linux-kernel - Re: devkmsg: was [PATCH next v2 3/3] printk: remove logbuf_lock, add syslog

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Message-ID: <X838OXY46bA2ozuo@alley>
Date:   Mon, 7 Dec 2020 10:56:09 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: devkmsg: was [PATCH next v2 3/3] printk: remove logbuf_lock, add
 syslog_lock

On Sun 2020-12-06 21:57:46, John Ogness wrote:
> On 2020-12-04, Petr Mladek <pmladek@...e.com> wrote:
> >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> >> index e9018c4e1b66..7385101210be 100644
> >> --- a/kernel/printk/printk.c
> >> +++ b/kernel/printk/printk.c
> >> @@ -785,7 +749,6 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence)
> >>  	if (offset)
> >>  		return -ESPIPE;
> >>  
> >> -	logbuf_lock_irq();
> >
> > user->seq manipulation is not longer safe from the atomicity point of
> > view.
> >
> > One solution would be to use atomic variable in struct devkmsg_user().
> > Another solution would be to synchronize it with user->lock like we do
> > in devkmsg_read().
> >
> > user->lock looks like an overhead. But it actually would make sense to
> > prevent seek in the middle of a read.
> 
> I would prefer using atomic64_t. Using user->lock could introduce some
> wacky regression.

OK, fair enough. User space might do crazy stuff.

Best Regards,
Petr