Hello, On Wed, Nov 29, 2023 at 11:01:04AM -0500, Waiman Long wrote: ... > > > Depending on how the cpumask operators are implemented, we may not have a > > > guarantee that testing CPU 2, for instance, will always return true. That is > > Can you please elaborate this part a bit? I'm having a difficult time > > imagining the sequence of operations where this would matter but that could > > easily be me not being familiar with the details. > > I may be a bit paranoid about incorrect result due to racing as I had been > burned before. Just testing a bit in the bitmask may probably be OK. I don't Setting and clearing a bit is as atomic as it gets, right? > think it will be a problem for x86, but I am less certain about other more > exotic architectures like arm64 or PPC which I am less familiar about. I add > a seqcount for synchronization just for the peace of mind. I can take the > seqcount out if you don't it is necessary. I just can't think of a case where this would be broken. The data being read and written is atomic. There's no way to break a bit operation into multiple pieces. It is possible to write a really bone-headed bitmask operations (like, if you shift the bits into place or sth) to make the bits go through unintended changes but that'd just be a flat-out broken implementation. Even for a bitmask where write accesses are synchronized through a spinlock, we should still be able to use test_bit() without holding the lock. This seems like a pretty basic assumption. Adding unnecessary synchronization confuses the readers. If we don't need it, we shouldn't have it. Thanks. -- tejun
