netdev - Re: [PATCH net-next] bnxt_en: replace PTP spinlock with seqlock

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Message-ID: <CACKFLinJE-QfyUSDpYYfKtKEypMKZX_y_rAm_nQCQz_cDh8YjQ@mail.gmail.com>
Date: Mon, 14 Oct 2024 23:20:36 -0700
From: Michael Chan <michael.chan@...adcom.com>
To: Vadim Fedorenko <vadfed@...a.com>
Cc: Vadim Fedorenko <vadim.fedorenko@...ux.dev>, Jakub Kicinski <kuba@...nel.org>, 
	"David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, 
	Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: Re: [PATCH net-next] bnxt_en: replace PTP spinlock with seqlock

On Mon, Oct 14, 2024 at 4:29 PM Vadim Fedorenko <vadfed@...a.com> wrote:
>
> We can see high contention on ptp_lock while doing RX timestamping
> on high packet rates over several queues. Spinlock is not effecient
> to protect timecounter for RX timestamps when reads are the most
> usual operations and writes are only occasional. It's better to use
> seqlock in such cases.
>
> Signed-off-by: Vadim Fedorenko <vadfed@...a.com>

> -/* Caller holds ptp_lock */
>  static int bnxt_refclk_read(struct bnxt *bp, struct ptp_system_timestamp *sts,
>                             u64 *ns)
>  {
>         struct bnxt_ptp_cfg *ptp = bp->ptp_cfg;
>         u32 high_before, high_now, low;
>
> +       /* Make sure the RESET bit is set */
> +       smp_mb__before_atomic();

This may not be sufficient.  MMIO read of any register (clock register
in this case) can hang the chip if it is undergoing reset.

>         if (test_bit(BNXT_STATE_IN_FW_RESET, &bp->state))
>                 return -EIO;

We could have missed the flag and got here while the chip is about to be reset.

I will review the patch in more detail tomorrow.  Thanks.

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