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2550aabf75
commit f6a1964771 upstream.
PL011's ->flush_buffer() implementation releases and reacquires the port
lock. Due to a race condition here, data can end up being added to the
circular buffer but neither being discarded nor being sent out. This
leads to, for example, tcdrain(2) waiting indefinitely.
Process A Process B
uart_flush_buffer()
- acquire lock
- circ_clear
- pl011_flush_buffer()
-- release lock
-- dmaengine_terminate_all()
uart_write()
- acquire lock
- add chars to circ buffer
- start_tx()
-- start DMA
- release lock
-- acquire lock
-- turn off DMA
-- release lock
// Data in circ buffer but DMA is off
According to the comment in the code, the releasing of the lock around
dmaengine_terminate_all() is to avoid a deadlock with the DMA engine
callback. However, since the time this code was written, the DMA engine
API documentation seems to have been clarified to say that
dmaengine_terminate_all() (in the identically implemented but
differently named dmaengine_terminate_async() variant) does not wait for
any running complete callback to be completed and can even be called
from a complete callback. So there is no possibility of deadlock if the
DMA engine driver implements this API correctly.
So we should be able to just remove this release and reacquire of the
lock to prevent the aforementioned race condition.
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Cc: stable <stable@vger.kernel.org>
Link: https://lore.kernel.org/r/20191118092547.32135-1-vincent.whitchurch@axis.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Linux kernel
============
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In order to build the documentation, use ``make htmldocs`` or
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https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
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.