From: kernelnewbies-bounces@kernelnewbies.org [mailto:kernelnewbies-bounces@kernelnewbies.org] On Behalf Of Malte Vesper Sent: Thursday, March 26, 2015 10:50 AM To: kernelnewbies Subject: Re: Atomics and memory barriers 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 You might want to try reading Documentation/memory-barriers.txt. It does a better job of explaining this stuff than atomic_ops.txt. That being said, none of this stuff is easy to understand. I’ve read both a few times and still don’t quite get all of it. Good luck, Jeff Haran