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When PTP timestamping is enabled on Tx, the controller inserts the Tx timestamp at the beginning of the frame buffer, between SFD and the L2 frame header. This means that the skb provided by the stack is required to have enough headroom otherwise a new skb needs to be created by the driver to accommodate the timestamp inserted by h/w. Up until now the driver was relying on skb_realloc_headroom() to create new skbs to accommodate PTP frames. Turns out that this method is not reliable in this context at least, as skb_realloc_headroom() for PTP frames can cause random crashes, mostly in subsequent skb_*() calls, when multiple concurrent TCP streams are run at the same time with the PTP flow on the same device (as seen in James' report). I also noticed that when the system is loaded by sending multiple TCP streams, the driver receives cloned skbs in large numbers. skb_cow_head() instead proves to be stable in this scenario, and not only handles cloned skbs too but it's also more efficient and widely used in other drivers. The commit introducing skb_realloc_headroom in the driver goes back to 2009, 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.