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

[John Ogness <john.ogness@linutronix.de>]

(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.

John Ogness