Date: Sat, 27 Aug 2022 00:13:30 UTC
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=265974 --- Comment #8 from Mark Johnston <markj@FreeBSD.org> --- (In reply to Pierre Habouzit from comment #6) I'm sorry, but I still don't quite see the problem. I spent some time trying to write litmus tests to explore interactions between readers, writers, and smr_poll(), but didn't find anything surprising (yet). For now I'm ignoring the deferred and lazy modes of SMR since they're not used in FreeBSD currently. I note that your litmus tests don't seem to take into account the fact that smr_advance() issues a atomic_thread_fence_rel() before updating the write sequence. (On arm64 this expands to DMB SY, the same as atomic_thread_fence_seq_cst().) With smr_enter() patched, both readers and writers issue a full memory barrier on arm64. What I don't really understand is why readers need to synchronize with smr_poll() at all. Even with the barriers you proposed, it's possible for a reader to store a snapshot of wr_seq that is older than the global rd_seq. This could happen if an interrupt or vmexit occurs between the load of wr_seq and the store to per-CPU memory. So long as smr_poll() does not advance rd_seq beyond that of any CPU actively executing a read section, it's ok, and I don't see how that can happen. What am I missing? Thanks for your patience and for taking the time to report these problems. I posted a first patch for review here: https://reviews.freebsd.org/D36370 -- You are receiving this mail because: You are the assignee for the bug.