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authorStefan Hajnoczi <stefanha@redhat.com>2018-03-22 15:28:33 +0000
committerStefan Hajnoczi <stefanha@redhat.com>2018-03-27 13:05:28 +0100
commitc40a2545700e9ad2ef67d5972484bbee4c83b2a6 (patch)
tree2c550ddaf9a59a58c3c07e40c02aded945384cd1 /util
parent67a74148d8828f611fc5a7a23277b8eceb4c9430 (diff)
coroutine: avoid co_queue_wakeup recursion
qemu_aio_coroutine_enter() is (indirectly) called recursively when processing co_queue_wakeup. This can lead to stack exhaustion. This patch rewrites co_queue_wakeup in an iterative fashion (instead of recursive) with bounded memory usage to prevent stack exhaustion. qemu_co_queue_run_restart() is inlined into qemu_aio_coroutine_enter() and the qemu_coroutine_enter() call is turned into a loop to avoid recursion. There is one change that is worth mentioning: Previously, when coroutine A queued coroutine B, qemu_co_queue_run_restart() entered coroutine B from coroutine A. If A was terminating then it would still stay alive until B yielded. After this patch B is entered by A's parent so that a A can be deleted immediately if it is terminating. It is safe to make this change since B could never interact with A if it was terminating anyway. Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Message-id: 20180322152834.12656-3-stefanha@redhat.com Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Diffstat (limited to 'util')
-rw-r--r--util/qemu-coroutine-lock.c34
-rw-r--r--util/qemu-coroutine.c94
2 files changed, 51 insertions, 77 deletions
diff --git a/util/qemu-coroutine-lock.c b/util/qemu-coroutine-lock.c
index 5a80c10690..27438a1858 100644
--- a/util/qemu-coroutine-lock.c
+++ b/util/qemu-coroutine-lock.c
@@ -68,40 +68,6 @@ void coroutine_fn qemu_co_queue_wait_impl(CoQueue *queue, QemuLockable *lock)
}
}
-/**
- * qemu_co_queue_run_restart:
- *
- * Enter each coroutine that was previously marked for restart by
- * qemu_co_queue_next() or qemu_co_queue_restart_all(). This function is
- * invoked by the core coroutine code when the current coroutine yields or
- * terminates.
- */
-void qemu_co_queue_run_restart(Coroutine *co)
-{
- Coroutine *next;
- QSIMPLEQ_HEAD(, Coroutine) tmp_queue_wakeup =
- QSIMPLEQ_HEAD_INITIALIZER(tmp_queue_wakeup);
-
- trace_qemu_co_queue_run_restart(co);
-
- /* Because "co" has yielded, any coroutine that we wakeup can resume it.
- * If this happens and "co" terminates, co->co_queue_wakeup becomes
- * invalid memory. Therefore, use a temporary queue and do not touch
- * the "co" coroutine as soon as you enter another one.
- *
- * In its turn resumed "co" can populate "co_queue_wakeup" queue with
- * new coroutines to be woken up. The caller, who has resumed "co",
- * will be responsible for traversing the same queue, which may cause
- * a different wakeup order but not any missing wakeups.
- */
- QSIMPLEQ_CONCAT(&tmp_queue_wakeup, &co->co_queue_wakeup);
-
- while ((next = QSIMPLEQ_FIRST(&tmp_queue_wakeup))) {
- QSIMPLEQ_REMOVE_HEAD(&tmp_queue_wakeup, co_queue_next);
- qemu_coroutine_enter(next);
- }
-}
-
static bool qemu_co_queue_do_restart(CoQueue *queue, bool single)
{
Coroutine *next;
diff --git a/util/qemu-coroutine.c b/util/qemu-coroutine.c
index 9eff7fd450..1ba4191b84 100644
--- a/util/qemu-coroutine.c
+++ b/util/qemu-coroutine.c
@@ -104,57 +104,65 @@ static void coroutine_delete(Coroutine *co)
void qemu_aio_coroutine_enter(AioContext *ctx, Coroutine *co)
{
- Coroutine *self = qemu_coroutine_self();
- CoroutineAction ret;
-
- /* Cannot rely on the read barrier for co in aio_co_wake(), as there are
- * callers outside of aio_co_wake() */
- const char *scheduled = atomic_mb_read(&co->scheduled);
+ QSIMPLEQ_HEAD(, Coroutine) pending = QSIMPLEQ_HEAD_INITIALIZER(pending);
+ Coroutine *from = qemu_coroutine_self();
- trace_qemu_aio_coroutine_enter(ctx, self, co, co->entry_arg);
+ QSIMPLEQ_INSERT_TAIL(&pending, co, co_queue_next);
- /* if the Coroutine has already been scheduled, entering it again will
- * cause us to enter it twice, potentially even after the coroutine has
- * been deleted */
- if (scheduled) {
- fprintf(stderr,
- "%s: Co-routine was already scheduled in '%s'\n",
- __func__, scheduled);
- abort();
- }
+ /* Run co and any queued coroutines */
+ while (!QSIMPLEQ_EMPTY(&pending)) {
+ Coroutine *to = QSIMPLEQ_FIRST(&pending);
+ CoroutineAction ret;
- if (co->caller) {
- fprintf(stderr, "Co-routine re-entered recursively\n");
- abort();
- }
+ /* Cannot rely on the read barrier for to in aio_co_wake(), as there are
+ * callers outside of aio_co_wake() */
+ const char *scheduled = atomic_mb_read(&to->scheduled);
- co->caller = self;
- co->ctx = ctx;
+ QSIMPLEQ_REMOVE_HEAD(&pending, co_queue_next);
- /* Store co->ctx before anything that stores co. Matches
- * barrier in aio_co_wake and qemu_co_mutex_wake.
- */
- smp_wmb();
+ trace_qemu_aio_coroutine_enter(ctx, from, to, to->entry_arg);
- ret = qemu_coroutine_switch(self, co, COROUTINE_ENTER);
-
- qemu_co_queue_run_restart(co);
+ /* if the Coroutine has already been scheduled, entering it again will
+ * cause us to enter it twice, potentially even after the coroutine has
+ * been deleted */
+ if (scheduled) {
+ fprintf(stderr,
+ "%s: Co-routine was already scheduled in '%s'\n",
+ __func__, scheduled);
+ abort();
+ }
- /* Beware, if ret == COROUTINE_YIELD and qemu_co_queue_run_restart()
- * has started any other coroutine, "co" might have been reentered
- * and even freed by now! So be careful and do not touch it.
- */
+ if (to->caller) {
+ fprintf(stderr, "Co-routine re-entered recursively\n");
+ abort();
+ }
- switch (ret) {
- case COROUTINE_YIELD:
- return;
- case COROUTINE_TERMINATE:
- assert(!co->locks_held);
- trace_qemu_coroutine_terminate(co);
- coroutine_delete(co);
- return;
- default:
- abort();
+ to->caller = from;
+ to->ctx = ctx;
+
+ /* Store to->ctx before anything that stores to. Matches
+ * barrier in aio_co_wake and qemu_co_mutex_wake.
+ */
+ smp_wmb();
+
+ ret = qemu_coroutine_switch(from, to, COROUTINE_ENTER);
+
+ /* Queued coroutines are run depth-first; previously pending coroutines
+ * run after those queued more recently.
+ */
+ QSIMPLEQ_PREPEND(&pending, &to->co_queue_wakeup);
+
+ switch (ret) {
+ case COROUTINE_YIELD:
+ break;
+ case COROUTINE_TERMINATE:
+ assert(!to->locks_held);
+ trace_qemu_coroutine_terminate(to);
+ coroutine_delete(to);
+ break;
+ default:
+ abort();
+ }
}
}