On Wed, 25 May 2016 15:16:40 +0200
Peter Zijlstra <[email protected]> wrote:
> > static char *get_trace_buf(void)
> > {
> > + struct trace_buffer_struct *buffer = this_cpu_ptr(trace_percpu_buffer);
> >
> > + if (!buffer || buffer->nesting >= 4)
> > return NULL;
>
> This is buggy fwiw; you need to unconditionally increment
> buffer->nesting to match the unconditional decrement.
>
> Otherwise 5 'increments' and 5 decrements will land you at -1.
As I said, I did a quick look and haven't reviewed it.
Peter, thanks for looking at it.
>
> >
> > + return &buffer->buffer[buffer->nesting++][0];
> > +}
> > +
> > +static void put_trace_buf(void)
> > +{
> > + this_cpu_dec(trace_percpu_buffer->nesting);
> > }
>
> So I don't know about tracing; but for perf this construct would not
> work 'properly'.
>
> The per context counter -- which is lost in this scheme -- guards
> against in-context recursion.
>
> Only if we nest from another context do we allow generation of a new
> event.
The ring buffer itself has a context check, where if you try to record
another event nested in the same context, it will simply return NULL.
But this buffer is only used for trace_printk() to manipulate a printf
format. It writes into here first, and then copies it into the tracing
ring buffer. If it happens at a nested event within the same context,
then trace_buffer_lock_reserve() will return NULL and the event wont be
recorded.
-- Steve
