Date: Mon, 26 Jun 2006 16:33:14 +0200 (CEST) From: Martin Blapp <mb@imp.ch> To: Gavin Atkinson <gavin.atkinson@ury.york.ac.uk> Cc: freebsd-stable@freebsd.org, "Wojciech A. Koszek" <wkoszek@freebsd.org>, csjp@freebsd.org, Robert Watson <rwatson@freebsd.org>, phk@freebsd.org, tmm@freebsd.org, Max Laier <max@love2party.net>, Patrick Guelat <patg@imp.ch> Subject: Re: Crash with FreeBSD 6.1 STABLE of today Message-ID: <20060626154810.S14714@godot.imp.ch> In-Reply-To: <1151328755.80434.17.camel@buffy.york.ac.uk> References: <20060621202508.S17514@godot.imp.ch> <20060623133915.S14714@godot.imp.ch> <1151078632.62769.30.camel@buffy.york.ac.uk> <200606231928.58063.max@love2party.net> <20060626115110.O14714@godot.imp.ch> <20060626133540.G14714@godot.imp.ch> <20060626151138.C14714@godot.imp.ch> <1151328755.80434.17.camel@buffy.york.ac.uk>
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Hi,
> Is the problem actually understood? Do we know what's racing with what?
> Given there only ever seems to be a single backtrace involved, as far as
> I can tell, it's ttymodem racing with tty_close - can those two
> functions alone be locked?
Correct: The only place there tp->t_session is set to NULL is in tty_close().
tp->t_pgrp = NULL;
tp->t_session = NULL;
tp->t_state = 0;
But I've seen this comment on peters page (http://people.freebsd.org/~peter/smp.html)
> At the moment I've hacked all the console code to obtain the scheduler mutex
> instead of spltty()'ing all over the place, because in a word: it can't
> spltty() any more. It's illegal because the console code may be called at any time (see
> the next section). This is a hack because it isn't MP safe against tty
> interrupts running on another cpu. Since the current tty interrupt is a fast
> interrupt, I'm not sure that we're any worse off (fastints aren't masked by the
> cpl anyway).
IMHO it explains why we need the proctree_lock (in early SMP times
scheduler mutex). Maybe this should be replaced with a proper tty subsystem
mutex ...)
After looking at our SMPnewgen Page, Poul Henning and Thomas Moestel
are working on locking the tty subsystem. I'll CC those :-)
Martin
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