On Tue, Mar 21, 2023 at 08:24:37PM +0000, Dr. David Alan Gilbert wrote:
> Hi Peter's,
> Peter M pointed me to a seg in a migration test in CI; I can reproduce
> it:
> * On an s390 host
How easy to reproduce?
> * only as part of a make check - running migration-test by itself
> doesn't trigger for me.
> * It looks like it's postcopy preempt
>
> (gdb) bt full
> #0 iov_size (iov=iov@entry=0x2aa00e60670, iov_cnt=<optimized out>) at
> ../util/iov.c:88
> len = 13517923312037845750
> i = 17305
> #1 0x000002aa004d068c in qemu_fflush (f=0x2aa00e58630) at
> ../migration/qemu-file.c:307
> local_error = 0x0
> #2 0x000002aa004d0e04 in qemu_fflush (f=<optimized out>) at
> ../migration/qemu-file.c:297
> #3 0x000002aa00613962 in postcopy_preempt_shutdown_file
> (s=s@entry=0x2aa00d1b4e0) at ../migration/ram.c:4657
> #4 0x000002aa004e12b4 in migration_completion (s=0x2aa00d1b4e0) at
> ../migration/migration.c:3469
> ret = <optimized out>
> current_active_state = 5
> must_precopy = 0
> can_postcopy = 0
> in_postcopy = true
> pending_size = 0
> __func__ = "migration_iteration_run"
> iter_state = <optimized out>
> s = 0x2aa00d1b4e0
> thread = <optimized out>
> setup_start = <optimized out>
> thr_error = <optimized out>
> urgent = <optimized out>
> #5 migration_iteration_run (s=0x2aa00d1b4e0) at ../migration/migration.c:3882
> must_precopy = 0
> can_postcopy = 0
> in_postcopy = true
> pending_size = 0
> __func__ = "migration_iteration_run"
> iter_state = <optimized out>
> s = 0x2aa00d1b4e0
> thread = <optimized out>
> setup_start = <optimized out>
> thr_error = <optimized out>
> urgent = <optimized out>
> #6 migration_thread (opaque=opaque@entry=0x2aa00d1b4e0) at
> ../migration/migration.c:4124
> iter_state = <optimized out>
> s = 0x2aa00d1b4e0
> --Type <RET> for more, q to quit, c to continue without paging--
> thread = <optimized out>
> setup_start = <optimized out>
> thr_error = <optimized out>
> urgent = <optimized out>
> #7 0x000002aa00819b8c in qemu_thread_start (args=<optimized out>) at
> ../util/qemu-thread-posix.c:541
> __cancel_buf =
> {__cancel_jmp_buf = {{__cancel_jmp_buf = {{__gregs =
> {4396782422080, 4393751543808, 4397299389454, 4396844235904, 2929182727824,
> 2929182933488, 4396843986792, 4397299389455, 33679382915066768,
> 33678512846981306}, __fpregs = {4396774031360, 8392704, 2929182933488, 0,
> 4396782422272, 2929172491858, 4396774031360, 1}}}, __mask_was_saved = 0}},
> __pad = {0x3ffb4a77a60, 0x0, 0x0, 0x0}}
> __cancel_routine = 0x2aa00819bf0 <qemu_thread_atexit_notify>
> __not_first_call = <optimized out>
> start_routine = 0x2aa004e08f0 <migration_thread>
> arg = 0x2aa00d1b4e0
> r = <optimized out>
> #8 0x000003ffb7b1e2e6 in start_thread () at /lib64/libc.so.6
> #9 0x000003ffb7aafdbe in thread_start () at /lib64/libc.so.6
>
> It looks like it's in the preempt test:
>
> (gdb) where
> #0 0x000003ffb17a0126 in __pthread_kill_implementation () from
> /lib64/libc.so.6
> #1 0x000003ffb1750890 in raise () from /lib64/libc.so.6
> #2 0x000003ffb172a340 in abort () from /lib64/libc.so.6
> #3 0x000002aa0041c130 in qtest_check_status (s=<optimized out>) at
> ../tests/qtest/libqtest.c:194
> #4 0x000003ffb1a3b5de in g_hook_list_invoke () from /lib64/libglib-2.0.so.0
> #5 <signal handler called>
> #6 0x000003ffb17a0126 in __pthread_kill_implementation () from
> /lib64/libc.so.6
> #7 0x000003ffb1750890 in raise () from /lib64/libc.so.6
> #8 0x000003ffb172a340 in abort () from /lib64/libc.so.6
> #9 0x000002aa00420318 in qmp_fd_receive (fd=<optimized out>) at
> ../tests/qtest/libqmp.c:80
> #10 0x000002aa0041d5ee in qtest_qmp_receive_dict (s=0x2aa01eb2700) at
> ../tests/qtest/libqtest.c:713
> #11 qtest_qmp_receive (s=0x2aa01eb2700) at ../tests/qtest/libqtest.c:701
> #12 qtest_vqmp (s=s@entry=0x2aa01eb2700, fmt=fmt@entry=0x2aa00487100 "{
> 'execute': 'query-migrate' }", ap=ap@entry=0x3ffc247cc68)
> at ../tests/qtest/libqtest.c:765
> #13 0x000002aa00413f1e in wait_command (who=who@entry=0x2aa01eb2700,
> command=command@entry=0x2aa00487100 "{ 'execute': 'query-migrate' }")
> at ../tests/qtest/migration-helpers.c:73
> #14 0x000002aa00414078 in migrate_query (who=who@entry=0x2aa01eb2700) at
> ../tests/qtest/migration-helpers.c:139
> #15 migrate_query_status (who=who@entry=0x2aa01eb2700) at
> ../tests/qtest/migration-helpers.c:161
> #16 0x000002aa00414480 in check_migration_status (ungoals=0x0,
> goal=0x2aa00495c7e "completed", who=0x2aa01eb2700) at
> ../tests/qtest/migration-helpers.c:177
> #17 wait_for_migration_status (who=0x2aa01eb2700, goal=<optimized out>,
> ungoals=0x0) at ../tests/qtest/migration-helpers.c:202
> #18 0x000002aa0041300e in migrate_postcopy_complete
> (from=from@entry=0x2aa01eb2700, to=to@entry=0x2aa01eb3000,
> args=args@entry=0x3ffc247cf48)
> at ../tests/qtest/migration-test.c:1137
> #19 0x000002aa004131a4 in test_postcopy_common (args=0x3ffc247cf48) at
> ../tests/qtest/migration-test.c:1162
> #20 test_postcopy_preempt () at ../tests/qtest/migration-test.c:1178
>
> Looking at the iov and file it's garbage; so it makes me think this is
> something like a flush on a closed file.
I didn't figure out how that could be closed, but I think there's indeed a
possible race that the qemufile can be accessed by both the return path
thread and the migration thread concurrently, while qemufile is not thread
safe on that.
What postcopy_preempt_shutdown_file() does was: the src uses this EOS to
kick the dest QEMU preempt thread out of the migration and shut it off.
After some thought I think this is unnecessary complexity, since postcopy
should end at the point where dest received all the data, then it sends a
SHUT to src. So potentially it's not good to have dest relying on anything
from src to shutdown anything (the preempt thread here) because it's the
dest qemu that makes the final decision to finish. Ideally the preempt
thread on dest should be able to shutdown itself.
The trick here is preempt thread will block at read() (aka, recvmsg()) at
the channel at that time and the only way to kick it out from that is a
shutdown() on dest. I attached a patch did it. I'm not 100% sure whether
it'll already resolve our problem but worth trying. This also made me
notice we forgot to enable SHUTDOWN feature on tls server when I was
running the patch 1 with qtest, so two patches needed.
--
Peter Xu
>From 0e317fa78e9671c119f6be78a0e0a36201517dc2 Mon Sep 17 00:00:00 2001
From: Peter Xu <pet...@redhat.com>
Date: Tue, 21 Mar 2023 19:58:42 -0400
Subject: [PATCH 1/2] io: tls: Inherit QIO_CHANNEL_FEATURE_SHUTDOWN on server
side
TLS iochannel will inherit io_shutdown() from the master ioc, however we
missed to do that on the server side.
This will e.g. allow qemu_file_shutdown() to work on dest QEMU too for
migration.
Signed-off-by: Peter Xu <pet...@redhat.com>
---
io/channel-tls.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/io/channel-tls.c b/io/channel-tls.c
index 5a7a3d48d6..9805dd0a3f 100644
--- a/io/channel-tls.c
+++ b/io/channel-tls.c
@@ -74,6 +74,9 @@ qio_channel_tls_new_server(QIOChannel *master,
ioc = QIO_CHANNEL_TLS(object_new(TYPE_QIO_CHANNEL_TLS));
ioc->master = master;
+ if (qio_channel_has_feature(master, QIO_CHANNEL_FEATURE_SHUTDOWN)) {
+ qio_channel_set_feature(QIO_CHANNEL(ioc),
QIO_CHANNEL_FEATURE_SHUTDOWN);
+ }
object_ref(OBJECT(master));
ioc->session = qcrypto_tls_session_new(
--
2.39.1
>From 73b267c17e689d0dafbde1d93c0125b81a43184e Mon Sep 17 00:00:00 2001
From: Peter Xu <pet...@redhat.com>
Date: Tue, 21 Mar 2023 19:11:22 -0400
Subject: [PATCH 2/2] migration: Fix potential race on postcopy_qemufile_src
postcopy_qemufile_src object should be owned by one thread, either the main
thread (e.g. when at the beginning, or at the end of migration), or by the
return path thread (when during a preempt enabled postcopy migration). If
that's not the case the access to the object might be racy.
postcopy_preempt_shutdown_file() can be potentially racy, because it's
called at the end phase of migration on the main thread, however during
which the return path thread hasn't yet been recycled; the recycle happens
in await_return_path_close_on_source() which is after this point.
It means, logically it's posslbe the main thread and the return path thread
are both operating on the same qemufile. While I don't think qemufile is
thread safe at all.
postcopy_preempt_shutdown_file() used to be needed because that's where we
send EOS to dest so that dest can safely shutdown the preempt thread.
To avoid the possible race, remove this only place that a race can happen.
Instead we figure out another way to safely close the preempt thread on
dest.
The core idea during postcopy on deciding "when to stop" is that dest will
send a postcopy SHUT message to src, telling src that all data is there.
Hence to shut the dest preempt thread maybe better to do it directly on
dest node.
This patch proposed such a way that we change postcopy_prio_thread_created
into PreemptThreadStatus, so that we kick the preempt thread on dest qemu
by a sequence of:
mis->preempt_thread_status = PREEMPT_THREAD_QUIT;
qemu_file_shutdown(mis->postcopy_qemufile_dst);
While here shutdown() is probably so far the easiest way to kick preempt
thread from a blocked qemu_get_be64(). Then it reads preempt_thread_status
to make sure it's not a network failure but a willingness to quit the
thread.
We could have avoided that extra status but just rely on migration status.
The problem is postcopy_ram_incoming_cleanup() is just called early enough
so we're still during POSTCOPY_ACTIVE no matter what.. So just make it
simple to have the status introduced.
Fixes: 36f62f11e4 ("migration: Postcopy preemption preparation on channel
creation")
Signed-off-by: Peter Xu <pet...@redhat.com>
---
migration/migration.c | 7 -------
migration/migration.h | 13 ++++++++++++-
migration/postcopy-ram.c | 20 +++++++++++++++-----
migration/ram.c | 6 ------
migration/ram.h | 1 -
5 files changed, 27 insertions(+), 20 deletions(-)
diff --git a/migration/migration.c b/migration/migration.c
index ae2025d9d8..bbc36100de 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -3459,16 +3459,9 @@ static void migration_completion(MigrationState *s)
}
} else if (s->state == MIGRATION_STATUS_POSTCOPY_ACTIVE) {
trace_migration_completion_postcopy_end();
-
qemu_mutex_lock_iothread();
qemu_savevm_state_complete_postcopy(s->to_dst_file);
qemu_mutex_unlock_iothread();
-
- /* Shutdown the postcopy fast path thread */
- if (migrate_postcopy_preempt()) {
- postcopy_preempt_shutdown_file(s);
- }
-
trace_migration_completion_postcopy_end_after_complete();
} else {
goto fail;
diff --git a/migration/migration.h b/migration/migration.h
index 2da2f8a164..7228163cc8 100644
--- a/migration/migration.h
+++ b/migration/migration.h
@@ -65,6 +65,12 @@ typedef struct {
bool all_zero;
} PostcopyTmpPage;
+typedef enum {
+ PREEMPT_THREAD_NONE = 0,
+ PREEMPT_THREAD_CREATED,
+ PREEMPT_THREAD_QUIT,
+} PreemptThreadStatus;
+
/* State for the incoming migration */
struct MigrationIncomingState {
QEMUFile *from_src_file;
@@ -124,7 +130,12 @@ struct MigrationIncomingState {
QemuSemaphore postcopy_qemufile_dst_done;
/* Postcopy priority thread is used to receive postcopy requested pages */
QemuThread postcopy_prio_thread;
- bool postcopy_prio_thread_created;
+ /*
+ * Always set by the main vm load thread only, but can be read by the
+ * postcopy preempt thread. "volatile" makes sure all reads will be
+ * uptodate across cores.
+ */
+ volatile PreemptThreadStatus preempt_thread_status;
/*
* Used to sync between the ram load main thread and the fast ram load
* thread. It protects postcopy_qemufile_dst, which is the postcopy
diff --git a/migration/postcopy-ram.c b/migration/postcopy-ram.c
index 41c0713650..263bab75ec 100644
--- a/migration/postcopy-ram.c
+++ b/migration/postcopy-ram.c
@@ -568,9 +568,14 @@ int postcopy_ram_incoming_cleanup(MigrationIncomingState
*mis)
{
trace_postcopy_ram_incoming_cleanup_entry();
- if (mis->postcopy_prio_thread_created) {
+ if (mis->preempt_thread_status == PREEMPT_THREAD_CREATED) {
+ /* Notify the fast load thread to quit */
+ mis->preempt_thread_status = PREEMPT_THREAD_QUIT;
+ if (mis->postcopy_qemufile_dst) {
+ qemu_file_shutdown(mis->postcopy_qemufile_dst);
+ }
qemu_thread_join(&mis->postcopy_prio_thread);
- mis->postcopy_prio_thread_created = false;
+ mis->preempt_thread_status = PREEMPT_THREAD_NONE;
}
if (mis->have_fault_thread) {
@@ -1203,7 +1208,7 @@ int postcopy_ram_incoming_setup(MigrationIncomingState
*mis)
*/
postcopy_thread_create(mis, &mis->postcopy_prio_thread, "fault-fast",
postcopy_preempt_thread, QEMU_THREAD_JOINABLE);
- mis->postcopy_prio_thread_created = true;
+ mis->preempt_thread_status = PREEMPT_THREAD_CREATED;
}
trace_postcopy_ram_enable_notify();
@@ -1652,6 +1657,11 @@ static void
postcopy_pause_ram_fast_load(MigrationIncomingState *mis)
trace_postcopy_pause_fast_load_continued();
}
+static bool preempt_thread_should_run(MigrationIncomingState *mis)
+{
+ return mis->preempt_thread_status != PREEMPT_THREAD_QUIT;
+}
+
void *postcopy_preempt_thread(void *opaque)
{
MigrationIncomingState *mis = opaque;
@@ -1671,11 +1681,11 @@ void *postcopy_preempt_thread(void *opaque)
/* Sending RAM_SAVE_FLAG_EOS to terminate this thread */
qemu_mutex_lock(&mis->postcopy_prio_thread_mutex);
- while (1) {
+ while (preempt_thread_should_run(mis)) {
ret = ram_load_postcopy(mis->postcopy_qemufile_dst,
RAM_CHANNEL_POSTCOPY);
/* If error happened, go into recovery routine */
- if (ret) {
+ if (ret && preempt_thread_should_run(mis)) {
postcopy_pause_ram_fast_load(mis);
} else {
/* We're done */
diff --git a/migration/ram.c b/migration/ram.c
index 96e8a19a58..8c316ffed2 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -4651,12 +4651,6 @@ static int ram_resume_prepare(MigrationState *s, void
*opaque)
return 0;
}
-void postcopy_preempt_shutdown_file(MigrationState *s)
-{
- qemu_put_be64(s->postcopy_qemufile_src, RAM_SAVE_FLAG_EOS);
- qemu_fflush(s->postcopy_qemufile_src);
-}
-
static SaveVMHandlers savevm_ram_handlers = {
.save_setup = ram_save_setup,
.save_live_iterate = ram_save_iterate,
diff --git a/migration/ram.h b/migration/ram.h
index 81cbb0947c..93b071a1a7 100644
--- a/migration/ram.h
+++ b/migration/ram.h
@@ -96,7 +96,6 @@ int64_t ramblock_recv_bitmap_send(QEMUFile *file,
const char *block_name);
int ram_dirty_bitmap_reload(MigrationState *s, RAMBlock *rb);
bool ramblock_page_is_discarded(RAMBlock *rb, ram_addr_t start);
-void postcopy_preempt_shutdown_file(MigrationState *s);
void *postcopy_preempt_thread(void *opaque);
/* ram cache */
--
2.39.1
