Date: Sun, 9 May 2010 19:04:15 +0200 From: Marius Strobl <marius@alchemy.franken.de> To: Attilio Rao <attilio@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r206878 - head/sys/kern Message-ID: <20100509170414.GA35602@alchemy.franken.de> In-Reply-To: <201004192327.o3JNRsK4047856@svn.freebsd.org> References: <201004192327.o3JNRsK4047856@svn.freebsd.org>
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On Mon, Apr 19, 2010 at 11:27:54PM +0000, Attilio Rao wrote: > Author: attilio > Date: Mon Apr 19 23:27:54 2010 > New Revision: 206878 > URL: http://svn.freebsd.org/changeset/base/206878 > > Log: > Fix a deadlock in the shutdown code: > When performing a smp_rendezvous() or more likely, on amd64 and i386, > a smp_tlb_shootdown() the caller will end up with the smp_ipi_mtx > spinlock held, busy-waiting for other CPUs to acknowledge the operation. > As long as CPUs are suspended (via cpu_reset()) between the active mask > read and IPI sending there can be a deadlock where the caller will wait > forever for a dead CPU to acknowledge the operation. > Please note that on CPU0 that is going to be someway heavier because of > the spinlocks being disabled earlier than quitting the machine. > > Fix this bug by calling cpu_reset() with the smp_ipi_mtx held. > Note that it is very likely that a saner offline/online CPUs mechanism > will help heavilly in fixing similar cases as it is likely more bugs > of this type may arise in the future. > This change causes a hang for me when running an SMP kernel on an UP machine or an MP machine with SMP disabled as in these cases smp_ipi_mtx isn't initialized (see mp_start()). The below patch fixes this for me. Marius Index: kern_shutdown.c =================================================================== --- kern_shutdown.c (revision 207463) +++ kern_shutdown.c (working copy) @@ -491,8 +491,8 @@ shutdown_reset(void *junk, int howto) /* * Acquiring smp_ipi_mtx here has a double effect: * - it disables interrupts avoiding CPU0 preemption - * by fast handlers (thus deadlocking against other CPUs) - * - it avoids deadlocks against smp_rendezvous() or, more + * by fast handlers (thus deadlocking against other CPUs) + * - it avoids deadlocks against smp_rendezvous() or, more * generally, threads busy-waiting, with this spinlock held, * and waiting for responses by threads on other CPUs * (ie. smp_tlb_shootdown()). @@ -500,9 +500,11 @@ shutdown_reset(void *junk, int howto) * For the !SMP case it just needs to handle the former problem. */ #ifdef SMP - mtx_lock_spin(&smp_ipi_mtx); + if (mtx_initialized(&smp_ipi_mtx) != 0) + mtx_lock_spin(&smp_ipi_mtx); + else #else - spinlock_enter(); + spinlock_enter(); #endif /* cpu_boot(howto); */ /* doesn't do anything at the moment */
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