Messages in this thread | | | Subject | Re: [PATCH V5 1/2] base/drivers/arch_topology: Replace mutex with READ_ONCE / WRITE_ONCE | From | Daniel Lezcano <> | Date | Thu, 29 Nov 2018 10:18:29 +0100 |
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On 29/11/2018 08:04, Juri Lelli wrote:
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>> 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 ?
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