On a second thought: Is it correct that atomic_set() without a writebarrier following it is still atomic (no torn writes), however not guaranteed to be visible to other threads? On 26/03/15 17:47, Malte Vesper wrote:
Hello, I have been reading up on atomics and struggle to grasp when exactly I need explicit memory barriers.
While the documentation (Documentat/atomic_ops.txt), talks about operations needing explicit barriers both sides, I assume this is only half true. If I for instance only want to use an atomic as a flag to show that some operation is complete I reckon that a barrier before is sufficient.
Furthermore I found the following lines: 288 <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L28...> If a caller requires memory barrier semantics around an atomic_t 289 <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L28...> operation which does not return a value, a set of interfaces are 290 <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L29...> defined which accomplish this: 291 <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L29...> 292 <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L29...> void smp_mb__before_atomic(void); 293 <http://users.sosdg.org/%7Eqiyong/lxr/source/Documentation/atomic_ops.txt#L29...> void smp_mb__after_atomic(void); note how it refers to "operation which does not return a value", why can't I use these for atomic operations that do return a value? What should I use instead, normal barriers like mb?
Please enlighten me Malte