netdev - Re: [PATCH net V4 2/2] igb: offer a PTP Hardware Clock instead of the timecompare method

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Message-ID: <20120323055927.GA2311@netboy.at.omicron.at>
Date:	Fri, 23 Mar 2012 06:59:29 +0100
From:	Richard Cochran <richardcochran@...il.com>
To:	chetan loke <loke.chetan@...il.com>
Cc:	"Keller, Jacob E" <jacob.e.keller@...el.com>,
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"e1000-devel@...ts.sourceforge.net" 
	<e1000-devel@...ts.sourceforge.net>,
	"Kirsher, Jeffrey T" <jeffrey.t.kirsher@...el.com>,
	"Ronciak, John" <john.ronciak@...el.com>,
	"john.stultz@...aro.org" <john.stultz@...aro.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH net V4 2/2] igb: offer a PTP Hardware Clock instead of
 the timecompare method

On Thu, Mar 22, 2012 at 07:13:42PM -0400, chetan loke wrote:

> /* users can keep re-trying - dont really care */

[Sigh] No, they do in fact care...

> igb_gettime_locking (...) {
>      unsigned int seq;
>      u64 ns;
>      do {
>      seq = read_seqbegin( &pigb->tmreg_seq_lock);
>      ns = timecounter_read(&pigb->tc);

And what happens here?

timecounter_read()
 timecounter_read_delta()
  tc->cc->read() == igb_825xx_systim_read()

   lo = rd32(E1000_SYSTIML); /* this read latches the time value */
   hi = rd32(E1000_SYSTIMH); /* and here is your race */

If two readers enter this code at nearly the same time, then they can
corrupt each other's hi/lo values.

>      } while (read_seqretry(&pigb->tmreg_seq_lock, seq));
>      // process ns
> }
> 
> 
> copyright 2012 - Chetan Loke <lokechetan@...il.com>
> 
> // trip cnt will ensure/enforce - evil adjtime user-space code can
> still not block us.
> // called from igb_tx[rx]_hwtstamp
> driver_rx_tx_path_locking( ... ) {
>      unsigned int seq, trip_cnt = 0;
>      u64 ns;
>      do {
>      seq = read_seqbegin( &pigb->tmreg_seq_lock);
>      trip_cnt++;
>      ns = timecounter_read(&pigb->tc);

Same here.

Thanks,
Richard
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