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The s390 common I/O layer (CIO) returns an unexpected -EBUSY return code when drivers try to start I/O while a path-verification (PV) process is pending. This can lead to failed device initialization attempts with symptoms like broken network connectivity after boot. Fix this by replacing the -EBUSY return code with a deferred condition code 1 reply to make path-verification handling consistent from a driver's point of view. The problem can be reproduced semi-regularly using the following process, while repeating steps 2-3 as necessary (example assumes an OSA device with bus-IDs 0.0.a000-0.0.a002 on CHPID 0.02): 1. echo 0.0.a000,0.0.a001,0.0.a002 >/sys/bus/ccwgroup/drivers/qeth/group 2. echo 0 > /sys/bus/ccwgroup/devices/0.0.a000/online 3. echo 1 > /sys/bus/ccwgroup/devices/0.0.a000/online ; \ echo on > /sys/devices/css0/chp0.02/status Background information: The common I/O layer starts path-verification I/Os when it receives indications about changes in a device path's availability. This occurs for example when hardware events indicate a change in channel-path status, or when a manual operation such as a CHPID vary or configure operation is performed. If a driver attempts to start I/O while a PV is running, CIO reports a successful I/O start (ccw_device_start() return code 0). Then, after completion of PV, CIO synthesizes an interrupt response that indicates an asynchronous status condition that prevented the start of the I/O (deferred condition code 1). If a PV indication arrives while a device is busy with driver-owned I/O, PV is delayed until after I/O completion was reported to the driver's interrupt handler. To ensure that PV can be started eventually, CIO reports a device busy condition (ccw_device_start() return code -EBUSY) if a driver tries to start another I/O while PV is pending. In some cases this -EBUSY return code causes device drivers to consider a device not operational, resulting in failed device initialization. Note: The code that introduced the problem was added in 2003. Symptoms started appearing with the following CIO commit that causes a PV indication when a device is removed from the cio_ignore list after the associated parent subchannel device was probed, but before online processing of the CCW device has started: |
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
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.