From: Geert Uytterhoeven <Geert.Uytterhoeven@sonycom.com>
To: Lou Langholtz <ldl@chpc.utah.edu>
Cc: Takashi Oe <toe@unlserve.unl.edu>, linuxppc-dev@lists.linuxppc.org
Subject: Re: [Fwd: Bug: 2.2.12 still hangs PPC after some PPP activity]
Date: Thu, 30 Sep 1999 07:33:43 +0200 (MET DST) [thread overview]
Message-ID: <Pine.GSO.4.10.9909300726510.8003-100000@begonia.sonytel.be> (raw)
In-Reply-To: <37F26FE5.98B8B00@chpc.utah.edu>
On Wed, 29 Sep 1999, Lou Langholtz wrote:
> Takashi Oe wrote:
>
> > . . .I'm fairly certain it's a bug. Good spotting. The last restore_flags()
> > shouldn't be there.
> >
> > --- macserial.c.ORIG Wed Sep 29 02:05:14 1999
> > +++ macserial.c Wed Sep 29 02:05:31 1999
> > @@ -1381,7 +1381,6 @@
> > if (info->xmit_cnt && !tty->stopped && !info->tx_stopped
> > && !info->tx_active)
> > transmit_chars(info);
> > - restore_flags(flags);
> > return ret;
> > }
> >
> > Takashi Oe
>
> Bummer...
>
> I just locked up my system and had to reboot. The above change then doesn't seem
> to fix the system hangs. On the other hand I never did see the "FB. overflow"
> message at least.
>
> I've been trying to search around HOWTOs and FAQs and mailing lists to get a
> better idea of wether nesting save_flags(flags); cli(); stuff...;
> save_flags(new_flags); cli(); restore_flags(new_flags); restore_flags(flags); is
> ever even ok. Haven't found anything conclusive yet though. Whatever the case, it
> doesn't seem like it'd be good practice anyhow. All the other serial support C
> files I've found so far seem to avoid this except for macserial.c.
Nesting `save_flags(); cli(); ... restore_flags();' is perfectly legal. That's
exactly the reason why `save_flags()' and `restore_flags()' were invented!
Normally, to disable interrupts to make sure a critical code section is not
interrupted, you do:
cli();
...
sti();
The problem with this construct is that the `sti()' will always re-enable the
interrupts, even when they were disabled when the `cli()' was called, like in
void func(void)
{
...
cli();
...
sti();
... // interrupts accidentally re-enabled here!
}
cli();
...
func();
...
sti();
So `save_flags()' saves the current interrupt mask, and `restore_flags()'
restores it. Problem solved by writing:
void func(void)
{
...
save_flags(flags);
cli();
...
restore_flags(flags);
... // interrupts accidentally re-enabled here!
}
For former AmigaOS programmers: it's a bit like the Disable()/Enable() Exec
functions, which allowed nestings:
int interrupt_disable_cnt = 0;
void Disable(void)
{
cli();
interrupt_disable_cnt++;
}
void Enable(void)
{
if (--interrupt_disable_cnt == 0)
sti();
}
Conclusion: use `cli()/sti()' when you're 100% sure interrupts were enabled
before the cli() was called, and `save_flags()/cli()/restore_flags()' when
interrupts may be disabled when entering your code.
Greetings,
Geert
--
Geert Uytterhoeven ----------------- Sony Suprastructure Center Europe (SUPC-E)
Geert.Uytterhoeven@sonycom.com ------------------- Sint Stevens Woluwestraat 55
Phone +32-2-7248648 Fax +32-2-7262686 ---------------- B-1130 Brussels, Belgium
** Sent via the linuxppc-dev mail list. See http://lists.linuxppc.org/
next prev parent reply other threads:[~1999-09-30 5:33 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <37F1AD4B.585E99AB@chpc.utah.edu>
1999-09-29 7:11 ` [Fwd: Bug: 2.2.12 still hangs PPC after some PPP activity] Geert Uytterhoeven
1999-09-29 7:16 ` Takashi Oe
1999-09-29 7:40 ` Geert Uytterhoeven
1999-09-29 9:13 ` Benjamin Herrenschmidt
1999-09-30 0:46 ` Takashi Oe
1999-09-30 8:50 ` Benjamin Herrenschmidt
1999-09-30 16:21 ` Takashi Oe
1999-09-30 16:35 ` Benjamin Herrenschmidt
1999-10-06 5:33 ` update on macserial/DMA Takashi Oe
1999-10-06 8:35 ` Benjamin Herrenschmidt
1999-10-06 13:47 ` Takashi Oe
1999-10-06 16:25 ` Benjamin Herrenschmidt
1999-10-06 16:50 ` Benjamin Herrenschmidt
1999-10-08 7:13 ` Lou Langholtz
1999-10-09 17:57 ` Lou Langholtz
[not found] ` <Pine.LNX.3.96LJ1.1b7.991005234830.1210A-100000@ofey.inetne br.com>
1999-10-07 9:48 ` Franz Sirl
1999-10-07 15:37 ` phandel
1999-10-12 7:20 ` [Fwd: Bug: 2.2.12 still hangs PPC after some PPP activity] Lou Langholtz
1999-09-29 16:44 ` Lou Langholtz
1999-09-29 17:02 ` Benjamin Herrenschmidt
1999-09-29 18:08 ` Lou Langholtz
1999-09-29 18:46 ` Lou Langholtz
1999-09-29 20:00 ` Lou Langholtz
1999-09-30 1:14 ` Takashi Oe
1999-09-30 5:33 ` Geert Uytterhoeven [this message]
1999-10-01 13:26 ` Franz Sirl
1999-10-01 13:46 ` Alvin Brattli
[not found] ` <Pine.OSF.3.96.991001153658.16772E-100000@mitra.phys.uit.no >
1999-10-01 14:08 ` Franz Sirl
1999-10-01 16:14 ` Lou Langholtz
[not found] <37F1C0A3.B9F25580@chpc.utah.edu>
1999-09-29 7:39 ` Geert Uytterhoeven
1999-09-10 16:51 Lou Langholtz
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