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fbc299437c
usb_kill_anchored_urbs() is commonly used to cancel all URBs on an
anchor just before releasing resources which the URBs rely on. By doing
so, users of this function rely on that no completer callbacks will take
place from any URB on the anchor after it returns.
However if this function is called in parallel with __usb_hcd_giveback_urb
processing a URB on the anchor, the latter may call the completer
callback after usb_kill_anchored_urbs() returns. This can lead to a
kernel panic due to use after release of memory in interrupt context.
The race condition is that __usb_hcd_giveback_urb() first unanchors the URB
and then makes the completer callback. Such URB is hence invisible to
usb_kill_anchored_urbs(), allowing it to return before the completer has
been called, since the anchor's urb_list is empty.
Even worse, if the racing completer callback resubmits the URB, it may
remain in the system long after usb_kill_anchored_urbs() returns.
Hence list_empty(&anchor->urb_list), which is used in the existing
while-loop, doesn't reliably ensure that all URBs of the anchor are gone.
A similar problem exists with usb_poison_anchored_urbs() and
usb_scuttle_anchored_urbs().
This patch adds an external do-while loop, which ensures that all URBs
are indeed handled before these three functions return. This change has
no effect at all unless the race condition occurs, in which case the
loop will busy-wait until the racing completer callback has finished.
This is a rare condition, so the CPU waste of this spinning is
negligible.
The additional do-while loop relies on usb_anchor_check_wakeup(), which
returns true iff the anchor list is empty, and there is no
__usb_hcd_giveback_urb() in the system that is in the middle of the
unanchor-before-complete phase. The @suspend_wakeups member of
struct usb_anchor is used for this purpose, which was introduced to solve
another problem which the same race condition causes, in commit
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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.