On Tue, Dec 08, 2015 at 07:57:38PM +0100, Ingo Molnar wrote:
> Btw., could we add more redundancy / debug code to the refcounting code? It
> seems
> to be a frequent source of very hard to find/fix races/bugs - so it should be
> ripe
> for some extra debug infrastructure ...
I'll try, but its not easy. The biggest problem so far is the
ctx::parent relation. We cannot easily track the reverse of that because
of locking (we'd need to hold two ctx->lock, and we cannot because
perf_event_context_sched_out()).
The other two relations we have reverse maps for:
task->perf_event_ctxp[] <-> ctx->task
event->ctx <-> ctx->event_list
Also, all 3 relations are (more or less) protected under ctx->lock:
- unclone_ctx():
removes the ctx->parent link, with ctx->lock held
- perf_remove_from_context():
removes the event from ctx::event_list, with ctx->lock held;
leaves event->ctx set, because nasty games with ctx migration,
it will get set to the new context, but can never be NULL.
- perf_event_exit_task_context():
clears task->perf_event_ctxp[], with ctx->lock held;
leaves ctx->task set, still looking into this.
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