On 09/04/16 23:20, Andriy Voskoboinyk wrote:
There is a rare, but reproducible deadlock for wlan(4) drivers:

Thread 1:
 * uhub_explore_handle_re_enumerate() (obtains enum_sx lock)
 * usbd_set_config_index()
 * usb_unconfigure()
 * usb_detach_device()
 * usb_detach_device_sub()
 * <device_detach()>
   typically <device_stop()> is executed here (prevents
    another possible deadlock?)
 * ieee80211_ifdetach()
 * ieee80211_vap_destroy()
 * <ic->ic_vap_delete>
 * ieee80211_vap_detach()
   here it calls ieee80211_stop() and waits for <any> -> INIT state

Thread 2 (started from thread 1):
 * ieee80211_newstate_cb()
 * vap->iv_newstate()
   here: if the driver will try to call usbd_do_request_flags()
   (typically via <drv_write_reg> / <drv_read_reg>) it will hang
   (because enum_sx lock is already held by thread 1).

Another way: execute some periodical task that will try to access
some registers (urtwn_temp_calib(), rum_ratectl_task(),
run_ratectl_cb()) while thread 1 is running - deadlock is
here too, since <device_detach> will wait for them indefinitely
(via ieee80211_draintask())

Right now the most obvious (and, probably, wrong) way is to just
detect & release all locks (usbd_enum_unlock()) for
ieee80211_ifdetach() / ieee80211_draintask() and re-acquire them
later (not tested yet).


I think the right solution is to let usbd_do_request_flags() use its own SX lock for synchronization, instead of re-using the enumeration SX lock. What do you think about that?


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