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migration: use bdrv_drain_all_begin/end() instead bdrv_drain_all() 

blk/bdrv_drain_all() only takes effect for a single instant and then
resumes block jobs, guest devices, and other external clients like the
NBD server.  This can be handy when performing a synchronous drain
before terminating the program, for example.

Monitor commands usually need to quiesce I/O across an entire code
region so blk/bdrv_drain_all() is not suitable.  They must use
bdrv_drain_all_begin/end() to mark the region.  This prevents new I/O
requests from slipping in or worse - block jobs completing and modifying
the graph.

I audited other blk/bdrv_drain_all() callers but did not find anything
that needs a similar fix.  This patch fixes the savevm/loadvm commands.
Although I haven't encountered a read world issue this makes the code
safer.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Stefan Hajnoczi 2017-05-22 14:57:04 +01:00 committed by Kevin Wolf
parent 17e2a4a47d
commit 8649f2f9b2
1 changed files with 15 additions and 3 deletions
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18
migration/savevm.c
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@ -2107,6 +2107,8 @@ int save_snapshot(const char *name, Error **errp)
}
vm_stop(RUN_STATE_SAVE_VM);
bdrv_drain_all_begin();
aio_context_acquire(aio_context);
memset(sn, 0, sizeof(*sn));
@ -2165,6 +2167,9 @@ int save_snapshot(const char *name, Error **errp)
if (aio_context) {
aio_context_release(aio_context);
}
bdrv_drain_all_end();
if (saved_vm_running) {
vm_start();
}
@ -2273,20 +2278,21 @@ int load_snapshot(const char *name, Error **errp)
}
/* Flush all IO requests so they don't interfere with the new state. */
bdrv_drain_all();
bdrv_drain_all_begin();
ret = bdrv_all_goto_snapshot(name, &bs);
if (ret < 0) {
error_setg(errp, "Error %d while activating snapshot '%s' on '%s'",
ret, name, bdrv_get_device_name(bs));
return ret;
goto err_drain;
}
/* restore the VM state */
f = qemu_fopen_bdrv(bs_vm_state, 0);
if (!f) {
error_setg(errp, "Could not open VM state file");
return -EINVAL;
ret = -EINVAL;
goto err_drain;
}
qemu_system_reset(SHUTDOWN_CAUSE_NONE);
@ -2296,6 +2302,8 @@ int load_snapshot(const char *name, Error **errp)
ret = qemu_loadvm_state(f);
aio_context_release(aio_context);
bdrv_drain_all_end();
migration_incoming_state_destroy();
if (ret < 0) {
error_setg(errp, "Error %d while loading VM state", ret);
@ -2303,6 +2311,10 @@ int load_snapshot(const char *name, Error **errp)
}
return 0;
err_drain:
bdrv_drain_all_end();
return ret;
}
void vmstate_register_ram(MemoryRegion *mr, DeviceState *dev)