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    SubjectRe: [PATCH v2] Input: evdev - Add EVIOC mechanism to extract the MT slot state
    On 01/06/2012 10:18 AM, Dmitry Torokhov wrote:
    > Hi Benjamin,
    >
    > On Fri, Jan 06, 2012 at 07:00:22PM +0100, Benjamin Tissoires wrote:
    >> Hi guys,
    >> I read somewhere in the code of Android a comment in which they
    >> complain about not being able to retrieve the slots states. So they
    >> assume they are all at 0.
    >> So this mechanism is good to have.
    >> However, back in January 2011, Dmitry raised the problem that this
    >> code was not thread safe.What happens if 2 applications ask for
    >> different slots values (let say X.org and utouch-frame)?
    >
    > 2 different processes should be fine; the problem would be if 2 threads
    > of the same process share the same file descriptor. So far the rest of
    > evdev copes just fine with multiple threads using the same fd (all
    > operations are atomic in this regard), setting ABS_MT_SLOT before
    > fetching the state break this property.

    How is this any different than two threads trying to set a different
    property, like the fuzz factor of an axis? This seems like something
    that should be guarded by a lock in userspace, essentially.

    -- Chase


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