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
