From: John Ogness <john.ogness@linutronix.de>
To: Petr Mladek <pmladek@suse.com>
Cc: 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
Subject: Re: [PATCH next v1 1/3] printk: track/limit recursion
Date: Tue, 23 Mar 2021 22:32:00 +0100 [thread overview]
Message-ID: <87czvp7e0f.fsf@jogness.linutronix.de> (raw)
In-Reply-To: <YFiuf/Kn9iLOwgNx@alley>
On 2021-03-22, Petr Mladek <pmladek@suse.com> wrote:
> On Wed 2021-03-17 00:33:24, John Ogness wrote:
>> Track printk() recursion and limit it to 3 levels per-CPU and per-context.
>
> Please, explain why it is added. I mean that it will
> allow remove printk_safe that provides recursion protection at the
> moment.
OK.
>> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
>> index 2f829fbf0a13..c666e3e43f0c 100644
>> --- a/kernel/printk/printk.c
>> +++ b/kernel/printk/printk.c
>> @@ -1940,6 +1940,71 @@ static void call_console_drivers(const char *ext_text, size_t ext_len,
>> }
>> }
>>
>> +/*
>> + * Recursion is tracked separately on each CPU. If NMIs are supported, an
>> + * additional NMI context per CPU is also separately tracked. Until per-CPU
>> + * is available, a separate "early tracking" is performed.
>> + */
>> +#ifdef CONFIG_PRINTK_NMI
>
> CONFIG_PRINTK_NMI is a shortcut for CONFIG_PRINTK && CONFIG_HAVE_NMI.
> It should be possible to use CONFIG_HAVE_NMI here because this should
> be in section where CONFIG_PRINTK is defined.
>
> This would make sense if it allows to remove CONFIG_PRINTK_NMI
> entirely. IMHO, it would be nice to remove one layer in the
> config options of possible.
OK. I will remove CONFIG_PRINTK_NMI for v2.
>> +#define PRINTK_CTX_NUM 2
>> +#else
>> +#define PRINTK_CTX_NUM 1
>> +#endif
>> +static DEFINE_PER_CPU(char [PRINTK_CTX_NUM], printk_count);
>> +static char printk_count_early[PRINTK_CTX_NUM];
>> +
>> +/*
>> + * Recursion is limited to keep the output sane. printk() should not require
>> + * more than 1 level of recursion (allowing, for example, printk() to trigger
>> + * a WARN), but a higher value is used in case some printk-internal errors
>> + * exist, such as the ringbuffer validation checks failing.
>> + */
>> +#define PRINTK_MAX_RECURSION 3
>> +
>> +/* Return a pointer to the dedicated counter for the CPU+context of the caller. */
>> +static char *printk_recursion_counter(void)
>> +{
>> + int ctx = 0;
>> +
>> +#ifdef CONFIG_PRINTK_NMI
>> + if (in_nmi())
>> + ctx = 1;
>> +#endif
>> + if (!printk_percpu_data_ready())
>> + return &printk_count_early[ctx];
>> + return &((*this_cpu_ptr(&printk_count))[ctx]);
>> +}
>
> It is not a big deal. But using an array for two contexts looks strange
> especially when only one is used on some architectures.
> Also &((*this_cpu_ptr(&printk_count))[ctx]) is quite tricky ;-)
>
> What do you think about the following, please?
>
> static DEFINE_PER_CPU(u8 printk_count);
> static u8 printk_count_early;
>
> #ifdef CONFIG_HAVE_NMI
> static DEFINE_PER_CPU(u8 printk_count_nmi);
> static u8 printk_count_nmi_early;
> #endif
>
> static u8 *printk_recursion_counter(void)
> {
> if (IS_ENABLED(CONFIG_HAVE_NMI) && in_nmi()) {
> if (printk_cpu_data_ready())
> return this_cpu_ptr(&printk_count_nmi);
> return printk_count_nmi_early;
> }
>
> if (printk_cpu_data_ready())
> return this_cpu_ptr(&printk_count);
> return printk_count_early;
> }
I can split it into explicit variables. But is the use of the IS_ENABLED
macro preferred over ifdef? I would prefer:
static u8 *printk_recursion_counter(void)
{
#ifdef CONFIG_HAVE_NMI
if (in_nmi()) {
if (printk_cpu_data_ready())
return this_cpu_ptr(&printk_count_nmi);
return printk_count_nmi_early;
}
#endif
if (printk_cpu_data_ready())
return this_cpu_ptr(&printk_count);
return printk_count_early;
}
Since @printk_count_nmi and @printk_count_nmi_early would not exist, I
would prefer the pre-processor removes that code block rather than
relying on compiler optimization.
John Ogness
next prev parent reply other threads:[~2021-03-23 21:32 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-16 23:33 [PATCH next v1 0/3] printk: remove safe buffers John Ogness
2021-03-16 23:33 ` [PATCH next v1 1/3] printk: track/limit recursion John Ogness
2021-03-21 5:34 ` Sergey Senozhatsky
2021-03-22 10:53 ` John Ogness
2021-03-22 11:13 ` Sergey Senozhatsky
2021-03-22 15:07 ` Petr Mladek
2021-03-22 14:49 ` Petr Mladek
2021-03-23 21:32 ` John Ogness [this message]
2021-03-24 8:41 ` Petr Mladek
2021-03-16 23:33 ` [PATCH next v1 2/3] printk: remove safe buffers John Ogness
2021-03-21 5:26 ` Sergey Senozhatsky
2021-03-22 11:16 ` John Ogness
2021-03-22 18:02 ` Petr Mladek
2021-03-22 21:58 ` John Ogness
2021-03-23 9:46 ` Petr Mladek
2021-03-23 10:47 ` Petr Mladek
2021-03-26 11:12 ` John Ogness
2021-03-29 10:04 ` Petr Mladek
2021-03-29 15:10 ` John Ogness
2021-03-29 15:13 ` John Ogness
2021-03-16 23:33 ` [PATCH next v1 3/3] printk: convert @syslog_lock to spin_lock John Ogness
2021-03-23 12:01 ` Petr Mladek
2021-03-26 11:23 ` John Ogness
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