Messages in this thread |  | | Date | Tue, 29 Aug 2017 21:49:48 +0200 | From | Peter Zijlstra <> | Subject | Re: WARNING: possible circular locking dependency detected |
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On Tue, Aug 29, 2017 at 07:40:44PM +0200, Thomas Gleixner wrote:
> One solution I'm looking into right now is to reverse the lock order and > actually make the hotplug code do: > > watchdog_lock(); > cpu_write_lock(); > > .... > cpu_write_unlock(); > watchdog_unlock(); > > and get rid of cpu_read_(un)lock() in the sysctl interface completely. I > know it's ugly, but we have other locks we take in the hotplug path as > well.
This is to serialize the sysctl against hotplug? I'm not immediately seeing why watchdog_lock needs to be the outer most lock, is that because of vfs locks or something?
> That solves that part of the issue, but it does not solve the > release_ds_buffers() problem. Though with the watchdog_lock() mechanism, it > allows me to do: > > ->park() := watchdog_disable() > perf_event_disable(percpuevt); > cleanup_event = percpuevt; > percpuevt = NULL; > and then > > watchdog_unlock() > if (cleanup_event) { > perf_event_release_ebent(cleanup_event); > cleanup_event = NULL; > } > mutex_unlock(&watchdog_mutex); > > That should do the trick nicely for both user space functions and the cpu > hotplug machinery. > > Though it's quite a rewrite of that mess, which is particularly non trivial > because that extra non perf implementation in arch/powerpc which has its > own NMI watchdog thingy wants its calls preserved. But AFAICT so far it > should just work. Famous last words.... > > Thoughts?
So I have a patch _somewhere_ that preserves the event<->cpu relation across hotplug and disable/enable would be sufficient. If you want I can try and dig that out and make it work again.
That would avoid having to do the destroy/create cycle of the watchdog events.
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