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    SubjectRe: [RFC PATCH] fpga: remove module reference counting from core components


    On 2023-10-30 09:32, Xu Yilun wrote:
    > On Fri, Oct 27, 2023 at 05:29:27PM +0200, Marco Pagani wrote:
    >> Remove unnecessary module reference counting from the core components
    >> of the subsystem. Low-level driver modules cannot be removed before
    >> core modules since they use their exported symbols.
    >
    > Could you help show the code for this conclusion?
    >
    > This is different from what I remember, a module cannot be removed when
    > its exported symbols are being used by other modules. IOW, the core
    > modules cannot be removed when there exist related low-level driver
    > modules. But the low-level driver modules could be removed freely
    > without other protecting mechanism.
    >

    My understanding was that we wanted to remove module reference counting
    from the fpga core and ease it from the responsibility of preventing
    low-level driver modules from being unloaded.

    If we want to keep reference counting in the fpga core, we could add a
    struct module *owner field in the struct fpga_manager_ops (and others
    core *_ops) so that the low-level driver can set it to THIS_MODULE.
    In this way, we can later use it in fpga_mgr_register() to bump up the
    refcount of the low-level driver module by calling
    try_module_get(mgr->mops->owner) directly when it registers the manager.
    Finally, fpga_mgr_unregister() would call module_put(mgr->mops->owner)
    to allow unloading the low-level driver module.

    In this way, it would no longer be necessary to call try_module_get()
    in fpga_mrg_get() since we could use a kref (included in the struct
    fpga_manager) to do refcounting for the in-kernel API users. Only when
    the kref reaches zero fpga_mgr_unregister() would succeed and put the
    low-level driver module.

    I think this approach would be safer since it would avoid the crash
    that can currently happen if the low-level driver module is removed
    right when executing try_module_get() in fpga_mrg_get(). The possible
    caveat is that it would be required to call fpga_mgr_unregister()
    before being able to remove the low-level driver module.

    >>
    >> For more context, refer to this thread:
    >> https://lore.kernel.org/linux-fpga/ZS6hhlvjUcqyv8zL@yilunxu-OptiPlex-7050
    >>
    >> Other changes:
    >>
    >> In __fpga_bridge_get(): do a (missing ?) get_device() and bind the
    >
    > I think get_device() is in (of)_fpga_bridge_get() -> class_find_device()
    > and put_device() correspond to it.
    >

    You are right. I missed that one.

    > But the code style here is rather misleading, the put_device() should be
    > moved out to (of)_fpga_bridge_get().
    >

    Right, I'll improve the (of)_fpga_bridge_get() style for the next version.

    >> image to the bridge only after the mutex has been acquired.
    >
    > This is good to me.
    >
    >>
    >> In __fpga_mgr_get(): do a get_device(). Currently, get_device() is
    >> called when allocating an image in fpga_image_info_alloc().
    >> However, since there are still two (of_)fpga_mgr_get() functions
    >> exposed by the core, I think they should behave as expected.
    >
    > Same as fpga bridge.
    >
    >>
    >> In fpga_region_get() / fpga_region_put(): call get_device() before
    >> acquiring the mutex and put_device() after having released the mutex
    >> to avoid races.
    >
    > Could you help elaborate more about the race?
    >

    I accidentally misused the word race. My concern was that memory might
    be released after the last put_device(), causing mutex_unlock() to be
    called on a mutex that does not exist anymore. It should not happen
    for the moment since the region does not use devres, but I think it
    still makes the code more brittle.

    > Thanks,
    > Yilun
    >
    >>
    >> Fixes: 654ba4cc0f3e ("fpga manager: ensure lifetime with of_fpga_mgr_get")
    >> Signed-off-by: Marco Pagani <marpagan@redhat.com>
    >> ---
    >> drivers/fpga/fpga-bridge.c | 24 +++++++-----------------
    >> drivers/fpga/fpga-mgr.c | 8 +-------
    >> drivers/fpga/fpga-region.c | 14 ++++----------
    >> 3 files changed, 12 insertions(+), 34 deletions(-)
    >>
    >> diff --git a/drivers/fpga/fpga-bridge.c b/drivers/fpga/fpga-bridge.c
    >> index a024be2b84e2..3bcc9c9849c5 100644
    >> --- a/drivers/fpga/fpga-bridge.c
    >> +++ b/drivers/fpga/fpga-bridge.c
    >> @@ -58,30 +58,21 @@ EXPORT_SYMBOL_GPL(fpga_bridge_disable);
    >> static struct fpga_bridge *__fpga_bridge_get(struct device *dev,
    >> struct fpga_image_info *info)
    >> {
    >> - struct fpga_bridge *bridge;
    >> - int ret = -ENODEV;
    >> -
    >> - bridge = to_fpga_bridge(dev);
    >> + struct fpga_bridge *bridge = to_fpga_bridge(dev);
    >>
    >> - bridge->info = info;
    >> + get_device(dev);
    >>
    >> if (!mutex_trylock(&bridge->mutex)) {
    >> - ret = -EBUSY;
    >> - goto err_dev;
    >> + dev_dbg(dev, "%s: FPGA Bridge already in use\n", __func__);
    >> + put_device(dev);
    >> + return ERR_PTR(-EBUSY);
    >> }
    >>
    >> - if (!try_module_get(dev->parent->driver->owner))
    >> - goto err_ll_mod;
    >> + bridge->info = info;
    >>
    >> dev_dbg(&bridge->dev, "get\n");
    >>
    >> return bridge;
    >> -
    >> -err_ll_mod:
    >> - mutex_unlock(&bridge->mutex);
    >> -err_dev:
    >> - put_device(dev);
    >> - return ERR_PTR(ret);
    >> }
    >>
    >> /**
    >> @@ -93,7 +84,7 @@ static struct fpga_bridge *__fpga_bridge_get(struct device *dev,
    >> * Return:
    >> * * fpga_bridge struct pointer if successful.
    >> * * -EBUSY if someone already has a reference to the bridge.
    >> - * * -ENODEV if @np is not an FPGA Bridge or can't take parent driver refcount.
    >> + * * -ENODEV if @np is not an FPGA Bridge.
    >> */
    >> struct fpga_bridge *of_fpga_bridge_get(struct device_node *np,
    >> struct fpga_image_info *info)
    >> @@ -146,7 +137,6 @@ void fpga_bridge_put(struct fpga_bridge *bridge)
    >> dev_dbg(&bridge->dev, "put\n");
    >>
    >> bridge->info = NULL;
    >> - module_put(bridge->dev.parent->driver->owner);
    >> mutex_unlock(&bridge->mutex);
    >> put_device(&bridge->dev);
    >> }
    >> diff --git a/drivers/fpga/fpga-mgr.c b/drivers/fpga/fpga-mgr.c
    >> index 06651389c592..6c355eafd18f 100644
    >> --- a/drivers/fpga/fpga-mgr.c
    >> +++ b/drivers/fpga/fpga-mgr.c
    >> @@ -670,14 +670,9 @@ static struct fpga_manager *__fpga_mgr_get(struct device *dev)
    >>
    >> mgr = to_fpga_manager(dev);
    >>
    >> - if (!try_module_get(dev->parent->driver->owner))
    >> - goto err_dev;
    >> + get_device(&mgr->dev);
    >>
    >> return mgr;
    >> -
    >> -err_dev:
    >> - put_device(dev);
    >> - return ERR_PTR(-ENODEV);
    >> }
    >>
    >> static int fpga_mgr_dev_match(struct device *dev, const void *data)
    >> @@ -727,7 +722,6 @@ EXPORT_SYMBOL_GPL(of_fpga_mgr_get);
    >> */
    >> void fpga_mgr_put(struct fpga_manager *mgr)
    >> {
    >> - module_put(mgr->dev.parent->driver->owner);
    >> put_device(&mgr->dev);
    >> }
    >> EXPORT_SYMBOL_GPL(fpga_mgr_put);
    >> diff --git a/drivers/fpga/fpga-region.c b/drivers/fpga/fpga-region.c
    >> index b364a929425c..c299956cafdc 100644
    >> --- a/drivers/fpga/fpga-region.c
    >> +++ b/drivers/fpga/fpga-region.c
    >> @@ -41,22 +41,17 @@ EXPORT_SYMBOL_GPL(fpga_region_class_find);
    >> * Return:
    >> * * fpga_region struct if successful.
    >> * * -EBUSY if someone already has a reference to the region.
    >> - * * -ENODEV if can't take parent driver module refcount.
    >> */
    >> static struct fpga_region *fpga_region_get(struct fpga_region *region)
    >> {
    >> struct device *dev = &region->dev;
    >>
    >> + get_device(dev);
    >> +
    >> if (!mutex_trylock(&region->mutex)) {
    >> dev_dbg(dev, "%s: FPGA Region already in use\n", __func__);
    >> - return ERR_PTR(-EBUSY);
    >> - }
    >> -
    >> - get_device(dev);
    >> - if (!try_module_get(dev->parent->driver->owner)) {
    >> put_device(dev);
    >> - mutex_unlock(&region->mutex);
    >> - return ERR_PTR(-ENODEV);
    >> + return ERR_PTR(-EBUSY);
    >> }
    >>
    >> dev_dbg(dev, "get\n");
    >> @@ -75,9 +70,8 @@ static void fpga_region_put(struct fpga_region *region)
    >>
    >> dev_dbg(dev, "put\n");
    >>
    >> - module_put(dev->parent->driver->owner);
    >> - put_device(dev);
    >> mutex_unlock(&region->mutex);
    >> + put_device(dev);
    >> }
    >>
    >> /**
    >> --
    >> 2.41.0
    >>
    >

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