Re: [CFT][PATCH v3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression

From: Oleg Nesterov <oleg@redhat.com>
To: Mike Christie <michael.christie@oracle.com>
Cc: axboe@kernel.dk, brauner@kernel.org, mst@redhat.com,
	linux-kernel@vger.kernel.org, linux@leemhuis.info,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	stefanha@redhat.com, nicolas.dichtel@6wind.com,
	virtualization@lists.linux-foundation.org,
	torvalds@linux-foundation.org
Subject: Re: [CFT][PATCH v3] fork, vhost: Use CLONE_THREAD to fix freezer/ps regression
Date: Tue, 6 Jun 2023 14:16:44 +0200	[thread overview]
Message-ID: <20230606121643.GD7542@redhat.com> (raw)
In-Reply-To: <03c07f48-8922-f563-560c-f0d4cc3e1279@oracle.com>

On 06/05, Mike Christie wrote:
>
> On 6/5/23 10:10 AM, Oleg Nesterov wrote:
> > On 06/03, michael.christie@oracle.com wrote:
> >>
> >> On 6/2/23 11:15 PM, Eric W. Biederman wrote:
> >> The problem is that as part of the flush the drivers/vhost/scsi.c code
> >> will wait for outstanding commands, because we can't free the device and
> >> it's resources before the commands complete or we will hit the accessing
> >> freed memory bug.
> >
> > ignoring send-fd/clone issues, can we assume that the final fput/release
> > should always come from vhost_worker's sub-thread (which shares mm/etc) ?
>
> I think I'm misunderstanding the sub-thread term.
>
> - Is it the task_struct's context that we did the
> kernel/vhost_taskc.c:vhost_task_create() from? Below it would be the
> thread we did VHOST_SET_OWNER from.

Yes,

> So it works like if we were using a kthread still:
>
> 1. Userapce thread0 opens /dev/vhost-$something.
> 2. thread0 does VHOST_SET_OWNER ioctl. This calls vhost_task_create() to
> create the task_struct which runs the vhost_worker() function which handles
> the work->fns.
> 3. If userspace now does a SIGKILL or just exits without doing a close() on
> /dev/vhost-$something, then when thread0 does exit_files() that will do the
> fput that does vhost-$something's file_operations->release.

So, at least in this simple case vhost_worker() can just exit after SIGKILL,
and thread0 can flush the outstanding commands when it calls vhost_dev_flush()
rather than wait for vhost_worker().

Right?

not that I think this can help in the general case ...

Oleg.

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