Hi, On Fri, Sep 16, 2011 at 10:41 AM, K. Macy <km...@freebsd.org> wrote: > On Fri, Sep 16, 2011 at 3:02 AM, Arnaud Lacombe <lacom...@gmail.com> wrote: >> Hi, >> >> On Wed, Sep 14, 2011 at 10:53 PM, Arnaud Lacombe <lacom...@gmail.com> wrote: >>> Hi Kip, >>> >>> I've got a few question about the buf_ring(9) API. >>> >>> 1) what means the 'drbr_' prefix. I can guess the two last letter, 'b' >>> and 'r', for Buffer Ring, but what about 'd' and 'r' ? >>> >>> 2) in `sys/sys/buf_ring.h', you defined 'struct buf_ring' as: >>> >>> struct buf_ring { >>> volatile uint32_t br_prod_head; >>> volatile uint32_t br_prod_tail; >>> int br_prod_size; >>> int br_prod_mask; >>> uint64_t br_drops; >>> uint64_t br_prod_bufs; >>> uint64_t br_prod_bytes; >> shouldn't those 3 fields be updated atomically, especially on 32bits >> platforms ? That might pose a problem as, AFAIK, FreeBSD do not have >> MI 64bits atomics operations... > > Between the point at which br_prod_tail == prod_head and when we > update br_prod_tail to point to prod_next we are the exclusive owners > of the fields in buf_ring. That is why we wait for any other > enqueueing threads to update br_prod_tail to point to prod_head before > continuing. > How do you enforce ordering ? I do not see anything particular forbidding the `br->br_prod_tail' to be committed first, leading other thread to believe they have access to the statistics, while the other thread has not yet committed its change.
Thanks, - Arnaud > Cheers > > /* > * If there are other enqueues in progress > * that preceeded us, we need to wait for them > * to complete > */ > while (br->br_prod_tail != prod_head) > cpu_spinwait(); > br->br_prod_bufs++; > br->br_prod_bytes += nbytes; > br->br_prod_tail = prod_next; > critical_exit(); > _______________________________________________ freebsd-hackers@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-hackers To unsubscribe, send any mail to "freebsd-hackers-unsubscr...@freebsd.org"