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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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