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Hardware related to the ixgbe driver is limited to handling a single Tx timestamp request at a time. Thus, the driver ignores requests for Tx timestamp while waiting for the current request to finish. It uses a state bit lock which enforces that only one timestamp request is honored at a time. Unfortunately this suffers from a simple race condition. The bit lock is not cleared until after skb_tstamp_tx() is called notifying applications of a new Tx timestamp. Even a well behaved application sending only one packet at a time and waiting for a response can wake up and send a new packet before the bit lock is cleared. This results in needlessly dropping some Tx timestamp requests. We can fix this by unlocking the state bit as soon as we read the Timestamp register, as this is the first point at which it is safe to unlock. To avoid issues with the skb pointer, we'll use a copy of the pointer and set the global variable in the driver structure to NULL first. This ensures that the next timestamp request does not modify our local copy of the skb pointer. This ensures that well behaved applications do not accidentally race with the unlock bit. Obviously an application which sends multiple Tx timestamp requests at once will still only timestamp one packet at a time. Unfortunately there is nothing we can do about this. Reported-by: David Mirabito <davidm@metamako.com> Signed-off-by: Jacob Keller <jacob.e.keller@intel.com> Tested-by: Andrew Bowers <andrewx.bowers@intel.com> Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
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.