linux-kernel - Re: [PATCH] printk-formats.txt: Add examples for %pS and %pF

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Date:   Tue, 15 Aug 2017 21:41:37 +0200
From:   Helge Deller <deller@....de>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        linux-kernel@...r.kernel.org, linux-parisc@...r.kernel.org
Subject: Re: [PATCH] printk-formats.txt: Add examples for %pS and %pF

On 15.08.2017 14:46, Steven Rostedt wrote:
> On Thu, 10 Aug 2017 19:35:33 +0200
> Helge Deller <deller@....de> wrote:
> 
>> Sometimes people seems unclear when to use the %pS or %pF printk format.
>> Adding some examples may help to avoid such mistakes.
>>
>> See for example commit 51d96dc2e2dc ("random: fix warning message on ia64 and
>> parisc") which fixed such a wrong format string.
>>
>> Signed-off-by: Helge Deller <deller@....de>
>>
>> diff --git a/Documentation/printk-formats.txt b/Documentation/printk-formats.txt
>> index 65ea591..be8c05b 100644
>> --- a/Documentation/printk-formats.txt
>> +++ b/Documentation/printk-formats.txt
>> @@ -73,6 +73,12 @@ actually function descriptors which must first be resolved. The ``F`` and
>>  ``f`` specifiers perform this resolution and then provide the same
>>  functionality as the ``S`` and ``s`` specifiers.
>>  
>> +Examples::
>> +
>> +	printk("Called from %pS.\n", __builtin_return_address(0));
>> +	printk("Called from %pS.\n", (void *)regs->ip);
>> +	printk("Called from %pF.\n", &gettimeofday);
> 
> Is the '&' really necessary? 
The '&' is not necessary. The compiler doesn't complain either.

> What about using the example:
> 	printk("Called in %pF.\n", __func__);

Very interesting!

This code:
void smp_cpus_done() {
printk("Called from %pF.\n", smp_cpus_done);
printk("Called from %pf.\n", smp_cpus_done);
printk("Called in %pS.\n", __func__);
printk("Called in %ps.\n", __func__);
printk("Called in %pF.\n", __func__);
printk("Called in %pf.\n", __func__);

gives:
 Called from smp_cpus_done+0x0/0x1b8.
 Called from smp_cpus_done.
 Called in __func__.28197+0x0/0x20.
 Called in __func__.28197.
 Called in 0x5041524953433332.
 Called in 0x5041524953433332.

So, the correct usage is:
printk("Called in %pS.\n", __func__);

But it should have printed
 Called from smp_cpus_done+0x0/0x1b8.
which means the (parisc?) printk resolver doesn't work correctly.

In assembly code a pointer to this object is handed to printk:
        .type   __func__.28197, @object
        .size   __func__.28197, 14
__func__.28197:
        .stringz        "smp_cpus_done"

I'll look into this problem.

Helge