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


    > ---
    > 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.


    Zhe

    > }
    >
    > #else /* CONFIG_EVENTFD */

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