netdev - Re: [PATCH v2 4/4] net: macb: Add hardware PTP support

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Message-ID: <20170607132817.GA28329@localhost.localdomain>
Date:   Wed, 7 Jun 2017 15:28:17 +0200
From:   Richard Cochran <richardcochran@...il.com>
To:     Rafal Ozieblo <rafalo@...ence.com>
Cc:     David Miller <davem@...emloft.net>,
        "nicolas.ferre@...el.com" <nicolas.ferre@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "devicetree@...r.kernel.org" <devicetree@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "harini.katakam@...inx.com" <harini.katakam@...inx.com>,
        "andrei.pistirica@...rochip.com" <andrei.pistirica@...rochip.com>
Subject: Re: [PATCH v2 4/4] net: macb: Add hardware PTP support

On Wed, Jun 07, 2017 at 11:13:36AM +0000, Rafal Ozieblo wrote:
> Please look at following call-stack:
> 
> 1. macb_interrupt()   // spin_lock(&bp->lock) is taken
> 2. macb_tx_interrupt()
> 3. macb_handle_txtstamp()
> 4. skb_tstamp_tx()
> 5. __skb_tstamp_tx()
> 6. skb_may_tx_timestamp()
> 7. read_lock_bh() // second lock is taken

Well, you can always drop the lock, or postpone the call to
skb_tstamp_tx() until after the spin lock is released...

> I know that those are different locks and different types. But this could lead 
> to deadlocks. This is the reason of warning I could see.

Can you please post the lockdep splat?

Thanks,
Richard