013 at 11:21 AM, Paul E. McKenney
<paul...@linux.vnet.ibm.com> wrote:
Looks like xadd() is x86-specific, but this is common code. One
approach would be to do xadd() for other arches, another approach
would be to make .rw be an atomic_t rather than a u32. Making it
be atomic_t is probably easiest. (The cmpxchg()s would then need
to be atomic_cmpxchg().)
Note that "xadd()" has different semantics from "atomic_add_return()".
xadd() returns the original value, while atomic_add_return() returns
the result of the addition.
In this case, we seem to want the xadd() semantics. I guess we can use
"atomic_add_return(val,&atomic)-val" and just assume that the compiler
gets it right (with the addition and the subtraction cancelling out).
Or maybe we should have a "atomic_add_return_original()" with xadd
semantics?
Linus
I will use the atomic_add_return() for the combined count. Actually what
the code is looking for in the returned value is the state of the writer
byte. So it doesn't really matter if _QR_BIAS has or hasn't been added
to the count.
-Longman
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