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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 get_next_command(). 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 sleep_thread(). 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; smp_wmb(); thread_wakeup_needed = 0; thread_wakeup_needed = 1; smp_rmb(); 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 waiter. Cc: <stable@vger.kernel.org> Acked-by: Alan Stern <stern@rowland.harvard.edu> Signed-off-by: Thinh Nguyen <thinhn@synopsys.com> Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com> |
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Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.