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fix another problem with GC and place messages

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When a place message is deserialized by simply adopting the page
containing the message, the adoption can trigger a garbage
collection, but there's still a pointer to a chain of objects
"in flight" in the thread, and a GC can discard the pairs that
form the chain.
This commit is contained in:
Matthew Flatt 2015-10-02 11:48:25 -06:00
parent d1a942be63
commit 93d9826936
1 changed files with 13 additions and 9 deletions
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22
racket/src/racket/src/place.c
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@ -2828,10 +2828,13 @@ static Scheme_Object *places_serialize(Scheme_Object *so, void **msg_memory, Sch
#endif
}
Scheme_Object *scheme_places_deserialize(Scheme_Object *so, void *msg_memory)
static Scheme_Object *places_deserialize(Scheme_Object *so, void *msg_memory, Scheme_Thread *from_p)
/* The caller must immediately drop any reference to `so' and
`msg_memory' after this function returns; otherwise, since the
`msg_memory' page may be deallocated, a GC could crash. */
`msg_memory' page may be deallocated, a GC could crash.
Also, we have to clear out the in-flight references in `from_p`
before the pages are discarded or adopted (where the latter
can trigger a GC, which creates the main problem) */
{
#if defined(MZ_USE_PLACES) && defined(MZ_PRECISE_GC)
Scheme_Object *new_so = so;
@ -2842,13 +2845,18 @@ Scheme_Object *scheme_places_deserialize(Scheme_Object *so, void *msg_memory)
/* small messages are deemed to be < 1k, this could be tuned in either direction */
if (GC_message_small_objects_size(msg_memory, 1024)) {
new_so = do_places_deep_copy(so, mzPDC_UNCOPY, 1, NULL, NULL);
from_p->place_channel_msg_in_flight = NULL;
from_p->place_channel_msg_chain_in_flight = NULL;
GC_dispose_short_message_allocator(msg_memory);
/* from this point, we must return immediately, so that any
reference to `so' can be dropped before GC. */
msg_memory = NULL;
}
else {
from_p->place_channel_msg_in_flight = NULL;
from_p->place_channel_msg_chain_in_flight = NULL;
GC_adopt_message_allocator(msg_memory);
scheme_collect_garbage(); // REMOVEME
msg_memory = NULL;
#if !defined(SHARED_TABLES)
new_so = do_places_deep_copy(so, mzPDC_DESER, 1, NULL, NULL);
@ -3549,9 +3557,9 @@ static Scheme_Object *place_async_try_receive_raw(Scheme_Place_Async_Channel *ch
static void cleanup_msg_memmory(void *thread) {
Scheme_Thread *p = thread;
if (p->place_channel_msg_in_flight) {
p->place_channel_msg_chain_in_flight = NULL;
GC_destroy_orphan_msg_memory(p->place_channel_msg_in_flight);
p->place_channel_msg_in_flight = NULL;
p->place_channel_msg_chain_in_flight = NULL;
}
}
@ -3575,9 +3583,7 @@ static Scheme_Object *place_async_try_receive(Scheme_Place_Async_Channel *ch, in
p->place_channel_msg_in_flight = msg_memory;
p->place_channel_msg_chain_in_flight = msg_chain;
log_received_msg(msg, msg_memory);
msg = scheme_places_deserialize(msg, msg_memory);
p->place_channel_msg_in_flight = NULL;
p->place_channel_msg_chain_in_flight = NULL;
msg = places_deserialize(msg, msg_memory, p);
}
END_ESCAPEABLE();
return msg;
@ -3602,9 +3608,7 @@ static Scheme_Object *place_channel_finish_ready(void *d, int argc, struct Schem
msg = *(Scheme_Object **)d;
BEGIN_ESCAPEABLE(cleanup_msg_memmory, p);
msg = scheme_places_deserialize(msg, p->place_channel_msg_in_flight);
p->place_channel_msg_in_flight = NULL;
p->place_channel_msg_chain_in_flight = NULL;
msg = places_deserialize(msg, p->place_channel_msg_in_flight, p);
END_ESCAPEABLE();
return msg;