On 2019-08-25, John Ogness <john.ogn...@linutronix.de> wrote: >>>>>> --- /dev/null >>>>>> +++ b/kernel/printk/ringbuffer.c >>>>>> +static bool assign_desc(struct prb_reserved_entry *e) >>>>>> +{ [...] >>>>>> + atomic_long_set_release(&d->id, atomic_long_read(&d->id) + >>>>>> + DESCS_COUNT(rb)); >>>>> >>>>> atomic_long_set_release() might be a bit confusing here. >>>>> There is no related acquire. >>> >>> As the comment states, this release is for prb_getdesc() users. The >>> only prb_getdesc() user is _dataring_pop(). (i.e. the descriptor's >>> ID is not what _dataring_pop() was expecting), then the tail must >>> have moved and _dataring_pop() needs to see that. Since there are no >>> data dependencies between descriptor ID and tail_pos, an explicit >>> memory barrier is used. More on this below. > >> + The two related barriers are in different source files >> and APIs: >> >> + assign_desc() in ringbuffer.c; ringbuffer API >> + _dataring_pop in dataring.c; dataring API > > Agreed. This is a consequence of the ID management being within the > high-level ringbuffer code. I could have added an smp_rmb() to the > NULL case in prb_getdesc(). Then both barriers would be in the same > file. However, this would mean smp_rmb() is called many times > (particularly by readers) when it is not necessary.
What I wrote here is wrong. prb_getdesc() is not called "many times (particularly by readers)". It is only called once within the writer function _dataring_pop(). Looking at this again, I think it would be better to move the smp_rmb() into the NULL case of prb_getdesc(). Then both barrier pairs are located (and documented) in the same file. This also simplifies the documentation by not saying "the caller's smp_rmb() everywhere". I would also change _dataring_pop() so that the smp_rmb() is located within the handling of the other two failed checks (begin_lpos != tail_lpos and !_datablock_valid()). Then the out: at the end is just return atomic_long_read(&dr->tail_lpos). After modifying the code in this way, I think it looks more straight forward and would have avoided your confusion: The RMB in dataring.c:_dataring_pop() matches the MB in dataring.c:dataring_push() and the RMB in ringbuffer.c:prb_getdesc() matches the SET_RELEASE in ringbuffer.c:assign_desc(). John Ogness