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

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Message-ID: <4AA38AC6.2010202@gmail.com>
Date:	Sun, 06 Sep 2009 12:11:18 +0200
From:	Marcin Slusarz <marcin.slusarz@...il.com>
To:	Pavel Machek <pavel@....cz>
CC:	Norbert van Bolhuis <nvbolhuis@...valley.nl>,
	linux-kernel@...r.kernel.org
Subject: Re: PROBLEM: CONFIG_NO_HZ could cause software timeouts

Pavel Machek wrote:
> On Sat 2009-09-05 20:19:46, Marcin Slusarz wrote:
>> 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...
> 
> Too long wait is expected, but AFAICS he's complaining about too short
> delay and that's a hard bug.

Yeah, I know. But conclusion is a bit fishy - 13 iterations don't necessarily mean 13us.
Bug might be elsewhere.

Marcin

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