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Use conditions instead of current_event_lock to fix a deadlock.

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Avi Bab correctly noted as bug 3044479 the fact that any thread
blocking on current_event_lock will do so while holding
th_base_lock, making it impossible for the currently running event's
callback to call any other functions that require th_base_lock.

This patch switches the current_event_lock code to instead use a
condition variable that we wait on if we're trying to mess with
a currently-executing event, and that we signal when we're done
executing a callback if anybody is waiting on it.
This commit is contained in:
Nick Mathewson 2010-08-17 13:18:18 -04:00
parent d4977b52f0
commit e0972c2138
2 changed files with 37 additions and 34 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

13
event-internal.h
View File

@ -216,9 +216,6 @@ struct event_base {
/** The total size of common_timeout_queues. */
int n_common_timeouts_allocated;
/** The event whose callback is executing right now */
struct event *current_event;
/** List of defered_cb that are active. We run these after the active
* events. */
struct deferred_cb_queue defer_queue;
@ -247,9 +244,13 @@ struct event_base {
unsigned long th_owner_id;
/** A lock to prevent conflicting accesses to this event_base */
void *th_base_lock;
/** A lock to prevent event_del from deleting an event while its
* callback is executing. */
void *current_event_lock;
/** The event whose callback is executing right now */
struct event *current_event;
/** A condition that gets signalled when we're done processing an
* event with waiters on it. */
void *current_event_cond;
/** Number of threads blocking on current_event_cond. */
int current_event_waiters;
#endif
#ifdef WIN32

58
event.c
View File

@ -604,8 +604,7 @@ event_base_new_with_config(const struct event_config *cfg)
EVTHREAD_ALLOC_LOCK(base->th_base_lock,
EVTHREAD_LOCKTYPE_RECURSIVE);
base->defer_queue.lock = base->th_base_lock;
EVTHREAD_ALLOC_LOCK(base->current_event_lock,
EVTHREAD_LOCKTYPE_RECURSIVE);
EVTHREAD_ALLOC_COND(base->current_event_cond);
r = evthread_make_base_notifiable(base);
if (r<0) {
event_base_free(base);
@ -731,8 +730,7 @@ event_base_free(struct event_base *base)
event_changelist_freemem(&base->changelist);
EVTHREAD_FREE_LOCK(base->th_base_lock, EVTHREAD_LOCKTYPE_RECURSIVE);
EVTHREAD_FREE_LOCK(base->current_event_lock,
EVTHREAD_LOCKTYPE_RECURSIVE);
EVTHREAD_FREE_COND(base->current_event_cond);
mm_free(base);
}
@ -1217,9 +1215,10 @@ event_process_active_single_queue(struct event_base *base,
ev->ev_res & EV_WRITE ? "EV_WRITE " : " ",
ev->ev_callback));
#ifndef _EVENT_DISABLE_THREAD_SUPPORT
base->current_event = ev;
EVBASE_ACQUIRE_LOCK(base, current_event_lock);
base->current_event_waiters = 0;
#endif
switch (ev->ev_closure) {
case EV_CLOSURE_SIGNAL:
@ -1236,9 +1235,14 @@ event_process_active_single_queue(struct event_base *base,
break;
}
EVBASE_RELEASE_LOCK(base, current_event_lock);
EVBASE_ACQUIRE_LOCK(base, th_base_lock);
#ifndef _EVENT_DISABLE_THREAD_SUPPORT
base->current_event = NULL;
if (base->current_event_waiters) {
base->current_event_waiters = 0;
EVTHREAD_COND_BROADCAST(base->current_event_cond);
}
#endif
if (base->event_break)
return -1;
@ -1879,7 +1883,6 @@ event_add_internal(struct event *ev, const struct timeval *tv,
struct event_base *base = ev->ev_base;
int res = 0;
int notify = 0;
int need_cur_lock;
EVENT_BASE_ASSERT_LOCKED(base);
_event_debug_assert_is_setup(ev);
@ -1908,10 +1911,13 @@ event_add_internal(struct event *ev, const struct timeval *tv,
* callback, and we are not the main thread, then we want to wait
* until the callback is done before we mess with the event, or else
* we can race on ev_ncalls and ev_pncalls below. */
need_cur_lock = (base->current_event == ev) &&
(ev->ev_events & EV_SIGNAL);
if (need_cur_lock)
EVBASE_ACQUIRE_LOCK(base, current_event_lock);
#ifndef _EVENT_DISABLE_THREAD_SUPPORT
if (base->current_event == ev && (ev->ev_events & EV_SIGNAL)
&& !EVBASE_IN_THREAD(base)) {
++base->current_event_waiters;
EVTHREAD_COND_WAIT(base->current_event_cond, base->th_base_lock);
}
#endif
if ((ev->ev_events & (EV_READ|EV_WRITE|EV_SIGNAL)) &&
!(ev->ev_flags & (EVLIST_INSERTED|EVLIST_ACTIVE))) {
@ -2016,9 +2022,6 @@ event_add_internal(struct event *ev, const struct timeval *tv,
_event_debug_note_add(ev);
if (need_cur_lock)
EVBASE_RELEASE_LOCK(base, current_event_lock);
return (res);
}
@ -2042,7 +2045,6 @@ event_del_internal(struct event *ev)
{
struct event_base *base;
int res = 0, notify = 0;
int need_cur_lock;
event_debug(("event_del: %p, callback %p",
ev, ev->ev_callback));
@ -2059,9 +2061,12 @@ event_del_internal(struct event *ev)
* when this function returns, it will be safe to free the
* user-supplied argument. */
base = ev->ev_base;
need_cur_lock = (base->current_event == ev);
if (need_cur_lock)
EVBASE_ACQUIRE_LOCK(base, current_event_lock);
#ifndef _EVENT_DISABLE_THREAD_SUPPORT
if (base->current_event == ev && !EVBASE_IN_THREAD(base)) {
++base->current_event_waiters;
EVTHREAD_COND_WAIT(base->current_event_cond, base->th_base_lock);
}
#endif
EVUTIL_ASSERT(!(ev->ev_flags & ~EVLIST_ALL));
@ -2104,9 +2109,6 @@ event_del_internal(struct event *ev)
if (res != -1 && notify && !EVBASE_IN_THREAD(base))
evthread_notify_base(base);
if (need_cur_lock)
EVBASE_RELEASE_LOCK(base, current_event_lock);
_event_debug_note_del(ev);
return (res);
@ -2129,7 +2131,6 @@ void
event_active_nolock(struct event *ev, int res, short ncalls)
{
struct event_base *base;
int need_cur_lock = 0;
/* We get different kinds of events, add them together */
if (ev->ev_flags & EVLIST_ACTIVE) {
@ -2144,16 +2145,17 @@ event_active_nolock(struct event *ev, int res, short ncalls)
ev->ev_res = res;
if (ev->ev_events & EV_SIGNAL) {
need_cur_lock = (base->current_event == ev);
if (need_cur_lock)
EVBASE_ACQUIRE_LOCK(base, current_event_lock);
#ifndef _EVENT_DISABLE_THREAD_SUPPORT
if (base->current_event == ev && !EVBASE_IN_THREAD(base)) {
++base->current_event_waiters;
EVTHREAD_COND_WAIT(base->current_event_cond, base->th_base_lock);
}
#endif
ev->ev_ncalls = ncalls;
ev->ev_pncalls = NULL;
}
event_queue_insert(base, ev, EVLIST_ACTIVE);
if (need_cur_lock)
EVBASE_RELEASE_LOCK(base, current_event_lock);
}
void