aio-wait: delegate polling of main AioContext if BQL not held
Any thread that is not a iothread returns NULL for qemu_get_current_aio_context().
As a result, it would also return true for
in_aio_context_home_thread(qemu_get_aio_context()), causing
AIO_WAIT_WHILE to invoke aio_poll() directly. This is incorrect
if the BQL is not held, because aio_poll() does not expect to
run concurrently from multiple threads, and it can actually
happen when savevm writes to the vmstate file from the
migration thread.
Therefore, restrict in_aio_context_home_thread to return true
for the main AioContext only if the BQL is held.
The function is moved to aio-wait.h because it is mostly used
there and to avoid a circular reference between main-loop.h
and block/aio.h.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200407140746.8041-5-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
index afeeb18..716d263 100644
--- a/include/block/aio-wait.h
+++ b/include/block/aio-wait.h
@@ -26,6 +26,7 @@
#define QEMU_AIO_WAIT_H
#include "block/aio.h"
+#include "qemu/main-loop.h"
/**
* AioWait:
@@ -124,4 +125,25 @@
*/
void aio_wait_bh_oneshot(AioContext *ctx, QEMUBHFunc *cb, void *opaque);
+/**
+ * in_aio_context_home_thread:
+ * @ctx: the aio context
+ *
+ * Return whether we are running in the thread that normally runs @ctx. Note
+ * that acquiring/releasing ctx does not affect the outcome, each AioContext
+ * still only has one home thread that is responsible for running it.
+ */
+static inline bool in_aio_context_home_thread(AioContext *ctx)
+{
+ if (ctx == qemu_get_current_aio_context()) {
+ return true;
+ }
+
+ if (ctx == qemu_get_aio_context()) {
+ return qemu_mutex_iothread_locked();
+ } else {
+ return false;
+ }
+}
+
#endif /* QEMU_AIO_WAIT_H */
diff --git a/include/block/aio.h b/include/block/aio.h
index cb19891..62ed954 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -133,12 +133,16 @@
AioHandlerList deleted_aio_handlers;
/* Used to avoid unnecessary event_notifier_set calls in aio_notify;
- * accessed with atomic primitives. If this field is 0, everything
- * (file descriptors, bottom halves, timers) will be re-evaluated
- * before the next blocking poll(), thus the event_notifier_set call
- * can be skipped. If it is non-zero, you may need to wake up a
- * concurrent aio_poll or the glib main event loop, making
- * event_notifier_set necessary.
+ * only written from the AioContext home thread, or under the BQL in
+ * the case of the main AioContext. However, it is read from any
+ * thread so it is still accessed with atomic primitives.
+ *
+ * If this field is 0, everything (file descriptors, bottom halves,
+ * timers) will be re-evaluated before the next blocking poll() or
+ * io_uring wait; therefore, the event_notifier_set call can be
+ * skipped. If it is non-zero, you may need to wake up a concurrent
+ * aio_poll or the glib main event loop, making event_notifier_set
+ * necessary.
*
* Bit 0 is reserved for GSource usage of the AioContext, and is 1
* between a call to aio_ctx_prepare and the next call to aio_ctx_check.
@@ -682,19 +686,6 @@
AioContext *qemu_get_current_aio_context(void);
/**
- * in_aio_context_home_thread:
- * @ctx: the aio context
- *
- * Return whether we are running in the thread that normally runs @ctx. Note
- * that acquiring/releasing ctx does not affect the outcome, each AioContext
- * still only has one home thread that is responsible for running it.
- */
-static inline bool in_aio_context_home_thread(AioContext *ctx)
-{
- return ctx == qemu_get_current_aio_context();
-}
-
-/**
* aio_context_setup:
* @ctx: the aio context
*
