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Re: [Qemu-devel] [RFC PATCH 0/2] establish nesting rule of BQL vs cpu-ex
From: |
Roman Kagan |
Subject: |
Re: [Qemu-devel] [RFC PATCH 0/2] establish nesting rule of BQL vs cpu-exclusive |
Date: |
Fri, 21 Jun 2019 12:49:07 +0000 |
User-agent: |
Mutt/1.11.4 (2019-03-13) |
On Thu, Jun 06, 2019 at 01:22:33PM +0000, Roman Kagan wrote:
> On Mon, May 27, 2019 at 11:05:38AM +0000, Roman Kagan wrote:
> > On Thu, May 23, 2019 at 12:31:16PM +0100, Alex Bennée wrote:
> > >
> > > Roman Kagan <address@hidden> writes:
> > >
> > > > I came across the following AB-BA deadlock:
> > > >
> > > > vCPU thread main thread
> > > > ----------- -----------
> > > > async_safe_run_on_cpu(self,
> > > > async_synic_update)
> > > > ... [cpu hot-add]
> > > > process_queued_cpu_work()
> > > > qemu_mutex_unlock_iothread()
> > > > [grab BQL]
> > > > start_exclusive() cpu_list_add()
> > > > async_synic_update() finish_safe_work()
> > > > qemu_mutex_lock_iothread() cpu_exec_start()
> > > >
> > > > ATM async_synic_update seems to be the only async safe work item that
> > > > grabs BQL. However it isn't quite obvious that it shouldn't; in the
> > > > past there were more examples of this (e.g.
> > > > memory_region_do_invalidate_mmio_ptr).
> > > >
> > > > It looks like the problem is generally in the lack of the nesting rule
> > > > for cpu-exclusive sections against BQL, so I thought I would try to
> > > > address that. This patchset is my feeble attempt at this; I'm not sure
> > > > I fully comprehend all the consequences (rather, I'm sure I don't) hence
> > > > RFC.
> > >
> > > Hmm I think this is an area touched by:
> > >
> > > Subject: [PATCH v7 00/73] per-CPU locks
> > > Date: Mon, 4 Mar 2019 13:17:00 -0500
> > > Message-Id: <address@hidden>
> > >
> > > which has stalled on it's path into the tree. Last time I checked it
> > > explicitly handled the concept of work that needed the BQL and work that
> > > didn't.
> >
> > I'm still trying to get my head around that patchset, but it looks like
> > it changes nothing in regards to cpu-exclusive sections and safe work,
> > so it doesn't make the problem go.
> >
> > > How do you trigger your deadlock? Just hot-pluging CPUs?
> >
> > Yes. The window is pretty narrow so I only saw it once although this
> > test (where the vms are started and stopped and the cpus are plugged in
> > and out) is in our test loop for quite a bit (probably 2+ years).
> >
> > Roman.
>
> ping?
ping?