linux-kernel - Re: [RFC PATCH] pstore: Switch pmsg_lock to an rt

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Date:   Wed, 14 Dec 2022 14:53:27 -0800
From:   Kees Cook <keescook@...omium.org>
To:     John Stultz <jstultz@...gle.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, Wei Wang <wvw@...gle.com>,
        Midas Chien <midaschieh@...gle.com>,
        Connor O'Brien <connoro@...gle.com>,
        Anton Vorontsov <anton@...msg.org>,
        Colin Cross <ccross@...roid.com>,
        Tony Luck <tony.luck@...el.com>, kernel-team@...roid.com
Subject: Re: [RFC PATCH] pstore: Switch pmsg_lock to an rt_mutex to avoid
 priority inversion

On Mon, Dec 05, 2022 at 08:32:53PM +0000, John Stultz wrote:
> Wei Wang reported seeing priority inversion caused latencies
> caused by contention on pmsg_lock, and suggested it be switched
> to a rt_mutex.
> 
> I was initially hesitant this would help, as the tasks in that
> trace all seemed to be SCHED_NORMAL, so the benefit would be
> limited to only nice boosting.
> 
> However, another similar issue was raised where the priority
> inversion was seen did involve a blocked RT task so it is clear
> this would be helpful in that case.
> 
> Feedback would be appreciate!

This looks fine to me. Is there an appropriate "Fixes:" tag that could
be used?

-Kees

> 
> Cc: Wei Wang <wvw@...gle.com>
> Cc: Midas Chien<midaschieh@...gle.com>
> Cc: Connor O'Brien <connoro@...gle.com>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Anton Vorontsov <anton@...msg.org>
> Cc: Colin Cross <ccross@...roid.com>
> Cc: Tony Luck <tony.luck@...el.com>
> Cc: kernel-team@...roid.com
> Reported-by: Wei Wang <wvw@...gle.com>
> Signed-off-by: John Stultz <jstultz@...gle.com>
> ---
>  fs/pstore/pmsg.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/pstore/pmsg.c b/fs/pstore/pmsg.c
> index d8542ec2f38c..18cf94b597e0 100644
> --- a/fs/pstore/pmsg.c
> +++ b/fs/pstore/pmsg.c
> @@ -7,9 +7,10 @@
>  #include <linux/device.h>
>  #include <linux/fs.h>
>  #include <linux/uaccess.h>
> +#include <linux/rtmutex.h>
>  #include "internal.h"
>  
> -static DEFINE_MUTEX(pmsg_lock);
> +static DEFINE_RT_MUTEX(pmsg_lock);
>  
>  static ssize_t write_pmsg(struct file *file, const char __user *buf,
>  			  size_t count, loff_t *ppos)
> @@ -28,9 +29,9 @@ static ssize_t write_pmsg(struct file *file, const char __user *buf,
>  	if (!access_ok(buf, count))
>  		return -EFAULT;
>  
> -	mutex_lock(&pmsg_lock);
> +	rt_mutex_lock(&pmsg_lock);
>  	ret = psinfo->write_user(&record, buf);
> -	mutex_unlock(&pmsg_lock);
> +	rt_mutex_unlock(&pmsg_lock);
>  	return ret ? ret : count;
>  }
>  
> -- 
> 2.39.0.rc0.267.gcb52ba06e7-goog
> 

-- 
Kees Cook