Re: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()

[Paolo Bonzini]

On 15/07/21 06:14, Jason Wang wrote:
> > This obviously does not fly with PREEMPT_RT.  If eventfd_signal is
> > preempted and an unrelated thread calls eventfd_signal, the result is
> > a spurious WARN.  To avoid this, protect the percpu variable with a
> > local_lock.
>
> But local_lock only disable migration not preemption.

On mainline PREEMPT_RT, local_lock is an array of per-CPU spinlocks. 
When two eventfd_signals run on the same CPU and one is preempted, the 
spinlocks avoid that the second sees eventfd_wake_count > 0.

Thanks,

Paolo

> Or anything I missed here?
>
> Thanks
>
>
> > Reported-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> > Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
> > Cc: stable@xxxxxxxxxxxxxxx
> > Cc: He Zhe <zhe.he@xxxxxxxxxxxxx>
> > Cc: Jens Axboe <axboe@xxxxxxxxx>
> > Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> >
> > diff --git a/fs/eventfd.c b/fs/eventfd.c
> > index e265b6dd4f34..7d27b6e080ea 100644
> > --- a/fs/eventfd.c
> > +++ b/fs/eventfd.c
> > @@ -12,6 +12,7 @@
> >  #include <linux/fs.h>
> >  #include <linux/sched/signal.h>
> >  #include <linux/kernel.h>
> > +#include <linux/local_lock.h>
> >  #include <linux/slab.h>
> >  #include <linux/list.h>
> >  #include <linux/spinlock.h>
> > @@ -25,6 +26,7 @@
> >  #include <linux/idr.h>
> >  #include <linux/uio.h>
> >
> > +static local_lock_t eventfd_wake_lock = 
> > INIT_LOCAL_LOCK(eventfd_wake_lock);
> >  DEFINE_PER_CPU(int, eventfd_wake_count);
> >
> >  static DEFINE_IDA(eventfd_ida);
> > @@ -71,8 +73,11 @@ __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)))
> > +    local_lock(&eventfd_wake_lock);
> > +    if (WARN_ON_ONCE(this_cpu_read(eventfd_wake_count))) {
> > +        local_unlock(&eventfd_wake_lock);
> >          return 0;
> > +    }
> >
> >      spin_lock_irqsave(&ctx->wqh.lock, flags);
> >      this_cpu_inc(eventfd_wake_count);
> > @@ -83,6 +88,7 @@ __u64 eventfd_signal(struct eventfd_ctx *ctx, __u64 n)
> >          wake_up_locked_poll(&ctx->wqh, EPOLLIN);
> >      this_cpu_dec(eventfd_wake_count);
> >      spin_unlock_irqrestore(&ctx->wqh.lock, flags);
> > +    local_unlock(&eventfd_wake_lock);
> >
> >      return n;
> >  }