Re: [PATCH 3/3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

["Eric W. Biederman" <ebiederm@xmission.com>]

"Michael S. Tsirkin" <mst@redhat.com> writes:

> On Sun, May 21, 2023 at 09:51:24PM -0500, Mike Christie wrote:
>> When switching from kthreads to vhost_tasks two bugs were added:
>> 1. The vhost worker tasks's now show up as processes so scripts doing ps
>> or ps a would not incorrectly detect the vhost task as another process.
>> 2. kthreads disabled freeze by setting PF_NOFREEZE, but vhost tasks's
>> didn't disable or add support for them.
>> 
>> To fix both bugs, this switches the vhost task to be thread in the
>> process that does the VHOST_SET_OWNER ioctl, and has vhost_worker call
>> get_signal to support SIGKILL/SIGSTOP and freeze signals. Note that
>> SIGKILL/STOP support is required because CLONE_THREAD requires
>> CLONE_SIGHAND which requires those 2 signals to be suppported.
>> 
>> This a modified version of patch originally written by Linus which
>> handles his review comment to himself to rename ignore_signals to
>> block_signals to better represent what it now does. And it includes a
>> change to vhost_worker() to support SIGSTOP/KILL and freeze, and it
>> drops the wait use per Oleg's review comment that it's no longer needed
>> when using CLONE_THREAD.
>> 
>> Fixes: 6e890c5d5021 ("vhost: use vhost_tasks for worker threads")
>> Signed-off-by: Mike Christie <michael.christie@oracle.com>
>> ---
>>  drivers/vhost/vhost.c      | 17 ++++++++++++++++-
>>  include/linux/sched/task.h |  2 +-
>>  kernel/fork.c              | 12 +++---------
>>  kernel/signal.c            |  1 +
>>  kernel/vhost_task.c        | 16 ++++------------
>>  5 files changed, 25 insertions(+), 23 deletions(-)
>> 
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index a92af08e7864..bf83e9340e40 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -338,6 +338,7 @@ static int vhost_worker(void *data)
>>  	struct vhost_worker *worker = data;
>>  	struct vhost_work *work, *work_next;
>>  	struct llist_node *node;
>> +	bool dead = false;
>>  
>>  	for (;;) {
>>  		/* mb paired w/ kthread_stop */
>> @@ -349,8 +350,22 @@ static int vhost_worker(void *data)
>>  		}
>>  
>>  		node = llist_del_all(&worker->work_list);
>> -		if (!node)
>> +		if (!node) {
>>  			schedule();
>> +			/*
>> +			 * When we get a SIGKILL our release function will
>> +			 * be called. That will stop new IOs from being queued
>> +			 * and check for outstanding cmd responses. It will then
>> +			 * call vhost_task_stop to tell us to return and exit.
>> +			 */
>> +			if (!dead && signal_pending(current)) {
>> +				struct ksignal ksig;
>> +
>> +				dead = get_signal(&ksig);
>> +				if (dead)
>> +					clear_thread_flag(TIF_SIGPENDING);
>
>
> Does get_signal actually return true only on SIGKILL then?

get_signal returns the signal that was dequeued, or 0 if no signal was
dequeued.

For these threads that block all but SIGSTOP and SIGKILL get_signal
should properly never return any signal.  As SIGSTOP and SIGKILL are
handled internally to get_signal.

However get_signal was modified to have a special case for io_uring
and now the vhost code so that extra cleanup can be performed, before
do_exit is called on the thread.  That special case causes get_signal
to return when with the value of SIGKILL when the process exits.

The process can exit with do_group_exit aka exit(2) or any fatal signal
not just SIGKILL, and get_signal on these threads will return.

Eric