diff options
Diffstat (limited to 'tools/libxl/libxl_event.c')
| -rw-r--r-- | tools/libxl/libxl_event.c | 90 |
1 files changed, 75 insertions, 15 deletions
diff --git a/tools/libxl/libxl_event.c b/tools/libxl/libxl_event.c index 19575053ff..1af64c8361 100644 --- a/tools/libxl/libxl_event.c +++ b/tools/libxl/libxl_event.c @@ -839,18 +839,26 @@ int libxl_osevent_beforepoll(libxl_ctx *ctx, int *nfds_io, static int afterpoll_check_fd(libxl__poller *poller, const struct pollfd *fds, int nfds, int fd, int events) - /* returns mask of events which were requested and occurred */ + /* Returns mask of events which were requested and occurred. Will + * return nonzero only once for each (poller,fd,events) + * combination, until the next beforepoll. If events from + * different combinations overlap, between one such combination + * and all distinct combinations will produce nonzero returns. */ { if (fd >= poller->fd_rindices_allocd) /* added after we went into poll, have to try again */ return 0; + events |= POLLERR | POLLHUP; + int i, revents = 0; for (i=0; i<3; i++) { - int slot = poller->fd_rindices[fd][i]; + int *slotp = &poller->fd_rindices[fd][i]; + int slot = *slotp; if (slot >= nfds) - /* stale slot entry; again, added afterwards */ + /* stale slot entry (again, added afterwards), */ + /* or slot for which we have already returned nonzero */ continue; if (fds[slot].fd != fd) @@ -858,11 +866,16 @@ static int afterpoll_check_fd(libxl__poller *poller, continue; assert(!(fds[slot].revents & POLLNVAL)); - revents |= fds[slot].revents; - } - /* we mask in case requested events have changed */ - revents &= (events | POLLERR | POLLHUP); + /* we mask in case requested events have changed */ + int slot_revents = fds[slot].revents & events; + if (!slot_revents) + /* this slot is for a different set of events */ + continue; + + revents |= slot_revents; + *slotp = INT_MAX; /* so that next time we'll see slot >= nfds */ + } return revents; } @@ -876,16 +889,63 @@ static void afterpoll_internal(libxl__egc *egc, libxl__poller *poller, EGC_GC; libxl__ev_fd *efd; - LIBXL_LIST_FOREACH(efd, &CTX->efds, entry) { - if (!efd->events) - continue; + /* + * Warning! Reentrancy hazards! + * + * Many parts of this function eventually call arbitrary callback + * functions which may modify the event handling data structures. + * + * Of the data structures used here: + * + * egc, poller, now + * are allocated by our caller and relate to the + * current thread and its call stack down into the + * event machinery; it is not freed until we return. + * So it is safe. + * + * fds is either what application passed into + * libxl_osevent_afterpoll (which, although this + * isn't explicitly stated, clearly must remain + * valid until libxl_osevent_afterpoll returns) or + * it's poller->fd_polls which is modified only by + * our (non-recursive) caller eventloop_iteration. + * + * CTX comes from our caller, and applications are + * forbidden from destroying it while we are running. + * So the ctx pointer itself is safe to use; now + * for its contents: + * + * CTX->etimes is used in a simple reentrancy-safe manner. + * + * CTX->efds is more complicated; see below. + */ - int revents = afterpoll_check_fd(poller,fds,nfds, efd->fd,efd->events); - if (revents) { - DBG("ev_fd=%p occurs fd=%d events=%x revents=%x", - efd, efd->fd, efd->events, revents); - efd->func(egc, efd, efd->fd, efd->events, revents); + for (;;) { + /* We restart our scan of fd events whenever we call a + * callback function. This is necessary because such + * a callback might make arbitrary changes to CTX->efds. + * We invalidate the fd_rindices[] entries which were used + * so that we don't call the same function again. */ + int revents; + + LIBXL_LIST_FOREACH(efd, &CTX->efds, entry) { + + if (!efd->events) + continue; + + revents = afterpoll_check_fd(poller,fds,nfds, + efd->fd,efd->events); + if (revents) + goto found_fd_event; } + /* no ordinary fd events, then */ + break; + + found_fd_event: + DBG("ev_fd=%p occurs fd=%d events=%x revents=%x", + efd, efd->fd, efd->events, revents); + + efd->func(egc, efd, efd->fd, efd->events, revents); } if (afterpoll_check_fd(poller,fds,nfds, poller->wakeup_pipe[0],POLLIN)) { |