c/pve-qemu-qoup
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity
b64c4dec1c
pve-qemu-qoup/debian/patches/pve/0066-migration-for-snapshots-hold-the-BQL-during-setup-ca.patch
Fiona Ebner bf251437e9 update submodule and patches to QEMU 8.0.0 
Many changes were necessary this time around:

* QAPI was changed to avoid redundant has_* variables, see commit
44ea9d9be3 ("qapi: Start to elide redundant has_FOO in generated C")
for details. This affected many QMP commands added by Proxmox too.

* Pending querying for migration got split into two functions, one to
estimate, one for exact value, see commit c8df4a7aef ("migration:
Split save_live_pending() into state_pending_*") for details. Relevant
for savevm-async and PBS dirty bitmap.

* Some block (driver) functions got converted to coroutines, so the
Proxmox block drivers needed to be adapted.

* Alloc track auto-detaching during PBS live restore got broken by
AioContext-related changes resulting in a deadlock. The current, hacky
method was replaced by a simpler one. Stefan apparently ran into a
problem with that when he wrote the driver, but there were
improvements in the stream job code since then and I didn't manage to
reproduce the issue. It's a separate patch "alloc-track: fix deadlock
during drop" for now, you can find the details there.

* Async snapshot-related changes:
  - The pending querying got adapted to the above-mentioned split and
  a patch is added to optimize it/make it more similar to what
  upstream code does.
  - Added initialization of the compression counters (for
    future-proofing).
  - It's necessary the hold the BQL (big QEMU lock = iothread mutex)
  during the setup phase, because block layer functions are used there
  and not doing so leads to racy, hard-to-debug crashes or hangs. It's
  necessary to change some upstream code too for this, a version of
  the patch "migration: for snapshots, hold the BQL during setup
  callbacks" is intended to be upstreamed.
  - Need to take the bdrv graph read lock before flushing.

* hmp_info_balloon was moved to a different file.

* Needed to include a new headers from time to time to still get the
correct functions.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
2023-05-22 15:09:14 +02:00

191 lines
6.4 KiB
Diff
Raw Blame History

From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Fiona Ebner <f.ebner@proxmox.com>
Date: Fri, 5 May 2023 13:39:53 +0200
Subject: [PATCH] migration: for snapshots, hold the BQL during setup callbacks
In spirit, this is a partial revert of commit 9b09503752 ("migration:
run setup callbacks out of big lock"), but only for the snapshot case.
For snapshots, the bdrv_writev_vmstate() function is used during setup
(in QIOChannelBlock backing the QEMUFile), but not holding the BQL
while calling it could lead to an assertion failure. To understand
how, first note the following:
1. Generated coroutine wrappers for block layer functions spawn the
coroutine and use AIO_WAIT_WHILE()/aio_poll() to wait for it.
2. If the host OS switches threads at an inconvenient time, it can
happen that a bottom half scheduled for the main thread's AioContext
is executed as part of a vCPU thread's aio_poll().
An example leading to the assertion failure is as follows:
main thread:
1. A snapshot-save QMP command gets issued.
2. snapshot_save_job_bh() is scheduled.
vCPU thread:
3. aio_poll() for the main thread's AioContext is called (e.g. when
the guest writes to a pflash device, as part of blk_pwrite which is a
generated coroutine wrapper).
4. snapshot_save_job_bh() is executed as part of aio_poll().
3. qemu_savevm_state() is called.
4. qemu_mutex_unlock_iothread() is called. Now
qemu_get_current_aio_context() returns 0x0.
5. bdrv_writev_vmstate() is executed during the usual savevm setup.
But this function is a generated coroutine wrapper, so it uses
AIO_WAIT_WHILE. There, the assertion
assert(qemu_get_current_aio_context() == qemu_get_aio_context());
will fail.
To fix it, ensure that the BQL is held during setup. To avoid changing
the behavior for migration too, introduce conditionals for the setup
callbacks that need the BQL and only take the lock if it's not already
held.
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
---
include/migration/register.h | 2 +-
migration/block-dirty-bitmap.c | 15 ++++++++++++---
migration/block.c | 15 ++++++++++++---
migration/ram.c | 16 +++++++++++++---
migration/savevm.c | 2 --
5 files changed, 38 insertions(+), 12 deletions(-)
diff --git a/include/migration/register.h b/include/migration/register.h
index a8dfd8fefd..fa9b0b0f10 100644
--- a/include/migration/register.h
+++ b/include/migration/register.h
@@ -43,9 +43,9 @@ typedef struct SaveVMHandlers {
* by other locks.
*/
int (*save_live_iterate)(QEMUFile *f, void *opaque);
+ int (*save_setup)(QEMUFile *f, void *opaque);
/* This runs outside the iothread lock! */
- int (*save_setup)(QEMUFile *f, void *opaque);
/* Note for save_live_pending:
* must_precopy:
* - must be migrated in precopy or in stopped state
diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c
index a6440929fa..69fab3275c 100644
--- a/migration/block-dirty-bitmap.c
+++ b/migration/block-dirty-bitmap.c
@@ -1214,10 +1214,17 @@ static int dirty_bitmap_save_setup(QEMUFile *f, void *opaque)
{
DBMSaveState *s = &((DBMState *)opaque)->save;
SaveBitmapState *dbms = NULL;
+ bool release_lock = false;
- qemu_mutex_lock_iothread();
+ /* For snapshots, the BQL is held during setup. */
+ if (!qemu_mutex_iothread_locked()) {
+ qemu_mutex_lock_iothread();
+ release_lock = true;
+ }
if (init_dirty_bitmap_migration(s) < 0) {
- qemu_mutex_unlock_iothread();
+ if (release_lock) {
+ qemu_mutex_unlock_iothread();
+ }
return -1;
}
@@ -1225,7 +1232,9 @@ static int dirty_bitmap_save_setup(QEMUFile *f, void *opaque)
send_bitmap_start(f, s, dbms);
}
qemu_put_bitmap_flags(f, DIRTY_BITMAP_MIG_FLAG_EOS);
- qemu_mutex_unlock_iothread();
+ if (release_lock) {
+ qemu_mutex_unlock_iothread();
+ }
return 0;
}
diff --git a/migration/block.c b/migration/block.c
index b2497bbd32..c9d55be642 100644
--- a/migration/block.c
+++ b/migration/block.c
@@ -716,21 +716,30 @@ static void block_migration_cleanup(void *opaque)
static int block_save_setup(QEMUFile *f, void *opaque)
{
int ret;
+ bool release_lock = false;
trace_migration_block_save("setup", block_mig_state.submitted,
block_mig_state.transferred);
- qemu_mutex_lock_iothread();
+ /* For snapshots, the BQL is held during setup. */
+ if (!qemu_mutex_iothread_locked()) {
+ qemu_mutex_lock_iothread();
+ release_lock = true;
+ }
ret = init_blk_migration(f);
if (ret < 0) {
- qemu_mutex_unlock_iothread();
+ if (release_lock) {
+ qemu_mutex_unlock_iothread();
+ }
return ret;
}
/* start track dirty blocks */
ret = set_dirty_tracking();
- qemu_mutex_unlock_iothread();
+ if (release_lock) {
+ qemu_mutex_unlock_iothread();
+ }
if (ret) {
return ret;
diff --git a/migration/ram.c b/migration/ram.c
index 79d881f735..0ecbbc3202 100644
--- a/migration/ram.c
+++ b/migration/ram.c
@@ -3117,8 +3117,16 @@ static void migration_bitmap_clear_discarded_pages(RAMState *rs)
static void ram_init_bitmaps(RAMState *rs)
{
- /* For memory_global_dirty_log_start below. */
- qemu_mutex_lock_iothread();
+ bool release_lock = false;
+
+ /*
+ * For memory_global_dirty_log_start below.
+ * For snapshots, the BQL is held during setup.
+ */
+ if (!qemu_mutex_iothread_locked()) {
+ qemu_mutex_lock_iothread();
+ release_lock = true;
+ }
qemu_mutex_lock_ramlist();
WITH_RCU_READ_LOCK_GUARD() {
@@ -3130,7 +3138,9 @@ static void ram_init_bitmaps(RAMState *rs)
}
}
qemu_mutex_unlock_ramlist();
- qemu_mutex_unlock_iothread();
+ if (release_lock) {
+ qemu_mutex_unlock_iothread();
+ }
/*
* After an eventual first bitmap sync, fixup the initial bitmap
diff --git a/migration/savevm.c b/migration/savevm.c
index aa54a67fda..fc6a82a555 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -1621,10 +1621,8 @@ static int qemu_savevm_state(QEMUFile *f, Error **errp)
memset(&compression_counters, 0, sizeof(compression_counters));
ms->to_dst_file = f;
- qemu_mutex_unlock_iothread();
qemu_savevm_state_header(f);
qemu_savevm_state_setup(f);
- qemu_mutex_lock_iothread();
while (qemu_file_get_error(f) == 0) {
if (qemu_savevm_state_iterate(f, false) > 0) {
Reference in New Issue View Git Blame Copy Permalink