[libvirt] [PATCH v2 3/3] util: event loop: document another reason to defer deletion

This is why we should not free callback object synchronously upon removing handle or timeout. Imagine: 1. loop thread, drops the lock and is about to call event callback 2. another thread, enters remove handle and frees callback object 3. loop thread, enters event callback, uses callback object BOOOM --- src/util/vireventpoll.c | 7 +++++++ 1 file changed, 7 insertions(+) diff --git a/src/util/vireventpoll.c b/src/util/vireventpoll.c index 81ecab4..802b679 100644 --- a/src/util/vireventpoll.c +++ b/src/util/vireventpoll.c @@ -176,6 +176,10 @@ void virEventPollUpdateHandle(int watch, int events) * Unregister a callback from a file handle * NB, it *must* be safe to call this from within a callback * For this reason we only ever set a flag in the existing list. + * Another reason is that as we drop the lock upon event callback + * invocation we can't free callback object if we called + * from a thread then loop thread. + * * Actual deletion will be done out-of-band */ int virEventPollRemoveHandle(int watch) @@ -295,6 +299,9 @@ void virEventPollUpdateTimeout(int timer, int frequency) * Unregister a callback for a timer * NB, it *must* be safe to call this from within a callback * For this reason we only ever set a flag in the existing list. + * Another reason is that as we drop the lock upon event callback + * invocation we can't free callback object if we called + * from a thread then loop thread. * Actual deletion will be done out-of-band */ int virEventPollRemoveTimeout(int timer) -- 1.8.3.1