On Tue, 19 Oct 2021 13:26:25 +0200 Paolo Bonzini <pbonz...@redhat.com> wrote:
> On 19/10/21 07:56, Greg Kurz wrote: > > The drain_rcu_call() function can be blocked as long as an RCU reader > > stays in a read-side critical section. This is typically what happens > > when a TCG vCPU is executing a busy loop. It can deadlock the QEMU > > monitor as reported in https://gitlab.com/qemu-project/qemu/-/issues/650 . > > > > This can be avoided by allowing drain_rcu_call() to enforce an RCU grace > > period. Since each reader might need to do specific actions to end a > > read-side critical section, do it with notifiers. > > > > Prepare ground for this by adding a notifier list to the RCU reader > > struct and use it in wait_for_readers() if drain_rcu_call() is in > > progress. An API is added for readers to register their notifiers. > > > > This is largely based on a draft from Paolo Bonzini. > > > > Suggested-by: Paolo Bonzini <pbonz...@redhat.com> > > Signed-off-by: Greg Kurz <gr...@kaod.org> > > --- > > include/qemu/rcu.h | 16 ++++++++++++++++ > > util/rcu.c | 22 +++++++++++++++++++++- > > 2 files changed, 37 insertions(+), 1 deletion(-) > > > > diff --git a/include/qemu/rcu.h b/include/qemu/rcu.h > > index 515d327cf11c..d8c4fd8686b4 100644 > > --- a/include/qemu/rcu.h > > +++ b/include/qemu/rcu.h > > @@ -27,6 +27,7 @@ > > #include "qemu/thread.h" > > #include "qemu/queue.h" > > #include "qemu/atomic.h" > > +#include "qemu/notify.h" > > #include "qemu/sys_membarrier.h" > > > > #ifdef __cplusplus > > @@ -66,6 +67,14 @@ struct rcu_reader_data { > > > > /* Data used for registry, protected by rcu_registry_lock */ > > QLIST_ENTRY(rcu_reader_data) node; > > + > > + /* > > + * NotifierList used to force an RCU grace period. Accessed under > > + * rcu_registry_lock. Note that the notifier is called _outside_ > > + * the thread! > > + */ > > + NotifierList force_rcu; > > + void *force_rcu_data; > > This is a bit ugly because the force_rcu_data is shared across all > notifiers. Sure right now we have only one, but still the data argument > should be in rcu_register_thread rather than rcu_add_force_rcu_notifier. > I don't quite see why we'd need more than one notifier, but indeed this isn't conceptually correct. > It's a pity because I liked the Notifier local variable... But after > thinking about it more and deleting some suggestions that won't work, > it's just easiest to have the notifier in CPUState. > Agreed. > Maybe even move the unregistration to the existing function > tcg_cpus_destroy, and add tcg_cpus_init that calls tcg_register_thread() > and rcu_add_force_rcu_notifier(). This way you don't have to export > tcg_cpus_force_rcu, and the tcg-accel-ops.h APIs are a bit more tidy. > I don't think we can do that because of round-robin : we only have one thread in this case but tcg_cpus_destroy() must still be called for all vCPUs. Also, a single notifier will work just fine no matter which vCPU is running when wait_for_readers() is called if I understand correctly how round-robin works. > Paolo > > > +void rcu_add_force_rcu_notifier(Notifier *n, void *data) > > +{ > > + qemu_mutex_lock(&rcu_registry_lock); > > + notifier_list_add(&rcu_reader.force_rcu, n); > > + rcu_reader.force_rcu_data = data; > > + qemu_mutex_unlock(&rcu_registry_lock); > > +} > > + > > +void rcu_remove_force_rcu_notifier(Notifier *n) > > +{ > > + qemu_mutex_lock(&rcu_registry_lock); > > + rcu_reader.force_rcu_data = NULL; > > + notifier_remove(n); > > + qemu_mutex_unlock(&rcu_registry_lock); > > +} > > + > > static void rcu_init_complete(void) > > { > > QemuThread thread; > > >
