linux-kernel - Re: [PATCH v5 2/2] printk: Hide console waiter logic into helpers

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Date:   Thu, 11 Jan 2018 13:03:41 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        rostedt@...e.goodmis.org, Byungchul Park <byungchul.park@....com>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tejun Heo <tj@...nel.org>, Pavel Machek <pavel@....cz>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 2/2] printk: Hide console waiter logic into helpers

On Wed 2018-01-10 12:52:20, Steven Rostedt wrote:
> On Wed, 10 Jan 2018 14:24:18 +0100
> Petr Mladek <pmladek@...e.com> wrote:
> 
> > The commit ("printk: Add console owner and waiter logic to load balance
> > console writes") made vprintk_emit() and console_unlock() even more
> > complicated.
> > 
> > This patch extracts the new code into 3 helper functions. They should
> > help to keep it rather self-contained. It will be easier to use and
> > maintain.
> > 
> > This patch just shuffles the existing code. It does not change
> > the functionality.
> > 
> > Signed-off-by: Petr Mladek <pmladek@...e.com>
> > ---
> >  kernel/printk/printk.c | 242 +++++++++++++++++++++++++++++--------------------
> >  1 file changed, 145 insertions(+), 97 deletions(-)
> > 
> > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> > index 7e6459abba43..6217c280e6c1 100644
> > --- a/kernel/printk/printk.c
> > +++ b/kernel/printk/printk.c
> > @@ -86,15 +86,8 @@ EXPORT_SYMBOL_GPL(console_drivers);
> >  static struct lockdep_map console_lock_dep_map = {
> >  	.name = "console_lock"
> >  };
> > -static struct lockdep_map console_owner_dep_map = {
> > -	.name = "console_owner"
> > -};
> >  #endif
> >  
> > -static DEFINE_RAW_SPINLOCK(console_owner_lock);
> > -static struct task_struct *console_owner;
> > -static bool console_waiter;
> > -
> >  enum devkmsg_log_bits {
> >  	__DEVKMSG_LOG_BIT_ON = 0,
> >  	__DEVKMSG_LOG_BIT_OFF,
> > @@ -1551,6 +1544,143 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
> >  }
> >  
> >  /*
> > + * Special console_lock variants that help to reduce the risk of soft-lockups.
> > + * They allow to pass console_lock to another printk() call using a busy wait.
> > + */
> > +
> > +#ifdef CONFIG_LOCKDEP
> > +static struct lockdep_map console_owner_dep_map = {
> > +	.name = "console_owner"
> > +};
> > +#endif
> > +
> > +static DEFINE_RAW_SPINLOCK(console_owner_lock);
> > +static struct task_struct *console_owner;
> > +static bool console_waiter;
> > +
> > +/**
> > + * console_lock_spinning_enable - mark beginning of code where another
> > + *	thread might safely busy wait
> > + *
> > + * This might be called in sections where the current console_lock owner
> 
> 
> "might be"? It has to be called in sections where the current
> console_lock owner can not sleep. It's basically saying "console lock is
> now acting like a spinlock".

I am afraid that both explanations are confusing. Your one sounds like
it must be called every time we enter non-preemptive context in
console_unlock. What about the following?

 * This is basically saying that "console lock is now acting like
 * a spinlock". It can be called _only_ in sections where the current
 * console_lock owner could not sleep. Also it must be ready to hand
 * over the lock at the end of the section.

> > + * cannot sleep. It is a signal that another thread might start busy
> > + * waiting for console_lock.
> > + */

All the other changes look good to me. I will use them in the next version.

Best Regards,
Petr