Date: Mon, 25 Feb 2002 19:19:22 -0800 (PST) From: Matthew Dillon <dillon@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! Message-ID: <200202260319.g1Q3JMf55372@apollo.backplane.com> References: <20020226121245.F42820-100000@gamplex.bde.org>
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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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