On Oct 23, 2010, at 9:42 AM, Nick Mathewson wrote: > On Sat, Oct 23, 2010 at 5:14 AM, Ralph Castain <r...@open-mpi.org> wrote: >> Hi folks >> >> I successfully updated our libevent integration in Open MPI, but have >> encountered a problem with one use-case that used to work and now doesn't. >> Before proceeding to devise a fix, I just wanted to confirm that I >> accurately understand the issue. >> >> The problem arises from this scenario: >> >> 1. we receive a command via a message that we receive in a file descriptor >> event. We "push" the command message into a timer event (duration zero time) >> to help break a threading issue, and then return from the file descriptor >> event. >> >> 2. the event library is called with LOOP_ONCE, causing the timer event to >> fire. >> >> 3. from within the timer event, the command causes us to execute a procedure >> that results in us having to wait for another event to occur. We "block" in >> that position, running a loop that includes a call to progress the event >> library (i.e., a call to event_loop(LOOP_ONCE)). >> >> This last step used to work just fine, yet now yields the following warning: >> >> warn] event_base_loop: reentrant invocation. Only one event_base_loop can >> run on each event_base at once.: Resource temporarily unavailable >> >> and no progression of the event library occurs. >> >> I can see a few ways around this problem, but all involve considerable code >> rewrite. Before I do so, I just wanted to confirm my understanding that this >> behavior changed (i.e., recursion is no longer allowed), and see if any >> alternative approaches to revising such codes are recommended. > > Hi, Ralph! I'll try to answer in more detail later on -- I can't > write too much now, but I wanted to drop you a line before you started > rewriting your code. > > I think that the warning you hit may be overzealous. It was meant to > detect errors that would previously pass silently and produce > hard-to-diagnose crashes, such as running event_base_dispatch on the > same event base from multiple threads at once. But I think your use > case may either a) work right, or b) be trivial to make work right, > and if so, we should fix Libevent not to warn about it. > > I'll look harder at the code here and try to see what's going on here, > but calling the loop from a callback really was working fine with > Libevent 1.4, then I think 2.0 should support it.
I appreciate the help - let me know if I can be of further assistance. Please note that the warning isn't the problem (though I would like to silence it) - the problem is that the events don't get progressed, and so we loop forever on the warning. > > Thanks for the heads-up, and thanks to everybody else who's hunting > for regressions and other bugs in 2.0.8-rc. > > yrs, > -- > Nick > *********************************************************************** > To unsubscribe, send an e-mail to majord...@freehaven.net with > unsubscribe libevent-users in the body. *********************************************************************** To unsubscribe, send an e-mail to majord...@freehaven.net with unsubscribe libevent-users in the body.
