Re: [PATCH] perf_event: Fix oops triggered by cpu offline/online

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2010-03-05 at 17:06 +1100, Paul Mackerras wrote:
> Anton Blanchard found that he could reliably make the kernel hit a
> BUG_ON in the slab allocator by taking a cpu offline and then online
> while a system-wide perf record session was running.
> 
> The reason is that when the cpu comes up, we completely reinitialize
> the ctx field of the struct perf_cpu_context for the cpu.  If there is
> a system-wide perf record session running, then there will be a struct
> perf_event that has a reference to the context, so its refcount will
> be 2.  (The perf_event has been removed from the context's group_entry
> and event_entry lists by perf_event_exit_cpu(), but that doesn't
> remove the perf_event's reference to the context and doesn't decrement
> the context's refcount.)
> 
> When the cpu comes up, perf_event_init_cpu() gets called, and it calls
> __perf_event_init_context() on the cpu's context.  That resets the
> refcount to 1.  Then when the perf record session finishes and the
> perf_event is closed, the refcount gets decremented to 0 and the
> context gets kfreed after an RCU grace period.  Since the context
> wasn't kmalloced -- it's part of a per-cpu variable -- bad things
> happen.
> 
> In fact we don't need to completely reinitialize the context when the
> cpu comes up.  It's sufficient to initialize the context once at boot,
> but we need to do it for all possible cpus.
> 
> This moves the context initialization to happen at boot time.  With
> this, we don't trash the refcount and the context never gets kfreed,
> and we don't hit the BUG_ON.
> 
> Reported-by: Anton Blanchard <anton@samba.org>
> Signed-off-by: Paul Mackerras <paulus@samba.org>
> Tested-by: Anton Blanchard <anton@samba.org>

Very nice catch!

Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>