Date: Tue, 17 Jan 2012 21:57:35 +0400 From: Gleb Smirnoff <glebius@FreeBSD.org> To: mdf@FreeBSD.org Cc: current@FreeBSD.org Subject: Re: new panic in cpu_reset() with WITNESS Message-ID: <20120117175735.GJ12760@FreeBSD.org> In-Reply-To: <CAMBSHm_iuFwV5Hm7ArzzQbyf1mKgjmC=FGkhqGMdYgCzuJcZZg@mail.gmail.com> References: <20120117110242.GD12760@glebius.int.ru> <CAMBSHm_iuFwV5Hm7ArzzQbyf1mKgjmC=FGkhqGMdYgCzuJcZZg@mail.gmail.com>
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On Tue, Jan 17, 2012 at 07:34:23AM -0800, mdf@freebsd.org wrote: m> 2012/1/17 Gleb Smirnoff <glebius@freebsd.org>: m> > New panic has been introduced somewhere between m> > r229851 and r229932, that happens on shutdown if m> > kernel has WITNESS and doesn't have WITNESS_SKIPSPIN. m> > m> > Uptime: 1h0m17s m> > Rebooting... m> > panic: mtx_lock_spin: recursed on non-recursive mutex cnputs_mtx @ /usr/src/head/sys/kern/kern_cons.c:500 m> > cpuid = 0 m> > KDB: enter: panic m> > [ thread pid 1 tid 100001 ] m> > Stopped at kdb_enter+0x3b: movq $0,0x514d32(%rip) m> > db> m> > db> bt m> > Tracing pid 1 tid 100001 td 0xfffffe0001d5e000 m> > kdb_enter() at kdb_enter+0x3b m> > panic() at panic+0x1c7 m> > _mtx_lock_spin_flags() at _mtx_lock_spin_flags+0x10f m> > cnputs() at cnputs+0x7a m> > putchar() at putchar+0x11f m> > kvprintf() at kvprintf+0x83 m> > vprintf() at vprintf+0x85 m> > printf() at printf+0x67 m> > witness_checkorder() at witness_checkorder+0x773 m> > _mtx_lock_spin_flags() at _mtx_lock_spin_flags+0x99 m> > uart_cnputc() at uart_cnputc+0x3e m> > cnputc() at cnputc+0x4c m> > cnputs() at cnputs+0x26 m> > putchar() at putchar+0x11f m> > kvprintf() at kvprintf+0x83 m> > vprintf() at vprintf+0x85 m> > printf() at printf+0x67 m> > cpu_reset() at cpu_reset+0x81 m> > kern_reboot() at kern_reboot+0x3a5 m> > --More--^M ^Msys_reboot() at sys_reboot+0x42 m> > amd64_syscall() at amd64_syscall+0x39e m> > Xfast_syscall() at Xfast_syscall+0xf7 m> > --- syscall (55, FreeBSD ELF64, sys_reboot), rip = 0x40ea3c, rsp = 0x7fffffffd6d8, rbp = 0x49 --- m> > db> m> > db> show locks m> > exclusive sleep mutex Giant (Giant) r = 0 (0xffffffff809bc560) locked @ /usr/src/head/sys/kern/kern_module.c:101 m> > exclusive spin mutex smp rendezvous (smp rendezvous) r = 0 (0xffffffff80a08840) locked @ /usr/src/head/sys/kern/kern_shutdown.c:542 m> > db> m> > m> > So the problem is that we are holding smp rendezvous mutex during the cpu_reset(). m> > No mutexes should be obtained after it. However, since cpu_reset() does priting m> > we obtain cnputs_mtx, and later obtain uart_hwmtx. The latter is hardcoded in m> > the subr_witness.c as mutex to obtain before smp rendezvous, this triggers m> > yet another printf from witness, that finally panics due to recursing on m> > cnputs_mtx. m> m> At $WORK we explicitly marked cnputs_mtx as NO_WITNESS since it didn't m> seem possible to fit it into the heirarchy in any sane way, since a m> print can come from basically anywhere. m> m> If anyone has a better fix, that'd be great, but I haven't been able m> to think of one. Setting NO_WITNESS on cnputs_mtx won't help for the above problem, IMHO. -- Totus tuus, Glebius.
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