From: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
linux-kernel@vger.kernel.org, Petr Mladek <pmladek@suse.com>,
Steven Rostedt <rostedt@goodmis.org>,
Daniel Wang <wonderfly@google.com>,
Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
Alan Cox <gnomes@lxorguk.ukuu.org.uk>,
Jiri Slaby <jslaby@suse.com>, Peter Feiner <pfeiner@google.com>,
linux-serial@vger.kernel.org,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Subject: Re: [RFC][PATCHv2 2/4] printk: move printk_safe macros to printk header
Date: Wed, 17 Oct 2018 22:36:38 +0900 [thread overview]
Message-ID: <20181017133638.GA426@tigerII.localdomain> (raw)
In-Reply-To: <20181017075728.GF3121@hirez.programming.kicks-ass.net>
On (10/17/18 09:57), Peter Zijlstra wrote:
> On Wed, Oct 17, 2018 at 01:32:51PM +0900, Sergey Senozhatsky wrote:
> > This probably will be a bit more hairy. logbuf is written to by many
> > sources and is read from by many sides, including user-space [both read()
> > and write()]. So we will need more flags/magic around memcpy(). A simple,
> > "grab the logbuf entry, set the proper offset to point to the next available
> > logbuf record and then do memcpy()" won't suffice. We need a flag for
> > "memcpy() complete, we can read this entry". Otherwise:
>
> Sure, but lockless buffers mostly have reserve and commit stages anyway.
> Exactly to avoid that problem.
Right. So it should be a lockless ringbuffer supporting multiple
parallel readers and multiple parallel writers from multiple contexts
(task, sortirq, irq, NMI); with sane wrap around, etc. And we probably
need to somehow keep the existing format of logbuf entries to avoid
breakages in crash tool, and so on. It will take a lot of time, I'm
afraid.
> > All right. OK. So we are on the same page here:
>
> > - Have more opinions on this. People please speak out.
> > - Have clear "let's do it" from Cc-ed people.
> >
> >
> > If we are really doing this, then let's split it and have
> > incremental changes. Namely, what I suggest is:
>
> I'd start by replacing logbuf with the lockless buffer and ripping out
> the current nmi/safe/etc.. bollocks.
>
> There is absolutely no point what so ever in doing anything until that
> is sorted.
Peter, you have your point, however, I think I have a slightly different
priority list. On the top of my list - deadlocks in printk(). That's the
whole reason I sent this series. Deadlocks are real. We have real reports;
in the best case in form of lockdep splats; in the worst case - nothing,
simply because the system deadlocks in printk(), and even more - printk()
can deadlock in panic(). So addressing deadlocks in printk() will fix a
rather big, real problem. Then we can have fun with the rest of the things
you have mentioned. What do you think?
-ss
next prev parent reply other threads:[~2018-10-17 13:36 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-10-16 5:04 [RFC][PATCHv2 0/4] less deadlock prone serial consoles Sergey Senozhatsky
2018-10-16 5:04 ` [RFC][PATCHv2 1/4] panic: avoid deadlocks in re-entrant console drivers Sergey Senozhatsky
2018-10-17 4:48 ` Sergey Senozhatsky
2018-10-23 11:07 ` Petr Mladek
2018-10-23 11:54 ` Sergey Senozhatsky
2018-10-23 12:04 ` Sergey Senozhatsky
2018-10-23 12:12 ` Sergey Senozhatsky
2018-10-25 9:06 ` Petr Mladek
2018-10-25 9:31 ` Sergey Senozhatsky
2018-10-25 8:29 ` Petr Mladek
2018-10-25 9:05 ` Sergey Senozhatsky
2018-10-25 10:10 ` [PATCHv3] " Sergey Senozhatsky
2018-10-25 10:51 ` kbuild test robot
2018-10-25 11:56 ` Sergey Senozhatsky
2018-10-31 12:27 ` Petr Mladek
2018-11-01 1:48 ` Sergey Senozhatsky
2018-11-01 8:08 ` Petr Mladek
2018-11-22 13:12 ` Petr Mladek
2018-12-12 0:53 ` Daniel Wang
2018-12-12 5:23 ` Sergey Senozhatsky
2018-12-12 5:59 ` Daniel Wang
2018-12-12 6:06 ` Sergey Senozhatsky
2018-12-12 6:09 ` Daniel Wang
2018-10-16 5:04 ` [RFC][PATCHv2 2/4] printk: move printk_safe macros to printk header Sergey Senozhatsky
2018-10-16 7:27 ` Peter Zijlstra
2018-10-16 11:40 ` Petr Mladek
2018-10-16 12:17 ` Peter Zijlstra
2018-10-17 10:50 ` Petr Mladek
2018-10-17 14:00 ` Peter Zijlstra
2018-10-22 14:30 ` Petr Mladek
2018-10-16 12:27 ` Sergey Senozhatsky
2018-10-16 12:38 ` Peter Zijlstra
2018-10-16 12:54 ` Peter Zijlstra
2018-10-16 14:21 ` Peter Zijlstra
2018-10-17 4:32 ` Sergey Senozhatsky
2018-10-17 7:57 ` Peter Zijlstra
2018-10-17 13:36 ` Sergey Senozhatsky [this message]
2018-10-23 6:25 ` Sergey Senozhatsky
2018-10-16 5:04 ` [RFC][PATCHv2 3/4] serial: introduce uart_port locking helpers Sergey Senozhatsky
2018-12-08 3:12 ` Sergey Senozhatsky
2018-12-12 11:08 ` Greg Kroah-Hartman
2018-10-16 5:04 ` [RFC][PATCHv2 4/4] tty: 8250: switch to " Sergey Senozhatsky
2018-10-16 7:23 ` [RFC][PATCHv2 0/4] less deadlock prone serial consoles Peter Zijlstra
2018-10-16 8:12 ` Sergey Senozhatsky
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