Re: Re: [PATCH v8 03/10] eventfd: Increase the recursion depth of eventfd_signal()

[Yongji Xie <xieyongji@bytedance.com>]

On Thu, Jun 17, 2021 at 4:34 PM He Zhe <zhe.he@windriver.com> wrote:
>
>
>
> On 6/15/21 10:13 PM, Xie Yongji wrote:
> > Increase the recursion depth of eventfd_signal() to 1. This
> > is the maximum recursion depth we have found so far, which
> > can be triggered with the following call chain:
> >
> >     kvm_io_bus_write                        [kvm]
> >       --> ioeventfd_write                   [kvm]
> >         --> eventfd_signal                  [eventfd]
> >           --> vhost_poll_wakeup             [vhost]
> >             --> vduse_vdpa_kick_vq          [vduse]
> >               --> eventfd_signal            [eventfd]
> >
> > Signed-off-by: Xie Yongji <xieyongji@bytedance.com>
> > Acked-by: Jason Wang <jasowang@redhat.com>
>
> The fix had been posted one year ago.
>
> https://lore.kernel.org/lkml/20200410114720.24838-1-zhe.he@windriver.com/
>

OK, so it seems to be a fix for the RT system if my understanding is
correct? Any reason why it's not merged? I'm happy to rebase my series
on your patch if you'd like to repost it.

BTW, I also notice another thread for this issue:

https://lore.kernel.org/linux-fsdevel/DM6PR11MB420291B550A10853403C7592FF349@DM6PR11MB4202.namprd11.prod.outlook.com/T/

>
> > ---
> >  fs/eventfd.c            | 2 +-
> >  include/linux/eventfd.h | 5 ++++-
> >  2 files changed, 5 insertions(+), 2 deletions(-)
> >
> > diff --git a/fs/eventfd.c b/fs/eventfd.c
> > index e265b6dd4f34..cc7cd1dbedd3 100644
> > --- a/fs/eventfd.c
> > +++ b/fs/eventfd.c
> > @@ -71,7 +71,7 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
> >        * it returns true, the eventfd_signal() call should be deferred to a
> >        * safe context.
> >        */
> > -     if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count)))
> > +     if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count) > EFD_WAKE_DEPTH))
> >               return 0;
> >
> >       spin_lock_irqsave(&ctx->wqh.lock, flags);
> > diff --git a/include/linux/eventfd.h b/include/linux/eventfd.h
> > index fa0a524baed0..886d99cd38ef 100644
> > --- a/include/linux/eventfd.h
> > +++ b/include/linux/eventfd.h
> > @@ -29,6 +29,9 @@
> >  #define EFD_SHARED_FCNTL_FLAGS (O_CLOEXEC | O_NONBLOCK)
> >  #define EFD_FLAGS_SET (EFD_SHARED_FCNTL_FLAGS | EFD_SEMAPHORE)
> >
> > +/* Maximum recursion depth */
> > +#define EFD_WAKE_DEPTH 1
> > +
> >  struct eventfd_ctx;
> >  struct file;
> >
> > @@ -47,7 +50,7 @@ DECLARE_PER_CPU(int, eventfd_wake_count);
> >
> >  static inline bool eventfd_signal_count(void)
> >  {
> > -     return this_cpu_read(eventfd_wake_count);
> > +     return this_cpu_read(eventfd_wake_count) > EFD_WAKE_DEPTH;
>
> count is just count. How deep is acceptable should be put
> where eventfd_signal_count is called.
>

The return value of this function is boolean rather than integer.
Please see the comments in eventfd_signal():

"then it should check eventfd_signal_count() before calling this
function. If it returns true, the eventfd_signal() call should be
deferred to a safe context."

Thanks,
Yongji