[Intel-wired-lan] [PATCH v4 02/15] i40e: fix race condition with PTP_TX_IN_PROGRESS bits

[Bowers, AndrewX]

> -----Original Message-----
> From: Intel-wired-lan [mailto:intel-wired-lan-bounces at lists.osuosl.org] On
> Behalf Of Jacob Keller
> Sent: Wednesday, May 3, 2017 10:29 AM
> To: Intel Wired LAN <intel-wired-lan at lists.osuosl.org>
> Cc: Richard Cochran <richardcochran at gmail.com>
> Subject: [Intel-wired-lan] [PATCH v4 02/15] i40e: fix race condition with
> PTP_TX_IN_PROGRESS bits
> 
> Hardware related to the i40e driver has a limitation on Tx PTP packets.
> This requires us to limit the driver to timestamping a single packet at once.
> This is done using a state bitlock which enforces that only one timestamp
> request is honored at a time.
> 
> Unfortunately this suffers from a 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 timestamped
> packet request 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 timestamp another
> packet.
> 
> 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.
> 
> Now, a well behaved application which has at most one outstanding
> timestamp request will not accidentally race with the driver unlock bit.
> Obviously an application attempting to timestamp faster than one request at
> a time will have some timestamp requests skipped. Unfortunately there is
> nothing we can do about that.
> 
> Reported-by: David Mirabito <davidm at metamako.com>
> Signed-off-by: Jacob Keller <jacob.e.keller at intel.com>
> ---
>  drivers/net/ethernet/intel/i40e/i40e_ptp.c | 14 +++++++++++---
>  1 file changed, 11 insertions(+), 3 deletions(-)

Tested-by: Andrew Bowers <andrewx.bowers at intel.com>