Messages in this thread | | | Date | Thu, 29 Nov 2018 10:58:03 +0100 | From | Juri Lelli <> | Subject | Re: [PATCH V5 1/2] base/drivers/arch_topology: Replace mutex with READ_ONCE / WRITE_ONCE |
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On 29/11/18 10:18, Daniel Lezcano wrote: > On 29/11/2018 08:04, Juri Lelli wrote: > > [ ... ] > > >> With or without this patch, it is the case: > >> > >> task1 task2 > >> | | > >> read("/sys/.../cpu1/cpu_capacity) | > >> | write("/sys/.../cpu1/cpu_capacity") > >> read("/sys/.../cpu2/cpu_capacity) | > >> > >> > >> There is no guarantee userspace can have a consistent view of the > >> capacity. As soon as it reads a capacity, it can be changed in its back. > > > > True, but w/o the mutex task1 could read different cpu_capacity values > > for a cluster (it actually can also with current implementation, we > > should grab the mutex in the read path as well if we want to avoid > > this). > > Even if the mutex is on the read path, the userspace can see different > capacities because it will read the cpu_capacity per cpu directory. > > The mutex will be take when reading cpu0/cpu_capacity, not for > cpu[0-9]/cpu_capacity. Between two reads, a write can happen because the > lock is released in between. > > Do you agree with the patch ? Or do you want me to drop it ?
I don't actually have cases at hand that are showing regression with it, I was just trying to understand if we might potentially hit problems in the future. So, I'm not against this patch. :-)
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