Re: [PATCH 11/11 v2] ftrace: Add recording of functions that caused recursion

[Petr Mladek]

On Mon 2020-11-02 12:09:07, Steven Rostedt wrote:
> On Mon, 2 Nov 2020 17:41:47 +0100
> Petr Mladek <pmladek@xxxxxxxx> wrote:
> 
> > On Fri 2020-10-30 17:31:53, Steven Rostedt wrote:
> > > From: "Steven Rostedt (VMware)" <rostedt@xxxxxxxxxxx>
> > > 
> > > This adds CONFIG_FTRACE_RECORD_RECURSION that will record to a file
> > > "recursed_functions" all the functions that caused recursion while a
> > > callback to the function tracer was running.
> > >   
> > 
> > > --- /dev/null
> > > +++ b/kernel/trace/trace_recursion_record.c
> > > +	if (index >= CONFIG_FTRACE_RECORD_RECURSION_SIZE)
> > > +		return;
> > > +
> > > +	for (i = index - 1; i >= 0; i--) {
> > > +		if (recursed_functions[i].ip == ip) {
> > > +			cached_function = ip;
> > > +			return;
> > > +		}
> > > +	}
> > > +
> > > +	cached_function = ip;
> > > +
> > > +	/*
> > > +	 * We only want to add a function if it hasn't been added before.
> > > +	 * Add to the current location before incrementing the count.
> > > +	 * If it fails to add, then increment the index (save in i)
> > > +	 * and try again.
> > > +	 */
> > > +	old = cmpxchg(&recursed_functions[index].ip, 0, ip);
> > > +	if (old != 0) {
> > > +		/* Did something else already added this for us? */
> > > +		if (old == ip)
> > > +			return;
> > > +		/* Try the next location (use i for the next index) */
> > > +		i = index + 1;  
> > 
> > What about
> > 
> > 		index++;
> > 
> > We basically want to run the code again with index + 1 limit.
> 
> But something else could update nr_records, and we want to use that if
> nr_records is greater than i.
> 
> Now, we could swap the use case, and have
> 
> 	int index = 0;
> 
> 	[..]
> 	i = atomic_read(&nr_records);
> 	if (i > index)
> 		index = i;
> 
> 	[..]
> 
> 		index++;
> 		goto again;
> 
> 
> > 
> > Maybe, it even does not make sense to check the array again
> > and we should just try to store the value into the next slot.
> 
> We do this dance to prevent duplicates.

I see.

My code was wrong. It reserved slot for the new "ip" by cmpxchg
on nr_records. The "ip" was stored later so that any parallel
call need not see that it is a dumplicate.

Your code reserves the slot by cmpxchg of "ip".
Any parallel call would fail to take the slot and see
the "ip" in the next iteration.

Best Regards,
Petr