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    Subject[PATCH AUTOSEL for 4.14 02/84] dma-buf/fence: Fix lock inversion within dma-fence-array
    Date
    From: Chris Wilson <chris@chris-wilson.co.uk>

    [ Upstream commit 03e4e0a9e02cf703da331ff6cfd57d0be9bf5692 ]

    Ages ago Rob Clark noted,

    "Currently with fence-array, we have a potential deadlock situation. If
    we fence_add_callback() on an array-fence, the array-fence's lock is
    acquired first, and in it's ->enable_signaling() callback, it will install
    cbs on it's array-member fences, so the array-member's lock is acquired
    second.

    But in the signal path, the array-member's lock is acquired first, and
    the array-fence's lock acquired second."

    Rob proposed either extensive changes to dma-fence to unnest the
    fence-array signaling, or to defer the signaling onto a workqueue. This
    is a more refined version of the later, that should keep the latency
    of the fence signaling to a minimum by using an irq-work, which is
    executed asap.

    Reported-by: Rob Clark <robdclark@gmail.com>
    Suggested-by: Rob Clark <robdclark@gmail.com>
    References: 1476635975-21981-1-git-send-email-robdclark@gmail.com
    Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
    Cc: Rob Clark <robdclark@gmail.com>
    Cc: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
    Cc: Sumit Semwal <sumit.semwal@linaro.org>
    Cc: Christian König <christian.koenig@amd.com>
    Reviewed-by: Christian König <christian.koenig@amd.com>
    Signed-off-by: Sumit Semwal <sumit.semwal@linaro.org>
    Link: https://patchwork.freedesktop.org/patch/msgid/20171114162719.30958-1-chris@chris-wilson.co.uk
    Signed-off-by: Sasha Levin <alexander.levin@microsoft.com>
    ---
    drivers/base/Kconfig | 1 +
    drivers/dma-buf/dma-fence-array.c | 14 ++++++++++++--
    include/linux/dma-fence-array.h | 3 +++
    3 files changed, 16 insertions(+), 2 deletions(-)

    diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
    index 2415ad9f6dd4..49fd50fccd48 100644
    --- a/drivers/base/Kconfig
    +++ b/drivers/base/Kconfig
    @@ -249,6 +249,7 @@ config DMA_SHARED_BUFFER
    bool
    default n
    select ANON_INODES
    + select IRQ_WORK
    help
    This option enables the framework for buffer-sharing between
    multiple drivers. A buffer is associated with a file using driver
    diff --git a/drivers/dma-buf/dma-fence-array.c b/drivers/dma-buf/dma-fence-array.c
    index 0350829ba62e..dd1edfb27b61 100644
    --- a/drivers/dma-buf/dma-fence-array.c
    +++ b/drivers/dma-buf/dma-fence-array.c
    @@ -31,6 +31,14 @@ static const char *dma_fence_array_get_timeline_name(struct dma_fence *fence)
    return "unbound";
    }

    +static void irq_dma_fence_array_work(struct irq_work *wrk)
    +{
    + struct dma_fence_array *array = container_of(wrk, typeof(*array), work);
    +
    + dma_fence_signal(&array->base);
    + dma_fence_put(&array->base);
    +}
    +
    static void dma_fence_array_cb_func(struct dma_fence *f,
    struct dma_fence_cb *cb)
    {
    @@ -39,8 +47,9 @@ static void dma_fence_array_cb_func(struct dma_fence *f,
    struct dma_fence_array *array = array_cb->array;

    if (atomic_dec_and_test(&array->num_pending))
    - dma_fence_signal(&array->base);
    - dma_fence_put(&array->base);
    + irq_work_queue(&array->work);
    + else
    + dma_fence_put(&array->base);
    }

    static bool dma_fence_array_enable_signaling(struct dma_fence *fence)
    @@ -136,6 +145,7 @@ struct dma_fence_array *dma_fence_array_create(int num_fences,
    spin_lock_init(&array->lock);
    dma_fence_init(&array->base, &dma_fence_array_ops, &array->lock,
    context, seqno);
    + init_irq_work(&array->work, irq_dma_fence_array_work);

    array->num_fences = num_fences;
    atomic_set(&array->num_pending, signal_on_any ? 1 : num_fences);
    diff --git a/include/linux/dma-fence-array.h b/include/linux/dma-fence-array.h
    index 332a5420243c..bc8940ca280d 100644
    --- a/include/linux/dma-fence-array.h
    +++ b/include/linux/dma-fence-array.h
    @@ -21,6 +21,7 @@
    #define __LINUX_DMA_FENCE_ARRAY_H

    #include <linux/dma-fence.h>
    +#include <linux/irq_work.h>

    /**
    * struct dma_fence_array_cb - callback helper for fence array
    @@ -47,6 +48,8 @@ struct dma_fence_array {
    unsigned num_fences;
    atomic_t num_pending;
    struct dma_fence **fences;
    +
    + struct irq_work work;
    };

    extern const struct dma_fence_ops dma_fence_array_ops;
    --
    2.14.1
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