On Mon, Apr 02, 2018 at 10:03:58AM -0500, Christopher Lameter wrote: > On Sun, 1 Apr 2018, Alan Cox wrote: > > > > Restartable sequences are atomic with respect to preemption > > > (making it atomic with respect to other threads running on the > > > same CPU), as well as signal delivery (user-space execution > > > contexts nested over the same thread). > > > > CPU generally means 'big lump with legs on it'. You are not atomic to the > > same CPU, because that CPU may have 30+ cores with 8 threads per core. > > > > It could do with some better terminology (hardware thread, CPU context ?) > > Well we call it a "CPU" in the scheduler context I think. We could use > better terminology throughout the kernel tools and source. Agreed, it has been "CPU" for "single hardware thread" for a very long time. People tend to use "core" for "group of hardware threads" and "socket" for "big lump with legs on it". > Hardware Execution Context? Should be even more fun when non-CPU hardware execution contexts show up in force within each core. ;-) But the terminology in place for non-CPU hardware execution contexts should be able to survive that event. > > > In a typical usage scenario, the thread registering the rseq > > > structure will be performing loads and stores from/to that > > > structure. It is however also allowed to read that structure > > > from other threads. The rseq field updates performed by the > > > kernel provide relaxed atomicity semantics, which guarantee > > > that other threads performing relaxed atomic reads of the cpu > > > number cache will always observe a consistent value. > > > > So what happens to your API if the kernel atomics get improved ? You are > > effectively exporting rseq behaviour from private to public. > > There is already a pretty complex coherency model guiding kernel atomics. > Improvements/changes to that are difficult and the effect will ripple > throughout the kernel. So I would suggest that these areas of the kernel > are pretty "petrified" (or written in stone). I suspect that there are much more pressing areas of confusion in any case! Thanx, Paul -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
