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Revert "async: Fix qemu main thread hang on weak ordered platfrom"

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This reverts commit 97ccfe792b24f1fd56ae051d6d1f7bef67f6b732.
This commit is contained in:
Ying Fang 2020-06-01 09:12:15 +00:00
parent 76977afd22
commit 7d5d99c2eb
3 changed files with 0 additions and 297 deletions
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120
aio-wait-delegate-polling-of-main-AioContext-if-BQL-not-held.patch
View File

@ -1,120 +0,0 @@
From a2bae876b7f694b12073bac8ad6668e4d975ad88 Mon Sep 17 00:00:00 2001
From: Ying Fang <fangying1@huawei.com>
Date: Fri, 10 Apr 2020 16:08:19 +0000
Subject: [PATCH 1/2] 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.
upstream_url: https://patchwork.kernel.org/patch/11482099/
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20200407140746.8041-5-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
include/block/aio-wait.h | 22 ++++++++++++++++++++++
include/block/aio.h | 29 ++++++++++-------------------
2 files changed, 32 insertions(+), 19 deletions(-)
diff --git a/include/block/aio-wait.h b/include/block/aio-wait.h
index afd0ff7e..d349e7e3 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_kick(void);
*/
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 */
diff --git a/include/block/aio.h b/include/block/aio.h
index 0ca25dfe..c527893b 100644
--- a/include/block/aio.h
+++ b/include/block/aio.h
@@ -61,12 +61,16 @@ struct AioContext {
QLIST_HEAD(, AioHandler) 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.
@@ -581,19 +585,6 @@ void aio_co_enter(AioContext *ctx, struct Coroutine *co);
*/
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
--
2.25.2

171
async-use-explicit-memory-barriers.patch
View File

@ -1,171 +0,0 @@
From 787af8ed2bc86dc8688727d62a251965d9c42e00 Mon Sep 17 00:00:00 2001
From: Ying Fang <fangying1@huawei.com>
Date: Fri, 10 Apr 2020 16:19:50 +0000
Subject: [PATCH 2/2] async: use explicit memory barriers
When using C11 atomics, non-seqcst reads and writes do not participate
in the total order of seqcst operations. In util/async.c and util/aio-posix.c,
in particular, the pattern that we use
write ctx->notify_me write bh->scheduled
read bh->scheduled read ctx->notify_me
if !bh->scheduled, sleep if ctx->notify_me, notify
needs to use seqcst operations for both the write and the read. In
general this is something that we do not want, because there can be
many sources that are polled in addition to bottom halves. The
alternative is to place a seqcst memory barrier between the write
and the read. This also comes with a disadvantage, in that the
memory barrier is implicit on strongly-ordered architectures and
it wastes a few dozen clock cycles.
Fortunately, ctx->notify_me is never written concurrently by two
threads, so we can assert that and relax the writes to ctx->notify_me.
The resulting solution works and performs well on both aarch64 and x86.
Note that the atomic_set/atomic_read combination is not an atomic
read-modify-write, and therefore it is even weaker than C11 ATOMIC_RELAXED;
on x86, ATOMIC_RELAXED compiles to a locked operation.
upstream_url: https://patchwork.kernel.org/patch/11482103/
Analyzed-by: Ying Fang <fangying1@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Tested-by: Ying Fang <fangying1@huawei.com>
Message-Id: <20200407140746.8041-6-pbonzini@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
util/aio-posix.c | 16 ++++++++++++++--
util/aio-win32.c | 17 ++++++++++++++---
util/async.c | 16 ++++++++++++----
3 files changed, 40 insertions(+), 9 deletions(-)
diff --git a/util/aio-posix.c b/util/aio-posix.c
index 6fbfa792..ca58b9a4 100644
--- a/util/aio-posix.c
+++ b/util/aio-posix.c
@@ -613,6 +613,11 @@ bool aio_poll(AioContext *ctx, bool blocking)
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
@@ -623,7 +628,13 @@ bool aio_poll(AioContext *ctx, bool blocking)
* 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);
@@ -668,7 +679,8 @@ bool aio_poll(AioContext *ctx, bool blocking)
}
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 a23b9c36..729d533f 100644
--- a/util/aio-win32.c
+++ b/util/aio-win32.c
@@ -321,6 +321,12 @@ bool aio_poll(AioContext *ctx, bool blocking)
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 @@ bool aio_poll(AioContext *ctx, bool blocking)
* 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 @@ bool aio_poll(AioContext *ctx, bool blocking)
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 afc17fb3..12b33204 100644
--- a/util/async.c
+++ b/util/async.c
@@ -221,7 +221,14 @@ aio_ctx_prepare(GSource *source, gint *timeout)
{
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));
@@ -239,7 +246,8 @@ aio_ctx_check(GSource *source)
AioContext *ctx = (AioContext *) source;
QEMUBH *bh;
- 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);
for (bh = ctx->first_bh; bh; bh = bh->next) {
@@ -344,10 +352,10 @@ LinuxAioState *aio_get_linux_aio(AioContext *ctx)
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);
}
--
2.25.2

6
qemu.spec
View File

@ -61,8 +61,6 @@ Patch0048: COLO-compare-Fix-incorrect-if-logic.patch
Patch0049: qcow2-bitmap-Fix-uint64_t-left-shift-overflow.patch Patch0049: qcow2-bitmap-Fix-uint64_t-left-shift-overflow.patch
Patch0050: pcie-Add-pcie-root-port-fast-plug-unplug-feature.patch Patch0050: pcie-Add-pcie-root-port-fast-plug-unplug-feature.patch
Patch0051: pcie-Compat-with-devices-which-do-not-support-Link-W.patch Patch0051: pcie-Compat-with-devices-which-do-not-support-Link-W.patch
Patch0052: aio-wait-delegate-polling-of-main-AioContext-if-BQL-not-held.patch
Patch0053: async-use-explicit-memory-barriers.patch
BuildRequires: flex BuildRequires: flex
BuildRequires: bison BuildRequires: bison
@ -398,10 +396,6 @@ getent passwd qemu >/dev/null || \
%endif %endif
%changelog %changelog
* Sat Apr 11 4 2020 Huawei Technologies Co., Ltd. <fangying1@huawei.com>
- aio-wait: delegate polling of main AioContext if BQL not held
- async: use explicit memory barriers
* Wed Mar 18 2020 Huawei Technologies Co., Ltd. <fangying1@huawei.com> * Wed Mar 18 2020 Huawei Technologies Co., Ltd. <fangying1@huawei.com>
- pcie: Add pcie-root-port fast plug/unplug feature - pcie: Add pcie-root-port fast plug/unplug feature
- pcie: Compat with devices which do not support Link Width - pcie: Compat with devices which do not support Link Width