----- On Jul 16, 2020, at 11:46 AM, Mathieu Desnoyers mathieu.desnoyers@xxxxxxxxxxxx wrote: > ----- 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. Actually, sorry, the above is not quite right. It's been a while since I looked into the details of membarrier. The smp_mb() at the beginning of membarrier() needs to be paired with a smp_mb() _after_ rq->curr is switched back to the user thread, so the memory barrier is between store to rq->curr and following user-space accesses. The smp_mb() at the end of membarrier() needs to be paired with the smp_mb__after_spinlock() at the beginning of schedule, which is between accesses to userspace memory and switching rq->curr to kthread. As to *why* this ordering is needed, I'd have to dig through additional scenarios from https://lwn.net/Articles/573436/. Or maybe Paul remembers ? Thanks, Mathieu > 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 -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
