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    SubjectRe: 5.13-rt1 + KVM = WARNING: at fs/eventfd.c:74 eventfd_signal()
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    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@redhat.com>
    >> Fixes: b5e683d5cab8 ("eventfd: track eventfd_signal() recursion depth")
    >> Cc: stable@vger.kernel.org
    >> Cc: He Zhe <zhe.he@windriver.com>
    >> Cc: Jens Axboe <axboe@kernel.dk>
    >> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
    >>
    >> 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;
    >>  }
    >>
    >

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