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    • Ioan-Adrian Ratiu's avatar
      HID: usbhid: fix recursive deadlock · 9e9d8b71
      Ioan-Adrian Ratiu authored
      commit e470127e upstream.
      
      The critical section protected by usbhid->lock in hid_ctrl() is too
      big and because of this it causes a recursive deadlock. "Too big" means
      the case statement and the call to hid_input_report() do not need to be
      protected by the spinlock (no URB operations are done inside them).
      
      The deadlock happens because in certain rare cases drivers try to grab
      the lock while handling the ctrl irq which grabs the lock before them
      as described above. For example newer wacom tablets like 056a:033c try
      to reschedule proximity reads from wacom_intuos_schedule_prox_event()
      calling hid_hw_request() -> usbhid_request() -> usbhid_submit_report()
      which tries to grab the usbhid lock already held by hid_ctrl().
      
      There are two ways to get out of this deadlock:
          1. Make the drivers work "around" the ctrl critical region, in the
          wacom case for ex. by delaying the scheduling of the proximity read
          request itself to a workqueue.
          2. Shrink the critical region so the usbhid lock protects only the
          instructions which modify usbhid state, calling hid_input_report()
          with the spinlock unlocked, allowing the device driver to grab the
          lock first, finish and then grab the lock afterwards in hid_ctrl().
      
      This patch implements the 2nd solution.
      Signed-off-by: default avatarIoan-Adrian Ratiu <adi@adirat.com>
      Signed-off-by: default avatarJiri Kosina <jkosina@suse.cz>
      Cc: Jason Gerecke <killertofu@gmail.com>
      Signed-off-by: default avatarLuis Henriques <luis.henriques@canonical.com>
      9e9d8b71