Re: [PATCH 1/4] printk: LOG_CONT and LOG_NEWLINE are separate

[Petr Mládek]

On Thu 2014-07-17 07:11:39, Alex Elder wrote:
> On 07/17/2014 03:39 AM, Petr Mládek wrote:
> > On Wed 2014-07-16 12:26:57, Alex Elder wrote:
> >> Two log record flags--LOG_CONT and LOG_NEWLINE--are never both set
> >> at the same time in a log record flags field.  What follows is a
> >> great deal of explanation that aims to prove this assertion.
> 
> Thank you so much for reviewing these patches.
> 
> Your confirmation of the fact that LOG_CONT and LOG_NEWLINE
> should not go together is very valuable to me.  I have a set
> of follow-on patches that rely on this, and I didn't want to
> go ahead with proposing them until I knew this was right.

To be honest. My statement was based on a common sense. I simply
cannot imagine situatiuon when a text ends with "\n" and is continuous
at the same time. IMHO, it is against any logic.

IMHO, it would make sense to have only one flag, either LOG_NEWLINE or
LOG_CONT. Well, I am not sure if we could remove it easily. AFAIK, the
ring buffer is read also by external tools, e.g. crash.

Some more experienced kernel developer should answer this.

> I have some responses to your feedback below.
> 
> > It makes perfect sense. If you found a situation where both flags were
> > set together, it would mean a bug. If a record ends with new line, it
> > is not continuous and vice versa.
> 
> At an abstract level this makes sense to me too, but the code
> is written to handle many combinations of flags that simply will
> never happen.  It obscures what's going on, or is supposed to be
> going on.  So to the reader, this appears much more complicated than
> it really is.

Yes, the code is too complicated and deserve simplification.

> 
> > [...]
> > 
> >> Signed-off-by: Alex Elder <elder@xxxxxxxxxx>
> >> ---
> >>  kernel/printk/printk.c | 12 +++++-------
> >>  1 file changed, 5 insertions(+), 7 deletions(-)
> >>
> >> diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
> >> index 13e839d..301ade3 100644
> >> --- a/kernel/printk/printk.c
> >> +++ b/kernel/printk/printk.c
> >> @@ -1006,11 +1006,9 @@ static size_t msg_print_text(const struct printk_log *msg, enum log_flags prev,
> >>  		prefix = false;
> >>  
> >>  	if (msg->flags & LOG_CONT) {
> >> -		if ((prev & LOG_CONT) && !(prev & LOG_NEWLINE))
> >> +		if (prev & LOG_CONT)
> >>  			prefix = false;
> >> -
> >> -		if (!(msg->flags & LOG_NEWLINE))
> >> -			newline = false;
> >> +		newline = false;
> >>  	}
> > 
> > Makes sense. I like it.
> > 
> >>  	do {
> >> @@ -1642,7 +1640,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> >>  	/* mark and strip a trailing newline */
> >>  	if (text_len && text[text_len-1] == '\n') {
> >>  		text_len--;
> >> -		lflags |= LOG_NEWLINE;
> >> +		lflags = LOG_NEWLINE;
> >>  	}
> >>  
> >>  	/* strip kernel syslog prefix and extract log level or control flags */
> >> @@ -1672,7 +1670,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> >>  		level = default_message_loglevel;
> >>  
> >>  	if (dict)
> >> -		lflags |= LOG_PREFIX|LOG_NEWLINE;
> >> +		lflags = LOG_PREFIX|LOG_NEWLINE;
> >>  
> >>  	if (!(lflags & LOG_NEWLINE)) {
> >>  		/*
> >> @@ -1688,7 +1686,7 @@ asmlinkage int vprintk_emit(int facility, int level,
> >>  		else
> >>  			printed_len += log_store(facility, level,
> >>  						 lflags | LOG_CONT, 0,
> >> -						 dict, dictlen, text, text_len);
> >> +						 NULL, 0, text, text_len);
> >>  	} else {
> >>  		bool stored = false;
> >>  
> > 
> > I am not sure that I like the last three changes. The logic is
> > correct. But I think that these micro-optimizations makes the code less
> > readable and prone to errors with reordering and other changes.
> 
> It is not an optimization.  I don't care about that.
> 
> It is replacing a variable with a constant, because I
> know by static analysis that the variable will always
> have constant value.  This makes it completely obvious
> that "dict" will *never* be NULL in this case, and as
> above, makes it more obvious what's happening.
> 
> (You'll see in my follow-on series that I rely on the
> assignment rather than |= in order to do some refactoring.)

As I said, I do not have strong opinion here. It might be interesting
to see the whole serie to get better picture.

> If someone chooses to reorder the code in a way that
> makes |= necessary (for example) will put that back
> again, because not doing so would introduce a bug.
> 
> > The original code does not harm. The new code is less obvious and will
> > force many people to think why it is correct. Even you might be in
> > doubts if you see it after few months :-)
> 
> Actually I think it's the opposite.
> 
> > Well, I do not have strong opinion here. Other people might see it
> > different. Forcing people to think is not a bad idea after all :-)
> 
> I may be naive, but I think it's a requirement if you're going
> to change code.

But it is easier when the code is readable and there are as few
surprises as possible. Also you never could have everything in your
head. The less dependncy is there, the easier is to work with it.

Regarding the above changes. The origial code used the typical
pattern. It set default value and then modified different flags according
to various tests. It did not need to take care much of other flags that
were not related to the given test.

If you add lated another flag and more tests, the final forced
assignent could break things.

Also the hardcoded NULL is not obvious. If you would want to modify the
code, you would need to think harder why it is there. IMHO, it does
not bring any real benefit unless you want to use it for some furher
optimization. But this is not visible from the patchset.


Please, take this as my personal opinion. I am not maintainer of this
code and the final decisions will not not be mine. Also I am still
learning good code patterns.

> Thanks again for the review.  If you're willing after reading my
> explanations, please offer an ACK or Reviewed-by (or further
> questions and suggestions).  I'll have responses to your others
> shortly.

I would like to see the bigger picture before :-)

Best Regards,
Petr
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