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migration: Fix blocking in POSTCOPY_DEVICE during package load

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The package_loaded event is not set in case MIG_RP_MSG_PONG does not
arrive on the source from the destination in the return path thread. The
migration thread would then be blocked waiting for package_loaded event
indefinitely in POSTCOPY_DEVICE state. Where as, in such a condition the
source VM can safely resume as the destination has not yet started. The
pong message can get lost in case of a network failure or destination
crash before sending the pong.

This patch removes the package_loaded event and uses rp_sem, instead of
kicking multiple events. The error is detected in case of network
failure or destination crash and rp_sem is set in the out path of the
return path thread. This will kick the migration thread out from a
condition of indefinitely waiting for rp_sem. The migration thread then
fails early and breaks from the migration loop to resume the vm on the
source side.

Fixes: 7b842fe354 ("migration: Introduce POSTCOPY_DEVICE state")
Signed-off-by: Pranav Tyagi <prtyagi@redhat.com>
Reviewed-by: Juraj Marcin <jmarcin@redhat.com>
Reviewed-by: Peter Xu <peterx@redhat.com>
Link: https://lore.kernel.org/r/20260423094438.43556-1-prtyagi@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>
This commit is contained in:
Pranav Tyagi
2026-04-23 15:14:38 +05:30
committed by Peter Xu
parent ac0cc20ad2
commit 1ae7d9c30d
2 changed files with 31 additions and 18 deletions
Download Patch File Download Diff File Expand all files Collapse all files

48
migration/migration.c
View File

@@ -1661,7 +1661,6 @@ int migrate_init(MigrationState *s, Error **errp)
migration_reset_vfio_bytes_transferred();
s->postcopy_package_loaded = false;
qemu_event_reset(&s->postcopy_package_loaded_event);
return 0;
}
@@ -2317,7 +2316,7 @@ static void *source_return_path_thread(void *opaque)
if (tmp32 == QEMU_VM_PING_PACKAGED_LOADED) {
trace_source_return_path_thread_postcopy_package_loaded();
ms->postcopy_package_loaded = true;
qemu_event_set(&ms->postcopy_package_loaded_event);
migration_rp_kick(ms);
}
break;
@@ -2388,16 +2387,21 @@ out:
trace_source_return_path_thread_bad_end();
}
if (ms->state == MIGRATION_STATUS_POSTCOPY_RECOVER) {
if (ms->state == MIGRATION_STATUS_POSTCOPY_RECOVER ||
ms->state == MIGRATION_STATUS_POSTCOPY_DEVICE) {
/*
* this will be extremely unlikely: that we got yet another network
* issue during recovering of the 1st network failure.. during this
* period the main migration thread can be waiting on rp_sem for
* this thread to sync with the other side.
* The migration thread can get stuck waiting for rp_sem if the
* return path fails to sync with the destination. This handles
* two specific cases:
*
* When this happens, explicitly kick the migration thread out of
* RECOVER stage and back to PAUSED, so the admin can try
* everything again.
* POSTCOPY_RECOVER: A failure occurs during a recovery attempt.
* We kick the migration thread back to PAUSED so the admin can
* retry.
*
* POSTCOPY_DEVICE: The MIG_RP_MSG_PONG is lost due to a
* network failure or destination crash. We kick the migration
* thread out of its wait so it can fail the migration and safely
* resume the VM on the source.
*/
migration_rp_kick(ms);
}
@@ -3226,12 +3230,24 @@ static MigIterateState migration_iteration_run(MigrationState *s)
if (s->state == MIGRATION_STATUS_POSTCOPY_DEVICE &&
(s->postcopy_package_loaded || complete_ready)) {
/*
* If package has been loaded, the event is set and we will
* immediatelly transition to POSTCOPY_ACTIVE. If we are ready for
* completion, we need to wait for destination to load the postcopy
* package before actually completing.
* We will immediately transition to POSTCOPY_ACTIVE.
* If we are ready for completion, we need to wait for
* destination to load the postcopy package before actually
* completing.
*/
qemu_event_wait(&s->postcopy_package_loaded_event);
while (!s->postcopy_package_loaded) {
if (migration_rp_wait(s)) {
/*
* Error happened. Migration thread was stuck waiting in
* POSTCOPY_DEVICE for rp_sem which was never set.
*/
migrate_set_state(&s->state,
MIGRATION_STATUS_POSTCOPY_DEVICE,
MIGRATION_STATUS_FAILING);
return MIG_ITERATE_BREAK;
}
}
/* Acknowledgement received from the destination */
migrate_set_state(&s->state, MIGRATION_STATUS_POSTCOPY_DEVICE,
MIGRATION_STATUS_POSTCOPY_ACTIVE);
}
@@ -3863,7 +3879,6 @@ static void migration_instance_finalize(Object *obj)
qemu_sem_destroy(&ms->rp_state.rp_pong_acks);
qemu_sem_destroy(&ms->postcopy_qemufile_src_sem);
error_free(ms->error);
qemu_event_destroy(&ms->postcopy_package_loaded_event);
}
static void migration_instance_init(Object *obj)
@@ -3885,7 +3900,6 @@ static void migration_instance_init(Object *obj)
qemu_sem_init(&ms->wait_unplug_sem, 0);
qemu_sem_init(&ms->postcopy_qemufile_src_sem, 0);
qemu_mutex_init(&ms->qemu_file_lock);
qemu_event_init(&ms->postcopy_package_loaded_event, 0);
}
/*

1
migration/migration.h
View File

@@ -512,7 +512,6 @@ struct MigrationState {
bool rdma_migration;
bool postcopy_package_loaded;
QemuEvent postcopy_package_loaded_event;
GSource *hup_source;