On Thu, Sep 07, 2023 at 01:28:55PM +0200, Paolo Bonzini wrote: > On 9/6/23 21:01, Stefan Hajnoczi wrote: > > It is not safe to call drain_call_rcu() from qmp_device_add() because > > some call stacks are not prepared for drain_call_rcu() to drop the Big > > QEMU Lock (BQL). > > > > For example, device emulation code is protected by the BQL but when it > > calls aio_poll() -> ... -> qmp_device_add() -> drain_call_rcu() then the > > BQL is dropped. See https://bugzilla.redhat.com/show_bug.cgi?id=2215192 for > > a > > concrete bug of this type. > > > > Another limitation of drain_call_rcu() is that it cannot be invoked within > > an > > RCU read-side critical section since the reclamation phase cannot complete > > until the end of the critical section. Unfortunately, call stacks have been > > seen where this happens (see > > https://bugzilla.redhat.com/show_bug.cgi?id=2214985). > > I think the root cause here is that do_qmp_dispatch_bh is called on the > wrong context, namely qemu_get_aio_context() instead of > iohandler_get_aio_context(). This is what causes it to move to the vCPU > thread. > > Auditing all subsystems that use iohandler_get_aio_context(), for example > via qemu_set_fd_handler(), together with bottom halves, would be a bit > daunting. > > I don't have any objection to this patch series actually, but I would like > to see if using the right AioContext also fixes the bug---and then treat > these changes as more of a cleanup. Coroutines are pretty pervasive in QEMU > and are not going away which, as you say in the updated docs, makes > drain_call_rcu_co() preferrable to drain_call_rcu().
While I agree that the issue would not happen if monitor commands only
ran in the iohandler AioContext, I don't think we can change that.
When Kevin implemented coroutine commands in commit 9ce44e2ce267 ("qmp:
Move dispatcher to a coroutine"), he used qemu_get_aio_context()
deliberately so that AIO_WAIT_WHILE() can make progress.
I'm not clear on the exact scenario though, because coroutines shouldn't
call AIO_WAIT_WHILE().
Kevin?
There is only one coroutine monitor command that calls the QEMU block
layer: qmp_block_resize(). If we're going to change how the AioContext
works then now is the time to do it before there are more commands that
need to be audited/refactored.
Stefan
>
> Paolo
>
>
> > This patch series introduces drain_call_rcu_co(), which does the same thing
> > as
> > drain_call_rcu() but asynchronously. By yielding back to the event loop we
> > can
> > wait until the caller drops the BQL and leaves its RCU read-side critical
> > section.
> >
> > Patch 1 changes HMP so that coroutine monitor commands yield back to the
> > event
> > loop instead of running inside a nested event loop.
> >
> > Patch 2 introduces the new drain_call_rcu_co() API.
> >
> > Patch 3 converts qmp_device_add() into a coroutine monitor command and uses
> > drain_call_rcu_co().
> >
> > I'm sending this as an RFC because I don't have confirmation yet that the
> > bugs
> > mentioned above are fixed by this patch series.
> >
> > Stefan Hajnoczi (3):
> > hmp: avoid the nested event loop in handle_hmp_command()
> > rcu: add drain_call_rcu_co() API
> > qmp: make qmp_device_add() a coroutine
> >
> > MAINTAINERS | 2 ++
> > docs/devel/rcu.txt | 21 ++++++++++++++++
> > qapi/qdev.json | 1 +
> > include/monitor/qdev.h | 3 ++-
> > include/qemu/rcu.h | 1 +
> > util/rcu-internal.h | 8 ++++++
> > monitor/hmp.c | 28 +++++++++++----------
> > monitor/qmp-cmds.c | 2 +-
> > softmmu/qdev-monitor.c | 34 +++++++++++++++++++++++---
> > util/rcu-co.c | 55 ++++++++++++++++++++++++++++++++++++++++++
> > util/rcu.c | 3 ++-
> > hmp-commands.hx | 1 +
> > util/meson.build | 2 +-
> > 13 files changed, 140 insertions(+), 21 deletions(-)
> > create mode 100644 util/rcu-internal.h
> > create mode 100644 util/rcu-co.c
> >
>
signature.asc
Description: PGP signature
