linux-kernel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Petr Mladek <pmladek@suse.com>
To: John Ogness <john.ogness@linutronix.de>
Cc: linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Sergey Senozhatsky <sergey.senozhatsky.work@gmail.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 PATCH v1 20/25] serial: 8250: implement write_atomic
Date: Fri, 8 Mar 2019 11:28:19 +0100	[thread overview]
Message-ID: <20190308102819.lirle4vlf7mdcetp@pathway.suse.cz> (raw)
In-Reply-To: <87a7i6ovt3.fsf@linutronix.de>

On Fri 2019-03-08 05:05:12, John Ogness wrote:
> On 2019-02-27, Petr Mladek <pmladek@suse.com> wrote:
> >>>> Implement a non-sleeping NMI-safe write_atomic console function in
> >>>> order to support emergency printk messages.
> >>>
> >>> OK, it would be safe when prb_lock() is the only lock taken
> >>> in the NMI handler.
> >> 
> >> Which is the case. As I wrote to you already [0], NMI contexts are
> >> _never_ allowed to do things that rely on waiting forever for other
> >> CPUs.
> >
> > Who says _never_? I agree that it is not reasonable. But the
> > history shows that it happens.
> 
> Right, which is why it would need to become policy.
> 
> The emergency messages (aka write_atomic) introduce a new requirement to
> the kernel because this callback must be callable from any context. The
> console drivers must have some way of synchronizing. The CPU-reentrant
> spin lock is the only solution I am aware of.

I am not in position to decide if this requirement is acceptable
or not. We would need an opinion from Linus at minimum.

But it is not acceptable from my point of view. Note that
many spinlocks might be safely used in NMI in principle.

You want to introduce a single spinlock just because
printk() could be called anywhere. Most other subsystems
do not have this problem because they a more self-contained.

> If the ringbuffer was fully lockless

Exactly, I think that a solution would be fully lockless logbuffer.


> we should be able to have per-console CPU-reentrant spin locks
> as long as the ordering is preserved, which I expect shouldn't
> be a problem. If any NMI context needed a spin lock for its own
> purposes, it would need to use the CPU-reentrant spin lock of
> the first console so as to preserve the ordering in case of a panic.

IMHO, this is not acceptable. It would be nice to have direct output
from NMI but the cost looks to high here.

A solution would be to defer the console to irq_work or so.
We might also consider using trylock in NMI and have
the irq_work just as a fallback.

Best Regars,
Petr

  parent reply	other threads:[~2019-03-08 10:28 UTC|newest]

Thread overview: 147+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-12 14:29 [RFC PATCH v1 00/25] printk: new implementation John Ogness
2019-02-12 14:29 ` [RFC PATCH v1 01/25] printk-rb: add printk ring buffer documentation John Ogness
2019-02-12 14:45   ` Greg Kroah-Hartman
2019-02-12 14:29 ` [RFC PATCH v1 02/25] printk-rb: add prb locking functions John Ogness
2019-02-13 15:45   ` Petr Mladek
2019-02-13 21:39     ` John Ogness
2019-02-14 10:33       ` Petr Mladek
2019-02-14 12:10         ` John Ogness
2019-02-15 10:26           ` Petr Mladek
2019-02-15 10:56             ` John Ogness
2019-03-07  2:12   ` Sergey Senozhatsky
2019-02-12 14:29 ` [RFC PATCH v1 03/25] printk-rb: define ring buffer struct and initializer John Ogness
2019-02-12 14:46   ` Greg Kroah-Hartman
2019-02-14 12:46     ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 04/25] printk-rb: add writer interface John Ogness
2019-02-14 15:16   ` Petr Mladek
2019-02-14 23:36     ` John Ogness
2019-02-15  1:19       ` John Ogness
2019-02-15 13:47       ` Petr Mladek
2019-02-17  1:32         ` John Ogness
2019-02-21 13:51           ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 05/25] printk-rb: add basic non-blocking reading interface John Ogness
2019-02-18 12:54   ` Petr Mladek
2019-02-19 21:44     ` John Ogness
2019-02-21 16:22       ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 06/25] printk-rb: add blocking reader support John Ogness
2019-02-18 14:05   ` Petr Mladek
2019-02-19 21:47     ` John Ogness
2019-02-12 14:29 ` [RFC PATCH v1 07/25] printk-rb: add functionality required by printk John Ogness
2019-02-12 17:15   ` Linus Torvalds
2019-02-13  9:20     ` John Ogness
2019-02-18 15:59   ` Petr Mladek
2019-02-19 22:08     ` John Ogness
2019-02-22  9:58       ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 08/25] printk: add ring buffer and kthread John Ogness
2019-02-12 15:47   ` Sergey Senozhatsky
2019-02-19 13:54   ` Petr Mladek
2019-03-04  7:38   ` Sergey Senozhatsky
2019-03-04 10:00     ` Sergey Senozhatsky
2019-03-04 11:07       ` Sergey Senozhatsky
2019-03-05 21:00         ` John Ogness
2019-03-06 15:57           ` Petr Mladek
2019-03-06 21:17             ` John Ogness
2019-03-06 22:22               ` John Ogness
2019-03-07  6:41                 ` Sergey Senozhatsky
2019-03-07  6:51                   ` Sergey Senozhatsky
2019-03-07 12:50               ` Petr Mladek
2019-03-07  5:15           ` Sergey Senozhatsky
2019-03-11 10:51             ` John Ogness
2019-03-12  9:58               ` Sergey Senozhatsky
2019-03-12 10:30               ` Petr Mladek
2019-03-07 12:06     ` John Ogness
2019-03-08  1:31       ` Sergey Senozhatsky
2019-03-08 10:04         ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 09/25] printk: remove exclusive console hack John Ogness
2019-02-19 14:03   ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 10/25] printk: redirect emit/store to new ringbuffer John Ogness
2019-02-20  9:01   ` Petr Mladek
2019-02-20 21:25     ` John Ogness
2019-02-22 14:43       ` Petr Mladek
2019-02-22 15:06         ` John Ogness
2019-02-22 15:25           ` Petr Mladek
2019-02-25 12:11       ` Petr Mladek
2019-02-25 16:41         ` John Ogness
2019-02-26  9:45           ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 11/25] printk_safe: remove printk safe code John Ogness
2019-02-22 10:37   ` Petr Mladek
2019-02-22 13:38     ` John Ogness
2019-02-22 15:15       ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 12/25] printk: minimize console locking implementation John Ogness
2019-02-25 13:44   ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 13/25] printk: track seq per console John Ogness
2019-02-25 14:59   ` Petr Mladek
2019-02-26  8:45     ` John Ogness
2019-02-26 13:11       ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 14/25] printk: do boot_delay_msec inside printk_delay John Ogness
2019-02-12 14:29 ` [RFC PATCH v1 15/25] printk: print history for new consoles John Ogness
2019-02-26 14:58   ` Petr Mladek
2019-02-26 15:22     ` John Ogness
2019-02-27  9:02       ` Petr Mladek
2019-02-27 10:02         ` John Ogness
2019-02-27 13:12           ` Petr Mladek
2019-03-04  9:24       ` Sergey Senozhatsky
2019-02-12 14:29 ` [RFC PATCH v1 16/25] printk: implement CON_PRINTBUFFER John Ogness
2019-02-26 15:38   ` Petr Mladek
2019-02-12 14:29 ` [RFC PATCH v1 17/25] printk: add processor number to output John Ogness
2019-02-13 22:29   ` John Ogness
2019-02-12 14:29 ` [RFC PATCH v1 18/25] console: add write_atomic interface John Ogness
2019-02-12 14:29 ` [RFC PATCH v1 19/25] printk: introduce emergency messages John Ogness
2019-03-07  7:30   ` Sergey Senozhatsky
2019-03-08 10:31     ` Petr Mladek
2019-03-11 12:04       ` John Ogness
2019-03-12  2:51         ` Sergey Senozhatsky
2019-03-12  2:58       ` Sergey Senozhatsky
2019-02-12 14:29 ` [RFC PATCH v1 20/25] serial: 8250: implement write_atomic John Ogness
2019-02-27  9:46   ` Petr Mladek
2019-02-27 10:32     ` John Ogness
2019-02-27 13:55       ` Petr Mladek
2019-03-08  4:05         ` John Ogness
2019-03-08  4:17           ` John Ogness
2019-03-08 10:28           ` Petr Mladek [this message]
2019-02-12 14:29 ` [RFC PATCH v1 21/25] printk: implement KERN_CONT John Ogness
2019-02-12 14:30 ` [RFC PATCH v1 22/25] printk: implement /dev/kmsg John Ogness
2019-02-12 14:30 ` [RFC PATCH v1 23/25] printk: implement syslog John Ogness
2019-02-12 14:30 ` [RFC PATCH v1 24/25] printk: implement kmsg_dump John Ogness
2019-02-12 14:30 ` [RFC PATCH v1 25/25] printk: remove unused code John Ogness
2019-03-08 14:02   ` Sebastian Andrzej Siewior
2019-03-11  2:46     ` Sergey Senozhatsky
2019-03-11  8:18       ` Sebastian Andrzej Siewior
2019-03-12  9:38         ` Petr Mladek
2019-02-13  1:31 ` [RFC PATCH v1 00/25] printk: new implementation Sergey Senozhatsky
2019-02-13 13:43   ` John Ogness
2019-03-04  6:39     ` Sergey Senozhatsky
2019-02-13  1:41 ` Sergey Senozhatsky
2019-02-13 14:15   ` John Ogness
2019-03-04  5:31     ` Sergey Senozhatsky
2019-02-13  2:55 ` Sergey Senozhatsky
2019-02-13 14:43   ` John Ogness
2019-03-04  5:23     ` Sergey Senozhatsky
2019-03-07  9:53       ` John Ogness
2019-03-08 10:00         ` Petr Mladek
2019-03-11 10:54         ` Sergey Senozhatsky
2019-03-12 12:38           ` Petr Mladek
2019-03-12 15:15             ` John Ogness
2019-03-13  2:15               ` Sergey Senozhatsky
2019-03-13  8:19                 ` John Ogness
2019-03-13  8:40                   ` Sebastian Siewior
2019-03-13  9:27                     ` Sergey Senozhatsky
2019-03-13 10:06                       ` Sergey Senozhatsky
2019-03-14  9:27                       ` Petr Mladek
2019-03-13  8:46                   ` Sergey Senozhatsky
2019-03-14  9:14               ` Petr Mladek
2019-03-14  9:35                 ` John Ogness
2019-03-13  2:00             ` Sergey Senozhatsky
2019-02-13 16:54 ` David Laight
2019-02-13 22:20   ` John Ogness
2020-01-20 23:05 ` Eugeniu Rosca
2020-01-21 23:56   ` John Ogness
2020-01-22  2:34     ` Eugeniu Rosca
2020-01-22  7:31       ` Geert Uytterhoeven
2020-01-22 16:58         ` Eugeniu Rosca
2020-01-22 19:48           ` Geert Uytterhoeven
2020-01-24 16:09             ` Eugeniu Rosca
2020-01-27 12:32               ` Petr Mladek
2020-01-27 13:45                 ` Eugeniu Rosca
2020-01-22 10:33       ` John Ogness
2020-01-24 12:13         ` Eugeniu Rosca

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20190308102819.lirle4vlf7mdcetp@pathway.suse.cz \
    --to=pmladek@suse.com \
    --cc=akpm@linux-foundation.org \
    --cc=gnomes@lxorguk.ukuu.org.uk \
    --cc=gregkh@linuxfoundation.org \
    --cc=john.ogness@linutronix.de \
    --cc=jslaby@suse.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-serial@vger.kernel.org \
    --cc=peterz@infradead.org \
    --cc=pfeiner@google.com \
    --cc=rostedt@goodmis.org \
    --cc=sergey.senozhatsky.work@gmail.com \
    --cc=sergey.senozhatsky@gmail.com \
    --cc=torvalds@linux-foundation.org \
    --cc=wonderfly@google.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).