* Frank Ch. Eigler ([email protected]) wrote:
> Mathieu Desnoyers <[email protected]> writes:
>
> > [...]
> > The problem is also in _stp_print_flush, not *only* in relay code:
> > void _stp_print_flush (void)
> > ...
> > spin_lock(&_stp_print_lock);
> > ...
> > spin_unlock(&_stp_print_lock);
> >
> > Those will turn into mutexes with -rt.
>
> Indeed, plus systemtap-generated locking code uses rwlocks,
> local_irq_save/restore or preempt_disable, in various places. Could
> someone point to a place that spells out what would be more
> appropriate way of ensuring atomicity while being compatible with -rt?
>
> - FChE
AFAIK, for your needs either:
- Use atomic ops
- Use per-cpu data with preempt disabling/irq disabling
- Use the original "real" spin locks/rwlocks (raw_*).
- Don't play with timers or wakeups, since this kernel code uses the
"standard" spin locks (sleepable in -rt).
You just don't want to sleep in the tracing code.
Make sure that the sub-buffer switch code respects that too: it is the
most tricky and yet less executed part of the tracing code, so it's easy
for bugs to slip there and yet be undetected for a while.
Since you will likely disable preemption, make sure your tracing code
executes in a deterministic time.
Mathieu
--
Mathieu Desnoyers
Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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