linux-kernel - Re: [PATCH v3 2/6] freezer,umh: Clean up freezer/initrd interaction

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Date:   Thu, 14 Oct 2021 10:13:05 +0100
From:   Will Deacon <will@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, oleg@...hat.com, mingo@...nel.org,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, mgorman@...e.de, linux-kernel@...r.kernel.org,
        tj@...nel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3 2/6] freezer,umh: Clean up freezer/initrd interaction

On Sat, Oct 09, 2021 at 12:07:56PM +0200, Peter Zijlstra wrote:
> handle_initrd() marks itself as PF_FREEZER_SKIP in order to ensure
> that the UMH, which is going to freeze the system, doesn't
> indefinitely wait for it's caller.
> 
> Rework things by adding UMH_FREEZABLE to indicate the completion is
> freezable.
> 
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> ---
>  include/linux/umh.h     |    9 +++++----
>  init/do_mounts_initrd.c |   10 +---------
>  kernel/umh.c            |    8 ++++++++
>  3 files changed, 14 insertions(+), 13 deletions(-)

This looks much better to me:

Acked-by: Will Deacon <will@...nel.org>

Just a question on the old code:

> --- a/include/linux/umh.h
> +++ b/include/linux/umh.h
> @@ -11,10 +11,11 @@
>  struct cred;
>  struct file;
>  
> -#define UMH_NO_WAIT	0	/* don't wait at all */
> -#define UMH_WAIT_EXEC	1	/* wait for the exec, but not the process */
> -#define UMH_WAIT_PROC	2	/* wait for the process to complete */
> -#define UMH_KILLABLE	4	/* wait for EXEC/PROC killable */
> +#define UMH_NO_WAIT	0x00	/* don't wait at all */
> +#define UMH_WAIT_EXEC	0x01	/* wait for the exec, but not the process */
> +#define UMH_WAIT_PROC	0x02	/* wait for the process to complete */
> +#define UMH_KILLABLE	0x04	/* wait for EXEC/PROC killable */
> +#define UMH_FREEZABLE	0x08	/* wait for EXEC/PROC freezable */
>  
>  struct subprocess_info {
>  	struct work_struct work;
> --- a/init/do_mounts_initrd.c
> +++ b/init/do_mounts_initrd.c
> @@ -79,19 +79,11 @@ static void __init handle_initrd(void)
>  	init_mkdir("/old", 0700);
>  	init_chdir("/old");
>  
> -	/*
> -	 * In case that a resume from disk is carried out by linuxrc or one of
> -	 * its children, we need to tell the freezer not to wait for us.
> -	 */
> -	current->flags |= PF_FREEZER_SKIP;
> -
>  	info = call_usermodehelper_setup("/linuxrc", argv, envp_init,
>  					 GFP_KERNEL, init_linuxrc, NULL, NULL);
>  	if (!info)
>  		return;
> -	call_usermodehelper_exec(info, UMH_WAIT_PROC);
> -
> -	current->flags &= ~PF_FREEZER_SKIP;

How was this supposed to work if it raced with the freezer checking the
flag?

Will