Re: [PATCH printk-rework 09/12] um: synchronize kmsg_dumper

[Richard Weinberger <richard@nod.at>]

John,

----- Ursprüngliche Mail -----
> Von: "John Ogness" <john.ogness@linutronix.de>
> An: "Petr Mladek" <pmladek@suse.com>
> CC: "Sergey Senozhatsky" <sergey.senozhatsky.work@gmail.com>, "Sergey Senozhatsky" <sergey.senozhatsky@gmail.com>,
> "Steven Rostedt" <rostedt@goodmis.org>, "tglx" <tglx@linutronix.de>, "linux-kernel" <linux-kernel@vger.kernel.org>,
> "Thomas Meyer" <thomas@m3y3r.de>, "richard" <richard@nod.at>
> Gesendet: Montag, 1. Februar 2021 17:51:14
> Betreff: Re: [PATCH printk-rework 09/12] um: synchronize kmsg_dumper

> (Added CC: Thomas Meyer, Richard Weinberger)
> 
> On 2021-02-01, Petr Mladek <pmladek@suse.com> wrote:
>>> In preparation for removing printk's @logbuf_lock, dumpers that have
>>> assumed to be protected against parallel calls must provide their own
>>> synchronization. Add a locally static spinlock to synchronize the
>>> kmsg_dump call and temporary buffer usage.
>>> 
>>> Signed-off-by: John Ogness <john.ogness@linutronix.de>
>>> ---
>>>  arch/um/kernel/kmsg_dump.c | 8 ++++++++
>>>  1 file changed, 8 insertions(+)
>>> 
>>> diff --git a/arch/um/kernel/kmsg_dump.c b/arch/um/kernel/kmsg_dump.c
>>> index f38349ad00ea..173999422ed8 100644
>>> --- a/arch/um/kernel/kmsg_dump.c
>>> +++ b/arch/um/kernel/kmsg_dump.c
>>> @@ -1,5 +1,6 @@
>>>  // SPDX-License-Identifier: GPL-2.0
>>>  #include <linux/kmsg_dump.h>
>>> +#include <linux/spinlock.h>
>>>  #include <linux/console.h>
>>>  #include <shared/init.h>
>>>  #include <shared/kern.h>
>>> @@ -9,8 +10,10 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
>>>  				enum kmsg_dump_reason reason,
>>>  				struct kmsg_dumper_iter *iter)
>>>  {
>>> +	static DEFINE_SPINLOCK(lock);
>>>  	static char line[1024];
>>>  	struct console *con;
>>> +	unsigned long flags;
>>>  	size_t len = 0;
>>>  
>>>  	/* only dump kmsg when no console is available */
>>> @@ -25,11 +28,16 @@ static void kmsg_dumper_stdout(struct kmsg_dumper *dumper,
>>>  	if (con)
>>>  		return;
>>>  
>>> +	if (!spin_trylock_irqsave(&lock, flags))
>>> +		return;
>>> +
>>>  	printf("kmsg_dump:\n");
>>>  	while (kmsg_dump_get_line(iter, true, line, sizeof(line), &len)) {
>>>  		line[len] = '\0';
>>>  		printf("%s", line);
>>>  	}
>>> +
>>> +	spin_unlock_irqrestore(&lock, flags);
>>
>> What exactly is synchronized here, please?
>> Access to @line buffer or @iter or both?
> 
> @line is being synchronized. @iter does not require synchronization.
> 
>> It looks to me that the access to @line buffer was not synchronized
>> before. kmsg_dump_get_line() used a lock internally but
>> it was not synchronized with the later printf("%s", line);
> 
> The line was previously synchronized for the kmsg_dump_get_line()
> call. But yes, it was not synchronized after the call, which is a bug if
> the dump is triggered on multiple CPUs simultaneously. The commit
> message should also mention that it is handling that bug.
> 
>> IMHO, this patch is not needed.
> 
> I am not familiar enough with ARCH=um to know if dumps can be triggered
> on multiple CPUs simultaneously. Perhaps ThomasM or Richard can chime in
> here.

Well, uml has no SMP support, so no parallel dumps. :-)

Thanks,
//richard