Merge remote-tracking branch 'remotes/stefanha/tags/block-pull-request' into staging
Pull request
Fixes for QEMU on aarch64 ARM hosts and fdmon-io_uring.
# gpg: Signature made Thu 09 Apr 2020 18:42:01 BST
# gpg: using RSA key 8695A8BFD3F97CDAAC35775A9CA4ABB381AB73C8
# gpg: Good signature from "Stefan Hajnoczi <stefanha@redhat.com>" [full]
# gpg: aka "Stefan Hajnoczi <stefanha@gmail.com>" [full]
# Primary key fingerprint: 8695 A8BF D3F9 7CDA AC35 775A 9CA4 ABB3 81AB 73C8
* remotes/stefanha/tags/block-pull-request:
async: use explicit memory barriers
aio-wait: delegate polling of main AioContext if BQL not held
aio-posix: signal-proof fdmon-io_uring
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
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
*
diff --git a/util/aio-posix.c b/util/aio-posix.c
index cd6cf0a..c3613d2 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -559,6 +559,11 @@
int64_t timeout;
int64_t start = 0;
+ /*
+ * There cannot be two concurrent aio_poll calls for the same AioContext (or
+ * an aio_poll concurrent with a GSource prepare/check/dispatch callback).
+ * We rely on this below to avoid slow locked accesses to ctx->notify_me.
+ */
assert(in_aio_context_home_thread(ctx));
/* aio_notify can avoid the expensive event_notifier_set if
@@ -569,7 +574,13 @@
* so disable the optimization now.
*/
if (blocking) {
- atomic_add(&ctx->notify_me, 2);
+ atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
+ /*
+ * Write ctx->notify_me before computing the timeout
+ * (reading bottom half flags, etc.). Pairs with
+ * smp_mb in aio_notify().
+ */
+ smp_mb();
}
qemu_lockcnt_inc(&ctx->list_lock);
@@ -590,7 +601,8 @@
}
if (blocking) {
- atomic_sub(&ctx->notify_me, 2);
+ /* Finish the poll before clearing the flag. */
+ atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me) - 2);
aio_notify_accept(ctx);
}
diff --git a/util/aio-win32.c b/util/aio-win32.c
index a23b9c3..729d533 100644
--- a/util/aio-win32.c
+++ b/util/aio-win32.c
@@ -321,6 +321,12 @@
int count;
int timeout;
+ /*
+ * There cannot be two concurrent aio_poll calls for the same AioContext (or
+ * an aio_poll concurrent with a GSource prepare/check/dispatch callback).
+ * We rely on this below to avoid slow locked accesses to ctx->notify_me.
+ */
+ assert(in_aio_context_home_thread(ctx));
progress = false;
/* aio_notify can avoid the expensive event_notifier_set if
@@ -331,7 +337,13 @@
* so disable the optimization now.
*/
if (blocking) {
- atomic_add(&ctx->notify_me, 2);
+ atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) + 2);
+ /*
+ * Write ctx->notify_me before computing the timeout
+ * (reading bottom half flags, etc.). Pairs with
+ * smp_mb in aio_notify().
+ */
+ smp_mb();
}
qemu_lockcnt_inc(&ctx->list_lock);
@@ -364,8 +376,7 @@
ret = WaitForMultipleObjects(count, events, FALSE, timeout);
if (blocking) {
assert(first);
- assert(in_aio_context_home_thread(ctx));
- atomic_sub(&ctx->notify_me, 2);
+ atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me) - 2);
aio_notify_accept(ctx);
}
diff --git a/util/async.c b/util/async.c
index b94518b..3165a28 100644
--- a/util/async.c
+++ b/util/async.c
@@ -249,7 +249,14 @@
{
AioContext *ctx = (AioContext *) source;
- atomic_or(&ctx->notify_me, 1);
+ atomic_set(&ctx->notify_me, atomic_read(&ctx->notify_me) | 1);
+
+ /*
+ * Write ctx->notify_me before computing the timeout
+ * (reading bottom half flags, etc.). Pairs with
+ * smp_mb in aio_notify().
+ */
+ smp_mb();
/* We assume there is no timeout already supplied */
*timeout = qemu_timeout_ns_to_ms(aio_compute_timeout(ctx));
@@ -268,7 +275,8 @@
QEMUBH *bh;
BHListSlice *s;
- atomic_and(&ctx->notify_me, ~1);
+ /* Finish computing the timeout before clearing the flag. */
+ atomic_store_release(&ctx->notify_me, atomic_read(&ctx->notify_me) & ~1);
aio_notify_accept(ctx);
QSLIST_FOREACH_RCU(bh, &ctx->bh_list, next) {
@@ -411,10 +419,10 @@
void aio_notify(AioContext *ctx)
{
/* Write e.g. bh->scheduled before reading ctx->notify_me. Pairs
- * with atomic_or in aio_ctx_prepare or atomic_add in aio_poll.
+ * with smp_mb in aio_ctx_prepare or aio_poll.
*/
smp_mb();
- if (ctx->notify_me) {
+ if (atomic_read(&ctx->notify_me)) {
event_notifier_set(&ctx->notifier);
atomic_mb_set(&ctx->notified, true);
}
diff --git a/util/fdmon-io_uring.c b/util/fdmon-io_uring.c
index b4d6109..d5a80ed 100644
--- a/util/fdmon-io_uring.c
+++ b/util/fdmon-io_uring.c
@@ -88,7 +88,10 @@
}
/* No free sqes left, submit pending sqes first */
- ret = io_uring_submit(ring);
+ do {
+ ret = io_uring_submit(ring);
+ } while (ret == -EINTR);
+
assert(ret > 1);
sqe = io_uring_get_sqe(ring);
assert(sqe);
@@ -282,7 +285,10 @@
fill_sq_ring(ctx);
- ret = io_uring_submit_and_wait(&ctx->fdmon_io_uring, wait_nr);
+ do {
+ ret = io_uring_submit_and_wait(&ctx->fdmon_io_uring, wait_nr);
+ } while (ret == -EINTR);
+
assert(ret >= 0);
return process_cq_ring(ctx, ready_list);
