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    SubjectRe: [PATCH 02/12] locking/rwsem: Exit early when held by an anonymous owner
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    On 1/14/20 11:12 AM, Christoph Hellwig wrote:
    > The rwsem code overloads the owner field with either a task struct or
    > negative magic numbers. Add a quick hack to catch these negative
    > values early on. Without this spinning on a writer that replaced the
    > owner with RWSEM_OWNER_UNKNOWN, rwsem_spin_on_owner can crash while
    > deferencing the task_struct ->on_cpu field of a -8 value.
    >
    > XXX: This might be a bit of a hack as the code otherwise doesn't use
    > the ERR_PTR family macros, better suggestions welcome.
    >
    > Signed-off-by: Christoph Hellwig <hch@lst.de>
    > ---
    > kernel/locking/rwsem.c | 2 ++
    > 1 file changed, 2 insertions(+)
    >
    > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
    > index 44e68761f432..6adc719a30a1 100644
    > --- a/kernel/locking/rwsem.c
    > +++ b/kernel/locking/rwsem.c
    > @@ -725,6 +725,8 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable)
    > state = rwsem_owner_state(owner, flags, nonspinnable);
    > if (state != OWNER_WRITER)
    > return state;
    > + if (IS_ERR(owner))
    > + return state;
    >
    > rcu_read_lock();
    > for (;;) {

    The owner field is just a pointer to the task structure with the lower 3
    bits served as flag bits. Setting owner to RWSEM_OWNER_UNKNOWN (-2) will
    stop optimistic spinning. So under what condition did the crash happen?

    Anyway, PeterZ is working on revising the percpu-rwsem implementation to
    more gracefully handle the frozen case. At the end, there will not be a
    need for the RWSEM_OWNER_UNKNOWN magic and it can be removed.

    Cheers,
    Longman

    RWSEM_OWNER_UNKNOWN

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