netdev - Re: [PATCH net v2 1/2] gianfar: Replace skb_realloc_headroom with skb_cow

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Date:   Fri, 30 Oct 2020 09:41:52 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Claudiu Manoil <claudiu.manoil@....com>
Cc:     netdev@...r.kernel.org, "David S . Miller" <davem@...emloft.net>,
        james.jurack@...tek.com
Subject: Re: [PATCH net v2 1/2] gianfar: Replace skb_realloc_headroom with
 skb_cow_head for PTP

On Thu, 29 Oct 2020 10:10:56 +0200 Claudiu Manoil wrote:
> 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 93c1285c5d92
> ("gianfar: reallocate skb when headroom is not enough for fcb").
> For practical purposes I'm referencing a newer commit (from 2012)
> that brings the code to its current structure (and fixes the PTP
> case).
> 
> Fixes: 9c4886e5e63b ("gianfar: Fix invalid TX frames returned on error queue when time stamping")
> Reported-by: James Jurack <james.jurack@...tek.com>
> Suggested-by: Jakub Kicinski <kuba@...nel.org>
> Signed-off-by: Claudiu Manoil <claudiu.manoil@....com>

Applied both, thank you!