On 6/4/19 2:21 PM, Waiman Long wrote: > On 6/4/19 2:14 PM, Peter Zijlstra wrote: >>> I worked on this owner merging patch mainly to alleviate the need to use >>> cmpxchg for reader lock. cmpxchg_double() is certainly one possible >>> solution though it won't work on older CPUs. We can have a config option >>> to use cmpxchg_double as it may increase the size of other structures >>> that embedded rwsem and impose additional alignment constraint. >> cmpxchg8b was introduced with the Pentium (for PAE IIRC, it enabled >> atomic 64bit PTEs, but Linux never used it for that) and every Intel/AMD >> thereafter has had it. AFAIK there's no x86_64 chip without cmpxchg16b. > Thank for the clarification. I actually didn't check when cmpxch8b was > introduced. I know it is a bit slower than regular cmpxchg. So we may > still need to do some performance analysis to see how it compares with > my current approach.
Using cmpxchg_double is actually more risky than I thought. I have been trying to try to use cmpxchg_double for down_write, but I kept getting kernel panics because the rwsem wasn't 16b-aligned. As rwsem is embedded in quite a large number of structures, they all have to align properly to make that work or the kernel will panic. That does seem too risky to me. So I am dropping the idea of trying to use it. Cheers, Longman
