• Thinh Nguyen's avatar
    usb: gadget: f_mass_storage: Serialize wake and sleep execution · dc9217b6
    Thinh Nguyen authored
    f_mass_storage has a memorry barrier issue with the sleep and wake
    functions that can cause a deadlock. This results in intermittent hangs
    during MSC file transfer. The host will reset the device after receiving
    no response to resume the transfer. This issue is seen when dwc3 is
    processing 2 transfer-in-progress events at the same time, invoking
    completion handlers for CSW and CBW. Also this issue occurs depending on
    the system timing and latency.
    To increase the chance to hit this issue, you can force dwc3 driver to
    wait and process those 2 events at once by adding a small delay (~100us)
    in dwc3_check_event_buf() whenever the request is for CSW and read the
    event count again. Avoid debugging with printk and ftrace as extra
    delays and memory barrier will mask this issue.
    Scenario which can lead to failure:
    1) The main thread sleeps and waits for the next command in
    2) bulk_in_complete() wakes up main thread for CSW.
    3) bulk_out_complete() tries to wake up the running main thread for CBW.
    4) thread_wakeup_needed is not loaded with correct value in
    5) Main thread goes to sleep again.
    The pattern is shown below. Note the 2 critical variables.
     * common->thread_wakeup_needed
     * bh->state
    	CPU 0 (sleep_thread)		CPU 1 (wakeup_thread)
    	==============================  ===============================
    					bh->state = BH_STATE_FULL;
    	thread_wakeup_needed = 0;	thread_wakeup_needed = 1;
    	if (bh->state != BH_STATE_FULL)
    		sleep again ...
    As pointed out by Alan Stern, this is an R-pattern issue. The issue can
    be seen when there are two wakeups in quick succession. The
    thread_wakeup_needed can be overwritten in sleep_thread, and the read of
    the bh->state maybe reordered before the write to thread_wakeup_needed.
    This patch applies full memory barrier smp_mb() in both sleep_thread()
    and wakeup_thread() to ensure the order which the thread_wakeup_needed
    and bh->state are written and loaded.
    However, a better solution in the future would be to use wait_queue
    method that takes care of managing memory barrier between waker and
    Cc: <stable@vger.kernel.org>
    Acked-by: default avatarAlan Stern <stern@rowland.harvard.edu>
    Signed-off-by: default avatarThinh Nguyen <thinhn@synopsys.com>
    Signed-off-by: default avatarFelipe Balbi <felipe.balbi@linux.intel.com>
f_mass_storage.c 95.3 KB