On Tue, Dec 17, 2024 at 9:05 PM Daniel Xu <dxu@xxxxxxxxx> wrote: > My understanding is that read_seqbegin() and read_seqretry() > can execute at any time. That means the read side access of > the sequence number can occur doing an increment. > > To prevent the reader from reading a partially written value, > we need the WRITE_ONCE() to ensure the relaxed atomic > write. Hi Daniel, This seems very odd to me. Assuming that the lock field is of an atomic type, no reader will ever see a partial write. By "atomic" I don't mean subject to atomic read-modify-write operations, but simply that the hardware always completes a write to the value before it is made visible to any observers. This should be true for all aligned integral types in all modern architectures. Also, I don't believe WRITE_ONCE() imposes any atomicity where none exists. I'm not a Linux kernel developer, but from what I can tell all it does is wrap a pointer with volatile, to ensure that the compiler does not optimize-away the write. In this case I don't see how the compiler can ignore seq->lock++ between a spin lock and a barrier. --Elad > > Thanks, > Daniel >
