From owner-freebsd-current  Mon Feb 25 19:19:44 2002
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Date: Mon, 25 Feb 2002 19:19:22 -0800 (PST)
From: Matthew Dillon <dillon@apollo.backplane.com>
Message-Id: <200202260319.g1Q3JMf55372@apollo.backplane.com>
To: Bruce Evans <bde@zeta.org.au>
Cc: Terry Lambert <tlambert2@mindspring.com>,
	Alfred Perlstein <alfred@FreeBSD.ORG>,
	Bosko Milekic <bmilekic@unixdaemons.com>,
	Seigo Tanimura <tanimura@r.dl.itc.u-tokyo.ac.jp>,
	<current@FreeBSD.ORG>, John Baldwin <jhb@FreeBSD.ORG>
Subject: Re: Patch for critical_enter()/critical_exit() & interrupt assembly
 revamp, please review!
References:  <20020226121245.F42820-100000@gamplex.bde.org>
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X-Loop: FreeBSD.ORG

    It kinda sounds like the interrupt/trap code that calls into
    kern/subr_trap.c should be responsible for placing us in a 
    critical section.  kern/subr_trap.c has no business making
    assumptions in regards to cpu_critical_enter()/cpu_critical_exit()
    (for the same reason that fork_exit() has no business making
    those kinds of assumptions).

    For the moment I'm not going to worry about it.  I'll just keep the
    cpu_critical_*() API intact for i386 for now.

					-Matt
					Matthew Dillon 
					<dillon@backplane.com>

:The use of cpu_critical*() in subr_trap.c is a bit trickier/kludgier.
:We need to return from the interrupt handler atomically with clearing
:the lazy-masking flag.  The hardware interrupt enable flag must be
:used for this on i386's.  The code for this is logically:
:
:In ast:
:	/* Prevent changes to flags while we are deciding if they are set. */
:	critical_enter();
:
:	while ((ke->ke_flags & (KEF_ASTPENDING | KEF_NEEDRESCHED)) != 0) {
:		critical_exit();
:		...
:		/* As above. */
:		critical_enter();
:	}
:
:In doreti:
:	/*
:	 * Must use MD code to prevent race window after critical_exit().
:	 * We switch from lazy sofware masking using td_critnest (or
:	 * whatever critical_enter() uses) to hardware masking using cli.
:	 */
:	cli();
:	critical_exit();
:	...
:	iretd();
:
:but this is optimized in -current by using cpu_critical*() instead of
:critical_enter*() in ast() and not doing the cli() and critical_exit()
:in doreti.  This depends on cpu_critical_enter() being precisely cli().
:
:Bruce
:


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