linux-kernel - Re: [PATCH v2 4/4] PM: hibernate: don't set PF_FREEZER_SKIP flag when manipulating /dev/snapshot

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Date:   Fri, 15 Jul 2022 19:46:15 +0200
From:   "Rafael J. Wysocki" <rafael@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Greg KH <gregkh@...uxfoundation.org>,
        Oliver Neukum <oneukum@...e.com>,
        Wedson Almeida Filho <wedsonaf@...gle.com>,
        "Rafael J. Wysocki" <rjw@...k.pl>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Len Brown <len.brown@...el.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 4/4] PM: hibernate: don't set PF_FREEZER_SKIP flag when
 manipulating /dev/snapshot

On Sun, Jul 10, 2022 at 4:26 AM Tetsuo Handa
<penguin-kernel@...ove.sakura.ne.jp> wrote:
>
> Since khungtaskd skips threads with PF_FREEZER_SKIP flag set, currently
> we can't report unbounded uninterruptible sleep when something went wrong
> while manipulating /dev/snapshot interface.
>
> Let's change snapshot_{open,read,write}() to use mutex_lock_killable()
> and change snapshot_release() to use mutex_lock(), so that khungtaskd can
> report unbounded uninterruptible sleep, by not setting PF_FREEZER_SKIP
> flag.
>
> Since /dev/snapshot is exclusive due to hibernate_acquire(), we could
> choose mutex_trylock() for snapshot_{open,read,write}() as with
> snapshot_ioctl(). But until we confirm that this patch does not
> break something, let's stay mutex_lock_killable().
>
> Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
> Cc: "Rafael J. Wysocki" <rafael@...nel.org>
> Cc: Len Brown <len.brown@...el.com>
> Cc: Pavel Machek <pavel@....cz>
> ---
> This patch is only compile tested. Need to review if somewhere depends
> on PF_FREEZER_SKIP flag being set.

Yes, it does.  The process operating the snapshot device cannot be
frozen, which is why it sets PF_FREEZER_SKIP in the first place.

>
>  kernel/power/user.c | 19 +++++++++++--------
>  1 file changed, 11 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/power/user.c b/kernel/power/user.c
> index 32dd5a855e8c..9936efa07022 100644
> --- a/kernel/power/user.c
> +++ b/kernel/power/user.c
> @@ -68,7 +68,8 @@ static int snapshot_open(struct inode *inode, struct file *filp)
>                 break;
>         }
>
> -       lock_system_sleep();
> +       if (mutex_lock_killable(&system_transition_mutex))
> +               return -EINTR;
>
>         if (!hibernate_acquire()) {
>                 error = -EBUSY;
> @@ -102,7 +103,7 @@ static int snapshot_open(struct inode *inode, struct file *filp)
>         data->dev = 0;
>
>   Unlock:
> -       unlock_system_sleep();
> +       mutex_unlock(&system_transition_mutex);
>
>         return error;
>  }
> @@ -111,7 +112,7 @@ static int snapshot_release(struct inode *inode, struct file *filp)
>  {
>         struct snapshot_data *data;
>
> -       lock_system_sleep();
> +       mutex_lock(&system_transition_mutex);
>
>         swsusp_free();
>         data = filp->private_data;
> @@ -128,7 +129,7 @@ static int snapshot_release(struct inode *inode, struct file *filp)
>                         PM_POST_HIBERNATION : PM_POST_RESTORE);
>         hibernate_release();
>
> -       unlock_system_sleep();
> +       mutex_unlock(&system_transition_mutex);
>
>         return 0;
>  }
> @@ -140,7 +141,8 @@ static ssize_t snapshot_read(struct file *filp, char __user *buf,
>         ssize_t res;
>         loff_t pg_offp = *offp & ~PAGE_MASK;
>
> -       lock_system_sleep();
> +       if (mutex_lock_killable(&system_transition_mutex))
> +               return -EINTR;
>
>         data = filp->private_data;
>         if (!data->ready) {
> @@ -161,7 +163,7 @@ static ssize_t snapshot_read(struct file *filp, char __user *buf,
>                 *offp += res;
>
>   Unlock:
> -       unlock_system_sleep();
> +       mutex_unlock(&system_transition_mutex);
>
>         return res;
>  }
> @@ -173,7 +175,8 @@ static ssize_t snapshot_write(struct file *filp, const char __user *buf,
>         ssize_t res;
>         loff_t pg_offp = *offp & ~PAGE_MASK;
>
> -       lock_system_sleep();
> +       if (mutex_lock_killable(&system_transition_mutex))
> +               return -EINTR;
>
>         data = filp->private_data;
>
> @@ -195,7 +198,7 @@ static ssize_t snapshot_write(struct file *filp, const char __user *buf,
>         if (res > 0)
>                 *offp += res;
>  unlock:
> -       unlock_system_sleep();
> +       mutex_unlock(&system_transition_mutex);
>
>         return res;
>  }
> --
> 2.18.4
>