Hi, On Mon, Sep 19, 2011 at 8:46 AM, K. Macy <km...@freebsd.org> wrote: > If the value lags next by one then it is ours. This rule applies to > all callers so the rule holds consistently. > I think you do not understand what I mean, which is that the following:
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(); at runtime, can be seen, memory-wise as: while (br->br_prod_tail != prod_head) cpu_spinwait(); br->br_prod_tail = prod_next; br->br_prod_bufs++; br->br_prod_bytes += nbytes; critical_exit(); That is, there is no memory barrier to enforce completion of the load/increment/store/load/load/addition/store operations before updating what other thread spin on. Yes, `br_prod_tail' is marked `volatile', but there is no guarantee that it will not be re-ordered wrt. non-volatile write (to `br_prod_bufs' and `br_prod_bytes'). - Arnaud > On Mon, Sep 19, 2011 at 5:53 AM, Arnaud Lacombe <lacom...@gmail.com> wrote: >> 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"