191 lines
6.4 KiB
Diff
191 lines
6.4 KiB
Diff
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
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From: Fiona Ebner <f.ebner@proxmox.com>
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Date: Fri, 5 May 2023 13:39:53 +0200
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Subject: [PATCH] migration: for snapshots, hold the BQL during setup callbacks
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In spirit, this is a partial revert of commit 9b09503752 ("migration:
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run setup callbacks out of big lock"), but only for the snapshot case.
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For snapshots, the bdrv_writev_vmstate() function is used during setup
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(in QIOChannelBlock backing the QEMUFile), but not holding the BQL
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while calling it could lead to an assertion failure. To understand
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how, first note the following:
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1. Generated coroutine wrappers for block layer functions spawn the
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coroutine and use AIO_WAIT_WHILE()/aio_poll() to wait for it.
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2. If the host OS switches threads at an inconvenient time, it can
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happen that a bottom half scheduled for the main thread's AioContext
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is executed as part of a vCPU thread's aio_poll().
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An example leading to the assertion failure is as follows:
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main thread:
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1. A snapshot-save QMP command gets issued.
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2. snapshot_save_job_bh() is scheduled.
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vCPU thread:
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3. aio_poll() for the main thread's AioContext is called (e.g. when
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the guest writes to a pflash device, as part of blk_pwrite which is a
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generated coroutine wrapper).
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4. snapshot_save_job_bh() is executed as part of aio_poll().
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3. qemu_savevm_state() is called.
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4. qemu_mutex_unlock_iothread() is called. Now
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qemu_get_current_aio_context() returns 0x0.
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5. bdrv_writev_vmstate() is executed during the usual savevm setup.
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But this function is a generated coroutine wrapper, so it uses
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AIO_WAIT_WHILE. There, the assertion
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assert(qemu_get_current_aio_context() == qemu_get_aio_context());
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will fail.
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To fix it, ensure that the BQL is held during setup. To avoid changing
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the behavior for migration too, introduce conditionals for the setup
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callbacks that need the BQL and only take the lock if it's not already
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held.
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Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
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---
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include/migration/register.h | 2 +-
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migration/block-dirty-bitmap.c | 15 ++++++++++++---
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migration/block.c | 15 ++++++++++++---
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migration/ram.c | 16 +++++++++++++---
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migration/savevm.c | 2 --
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5 files changed, 38 insertions(+), 12 deletions(-)
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diff --git a/include/migration/register.h b/include/migration/register.h
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index 90914f32f5..c728fd9120 100644
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--- a/include/migration/register.h
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+++ b/include/migration/register.h
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@@ -43,9 +43,9 @@ typedef struct SaveVMHandlers {
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* by other locks.
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*/
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int (*save_live_iterate)(QEMUFile *f, void *opaque);
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+ int (*save_setup)(QEMUFile *f, void *opaque);
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/* This runs outside the iothread lock! */
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- int (*save_setup)(QEMUFile *f, void *opaque);
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/* Note for save_live_pending:
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* must_precopy:
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* - must be migrated in precopy or in stopped state
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diff --git a/migration/block-dirty-bitmap.c b/migration/block-dirty-bitmap.c
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index 285dd1d148..f7ee5a74d9 100644
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--- a/migration/block-dirty-bitmap.c
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+++ b/migration/block-dirty-bitmap.c
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@@ -1219,10 +1219,17 @@ static int dirty_bitmap_save_setup(QEMUFile *f, void *opaque)
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{
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DBMSaveState *s = &((DBMState *)opaque)->save;
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SaveBitmapState *dbms = NULL;
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+ bool release_lock = false;
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- qemu_mutex_lock_iothread();
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+ /* For snapshots, the BQL is held during setup. */
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+ if (!qemu_mutex_iothread_locked()) {
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+ qemu_mutex_lock_iothread();
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+ release_lock = true;
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+ }
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if (init_dirty_bitmap_migration(s) < 0) {
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- qemu_mutex_unlock_iothread();
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+ if (release_lock) {
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+ qemu_mutex_unlock_iothread();
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+ }
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return -1;
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}
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@@ -1230,7 +1237,9 @@ static int dirty_bitmap_save_setup(QEMUFile *f, void *opaque)
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send_bitmap_start(f, s, dbms);
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}
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qemu_put_bitmap_flags(f, DIRTY_BITMAP_MIG_FLAG_EOS);
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- qemu_mutex_unlock_iothread();
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+ if (release_lock) {
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+ qemu_mutex_unlock_iothread();
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+ }
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return 0;
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}
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diff --git a/migration/block.c b/migration/block.c
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index 86c2256a2b..8423e0c9f9 100644
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--- a/migration/block.c
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+++ b/migration/block.c
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@@ -725,21 +725,30 @@ static void block_migration_cleanup(void *opaque)
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static int block_save_setup(QEMUFile *f, void *opaque)
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{
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int ret;
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+ bool release_lock = false;
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trace_migration_block_save("setup", block_mig_state.submitted,
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block_mig_state.transferred);
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- qemu_mutex_lock_iothread();
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+ /* For snapshots, the BQL is held during setup. */
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+ if (!qemu_mutex_iothread_locked()) {
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+ qemu_mutex_lock_iothread();
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+ release_lock = true;
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+ }
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ret = init_blk_migration(f);
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if (ret < 0) {
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- qemu_mutex_unlock_iothread();
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+ if (release_lock) {
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+ qemu_mutex_unlock_iothread();
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+ }
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return ret;
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}
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/* start track dirty blocks */
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ret = set_dirty_tracking();
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- qemu_mutex_unlock_iothread();
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+ if (release_lock) {
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+ qemu_mutex_unlock_iothread();
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+ }
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if (ret) {
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return ret;
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diff --git a/migration/ram.c b/migration/ram.c
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index 6e1514f69f..6a1aec7031 100644
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--- a/migration/ram.c
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+++ b/migration/ram.c
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@@ -2896,8 +2896,16 @@ static void migration_bitmap_clear_discarded_pages(RAMState *rs)
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static void ram_init_bitmaps(RAMState *rs)
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{
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- /* For memory_global_dirty_log_start below. */
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- qemu_mutex_lock_iothread();
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+ bool release_lock = false;
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+
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+ /*
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+ * For memory_global_dirty_log_start below.
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+ * For snapshots, the BQL is held during setup.
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+ */
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+ if (!qemu_mutex_iothread_locked()) {
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+ qemu_mutex_lock_iothread();
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+ release_lock = true;
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+ }
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qemu_mutex_lock_ramlist();
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WITH_RCU_READ_LOCK_GUARD() {
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@@ -2909,7 +2917,9 @@ static void ram_init_bitmaps(RAMState *rs)
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}
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}
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qemu_mutex_unlock_ramlist();
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- qemu_mutex_unlock_iothread();
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+ if (release_lock) {
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+ qemu_mutex_unlock_iothread();
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+ }
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/*
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* After an eventual first bitmap sync, fixup the initial bitmap
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diff --git a/migration/savevm.c b/migration/savevm.c
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index d60c4f487a..3c015722f7 100644
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--- a/migration/savevm.c
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+++ b/migration/savevm.c
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@@ -1625,10 +1625,8 @@ static int qemu_savevm_state(QEMUFile *f, Error **errp)
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reset_vfio_bytes_transferred();
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ms->to_dst_file = f;
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- qemu_mutex_unlock_iothread();
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qemu_savevm_state_header(f);
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qemu_savevm_state_setup(f);
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- qemu_mutex_lock_iothread();
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while (qemu_file_get_error(f) == 0) {
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if (qemu_savevm_state_iterate(f, false) > 0) {
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