linux-kernel - Re: PROBLEM: CONFIG_NO_HZ could cause software timeouts

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Message-ID: <4AA2ABC2.1060803@gmail.com>
Date:	Sat, 05 Sep 2009 20:19:46 +0200
From:	Marcin Slusarz <marcin.slusarz@...il.com>
To:	Norbert van Bolhuis <nvbolhuis@...valley.nl>
CC:	linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: CONFIG_NO_HZ could cause software timeouts

Norbert van Bolhuis wrote:
> 
> The problem occurs when e.g. drivers use time_after(jiffes, timeout).
> 
> CONFIG_NO_HZ could make jiffies advance by more than 1.
> This is done by:
> tick_nohz_update_jiffies->tick_do_update_jiffies64->do_timer
> 
> If drivers use a timeout value of jiffies+1,
> "time_after(jiffies, timeout)" will be true after 1 interrupt
> (given that it advances jiffies by at least 2).
> 
> This is exactly what happens in cfi_cmdset_0002.c:do_write_buffer
> for our case (Powerpc MPC8313, linux-2.6.28, CONFIG_HZ=250,
> CONFIG_NO_HZ=y).
> 
> do_write_buffer does the following:
>  unsigned long uWriteTimeout = ( HZ / 1000 ) + 1;
>  ...
>  timeo = jiffies + uWriteTimeout;
>  ...
>  for (;;) {
>   ...
>   if (time_after(jiffies, timeo) && !chip_ready(map, adr))
>    break;
>   if (chip_ready(map, adr)) {
>    xip_enable(map, chip, adr);
>    goto op_done;
>   }
>   UDELAY(map, chip, adr, 1);
>  }
>  /* software timeout */
>  ret = -EIO;
> opdone:
>  ...
> 
> I've seen a few software timeouts after the for-loop
> looped only 13 times (= 13 us delay, i.s.o. the expected 1 ms). Typically

Are you sure? UDELAY may call schedule(), which can return to this thread
after much longer time than 13us...


Marcin
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