From: Helge Deller <deller@gmx.de>
To: Petr Mladek <pmladek@suse.com>
Cc: John Ogness <john.ogness@linutronix.de>,
Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Thomas Gleixner <tglx@linutronix.de>,
linux-kernel@vger.kernel.org, linux-parisc@vger.kernel.org
Subject: Re: [PATCH printk-rework 08/14] printk: add syslog_lock
Date: Tue, 23 Feb 2021 15:45:52 +0100 [thread overview]
Message-ID: <101aee06-2f7a-0c3a-44a1-449cba7ad7c1@gmx.de> (raw)
In-Reply-To: <YDUP5J+AJwYjx5P4@alley>
On 2/23/21 3:23 PM, Petr Mladek wrote:
> On Tue 2021-02-23 13:22:22, Helge Deller wrote:
>> On 2/22/21 5:28 PM, Petr Mladek wrote:
>>> On Sun 2021-02-21 22:39:42, Helge Deller wrote:
>>>> On 2/19/21 5:33 PM, John Ogness wrote:
>>>>> Added CC: linux-parisc@vger.kernel.org
>>>>>
>>>>> On 2021-02-19, John Ogness <john.ogness@linutronix.de> wrote:
>>>>>>>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>>>>>>>> index 20c21a25143d..401df370832b 100644
>>>>>>>> --- a/kernel/printk/printk.c
>>>>>>>> +++ b/kernel/printk/printk.c
>>>>>>>> +/* Return a consistent copy of @syslog_seq. */
>>>>>>>> +static u64 read_syslog_seq_irq(void)
>>>>>>>> +{
>>>>>>>> + u64 seq;
>>>>>>>> +
>>>>>>>> + raw_spin_lock_irq(&syslog_lock);
>>>>>>>> + seq = syslog_seq;
>>>>>>>> + raw_spin_unlock_irq(&syslog_lock);
>>>>>>>
>>>>>>> Is there any particular reason to disable interrupts here?
>>>>>>>
>>>>> I found a possible call chain in interrupt context. From arch/parisc
>>>>> there is the interrupt handler:
>>>>>
>>>> Yes, handle_interruption() is the irq handler, running with irqs off.
>>>> HPMC is the crash handler - it's called when the kernel will stop
>>>> anyway. pdc_console is a very basic firmware console which prints
>>>> the last messages before the machine halts on fatal errors.
>>>> So, this code it's not the typical use case....
>>>
>>> Thanks for information.
>>>
>>> Is this code supposed to work only during early boot or anytime,
>>> please?
>>
>> No.
>> It's only called when the kernel completely crashes, when all
>> spinlocks should get busted and so on.
>> It's the emergency way to get some info out at least.
>
> OK.
>
>>> Note that it is not safe because register_console() takes
>>> console_lock() which is a sleeping lock.
>>
>> As I said, in that stage the plan is to bust all spinlocks.
>
> Just to be sure. Note that that register_console() does not bust
> console_lock in panic.
Ok.
> bust_spinlocks() just increments oops_in_progress counter. It has
> effect only when the caller checks this variable and use trylock
> when it is set. For example, see serial8250_console_write():
>
> void serial8250_console_write(struct uart_8250_port *up, const char *s,
> unsigned int count)
> {
> int locked = 1;
>
> if (oops_in_progress)
> locked = spin_trylock_irqsave(&port->lock, flags);
> else
> spin_lock_irqsave(&port->lock, flags);
>
>
> ...
>
>
> if (locked)
> spin_unlock_irqrestore(&port->lock, flags);
> }
>
> register_console() does not check oops_in_progress at the moment
> and might get blocked on console_sem.
>
> We could add the checks for oops_in_progress into register_console().
> But I am not sure if it is worth it.
It's not worth it just because of parisc.
I haven't seen any such crash in years, so the current implementation
is probably untested and outdated.
> It seems that you used this code for ages. The risk of the deadlock
> is small. It likely works most of the time. The upcoming printk rework
> should allow a cleaner solution.
Yes, it would be great if you can include such a "hard-panic/crash-dump-case"
in the rework.
Helge
next prev parent reply other threads:[~2021-02-23 14:48 UTC|newest]
Thread overview: 43+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-02-18 8:18 [PATCH printk-rework 00/14] printk: remove logbuf_lock John Ogness
2021-02-18 8:18 ` [PATCH printk-rework 01/14] printk: limit second loop of syslog_print_all John Ogness
2021-02-18 16:15 ` Petr Mladek
2021-02-18 8:18 ` [PATCH printk-rework 02/14] printk: kmsg_dump: remove unused fields John Ogness
2021-02-18 16:18 ` Petr Mladek
2021-02-18 8:18 ` [PATCH printk-rework 03/14] printk: refactor kmsg_dump_get_buffer() John Ogness
2021-02-18 8:18 ` [PATCH printk-rework 04/14] printk: consolidate kmsg_dump_get_buffer/syslog_print_all code John Ogness
2021-02-19 12:28 ` Petr Mladek
2021-02-18 8:18 ` [PATCH printk-rework 05/14] printk: introduce CONSOLE_LOG_MAX for improved multi-line support John Ogness
2021-02-19 12:44 ` Petr Mladek
2021-02-18 8:18 ` [PATCH printk-rework 06/14] printk: use seqcount_latch for clear_seq John Ogness
2021-02-18 8:18 ` [PATCH printk-rework 07/14] printk: use atomic64_t for devkmsg_user.seq John Ogness
2021-02-19 12:59 ` Petr Mladek
2021-02-18 8:18 ` [PATCH printk-rework 08/14] printk: add syslog_lock John Ogness
2021-02-19 13:30 ` Petr Mladek
2021-02-19 14:45 ` John Ogness
2021-02-19 16:33 ` John Ogness
2021-02-21 21:39 ` Helge Deller
2021-02-22 16:28 ` Petr Mladek
2021-02-23 12:22 ` Helge Deller
2021-02-23 14:23 ` Petr Mladek
2021-02-23 14:45 ` Helge Deller [this message]
2021-02-22 16:05 ` Petr Mladek
2021-02-22 16:43 ` John Ogness
2021-02-23 14:38 ` Petr Mladek
2021-02-18 8:18 ` [PATCH printk-rework 09/14] printk: introduce a kmsg_dump iterator John Ogness
2021-02-19 15:56 ` Petr Mladek
2021-02-19 16:50 ` Michael Kelley
2021-02-19 17:57 ` synchronization model: was: " Petr Mladek
2021-02-24 12:27 ` John Ogness
2021-02-24 15:40 ` John Ogness
2021-02-25 12:33 ` Petr Mladek
2021-02-26 8:36 ` John Ogness
2021-02-26 9:48 ` Petr Mladek
2021-02-18 8:18 ` [PATCH printk-rework 10/14] um: synchronize kmsg_dumper John Ogness
2021-02-18 8:18 ` [PATCH printk-rework 11/14] printk: remove logbuf_lock John Ogness
2021-02-23 11:44 ` Petr Mladek
2021-02-18 8:18 ` [PATCH printk-rework 12/14] printk: kmsg_dump: remove _nolock() variants John Ogness
2021-02-23 11:51 ` Petr Mladek
2021-02-18 8:18 ` [PATCH printk-rework 13/14] printk: kmsg_dump: use kmsg_dump_rewind John Ogness
2021-02-23 11:53 ` Petr Mladek
2021-02-18 8:18 ` [PATCH printk-rework 14/14] printk: console: remove unnecessary safe buffer usage John Ogness
2021-02-23 12:55 ` Petr Mladek
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