----- On Jul 16, 2020, at 12:42 AM, Nicholas Piggin npiggin@xxxxxxxxx wrote: > I should be more complete here, especially since I was complaining > about unclear barrier comment :) > > > CPU0 CPU1 > a. user stuff 1. user stuff > b. membarrier() 2. enter kernel > c. smp_mb() 3. smp_mb__after_spinlock(); // in __schedule > d. read rq->curr 4. rq->curr switched to kthread > e. is kthread, skip IPI 5. switch_to kthread > f. return to user 6. rq->curr switched to user thread > g. user stuff 7. switch_to user thread > 8. exit kernel > 9. more user stuff > > What you're really ordering is a, g vs 1, 9 right? > > In other words, 9 must see a if it sees g, g must see 1 if it saw 9, > etc. > > Userspace does not care where the barriers are exactly or what kernel > memory accesses might be being ordered by them, so long as there is a > mb somewhere between a and g, and 1 and 9. Right? This is correct. Note that the accesses to user-space memory can be done either by user-space code or kernel code, it doesn't matter. However, in order to be considered as happening before/after either membarrier or the matching compiler barrier, kernel code needs to have causality relationship with user-space execution, e.g. user-space does a system call, or returns from a system call. In the case of io_uring, submitting a request or returning from waiting on request completion appear to provide this causality relationship. Thanks, Mathieu -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
