[DTrace-devel] reread event from buffer after exit() action

[Eugene Loh]

Consider the following two runs:

     # dtrace -n 'BEGIN { trace(345); exit(0) }'
     dtrace: description 'BEGIN ' matched 1 probe
     CPU     ID                    FUNCTION:NAME
       2      1                           :BEGIN         345

     # dtrace -n 'BEGIN { trace(345); exit(0) } write:entry'
     dtrace: description 'BEGIN ' matched 2 probes
     CPU     ID                    FUNCTION:NAME
       2      1                           :BEGIN         345
       2      1                           :BEGIN         345

The second one has doubled output.  Unsurprisingly, legacy DTrace does 
not do this.

Here is what I think is happening.  If we hit an exit() action, 
dt_consume_one() will return DONE:

   2318 dt_consume_one(...) {
   2397     for (i = 0; i < pdat->dtpda_ddesc->dtdd_nrecs; i++) {
   2401         rec = &pdat->dtpda_ddesc->dtdd_recs[i];
   2402         if (rec->dtrd_action == DTRACEACT_EXIT)
   2403             done = 1;
   2437     }
   2447     return done ? DTRACE_WORKSTATUS_DONE : DTRACE_WORKSTATUS_OKAY;
   2465 }

That's fine, but then in dt_consume_cpu(), the non-OKAY return value at 
line 2538 means the read tail of the cyclic buffer is not updated (lines 
2541 and 2544):

   2502     head = ring_buffer_read_head(rb_page);
   2503     tail = rb_page->data_tail;
   2505     if (head == tail)
   2506         break;
   2508     do {
   2535         rval = dt_consume_one(...);
   2538         if (rval != DTRACE_WORKSTATUS_OKAY)
   2539             return rval;
   2541         tail += hdr->size;
   2542     } while (tail != head);
   2544     ring_buffer_write_tail(rb_page, tail);

Going up the food chain, we get to cmd/dtrace.c:

   1515 do {
   1528     if (done == DONE_SAW_EXIT) {
   1530         done = DONE_SAW_END;
   1531         dtrace_stop(g_dtp);
   1533     }
   1535     switch (dtrace_work(g_dtp, g_ofp, chew, chewrec, NULL)) {
   1536     case DTRACE_WORKSTATUS_DONE:
   1537         if (done != DONE_SAW_END)
   1538             done = DONE_SAW_EXIT;
   1539         break;
   1540     case DTRACE_WORKSTATUS_OKAY:
   1541         break;
   1545     }
   1549 } while (done != DONE_SAW_END);

We call dtrace_work(), and the exit() action causes done=EXIT at line 
1538.  Then we cycle through the loop and at line 1530 set done=END.  We 
call dtrace_work() again and reread the same BEGIN event since the read 
tail was never updated.

I could imagine:
*) updating the read tail of the cyclic buffer in dt_consume_cpu(), even 
if dt_consume_one() does not return OKAY
*) not calling dtrace_work() if done==END