Date: Tue, 19 Oct 2004 14:34:01 -0700 From: Julian Elischer <julian@elischer.org> To: John Baldwin <jhb@freebsd.org> Cc: Robert Watson <rwatson@freebsd.org> Subject: Re: Preemption-related bug in propagate_priority(): it's OK to hold Giant but not be running Message-ID: <41758849.5080500@elischer.org> In-Reply-To: <200410191718.37092.jhb@FreeBSD.org> References: <Pine.NEB.3.96L.1041017151317.29749A-100000@fledge.watson.org> <200410191703.52737.jhb@FreeBSD.org> <200410191718.37092.jhb@FreeBSD.org>
next in thread | previous in thread | raw e-mail | index | archive | help
John Baldwin wrote: >On Tuesday 19 October 2004 05:03 pm, John Baldwin wrote: > > >>On Sunday 17 October 2004 03:17 pm, Robert Watson wrote: >> >> >>>Ran into an interesting problem relating to WITNESS and a >>>multiply-destroyed mutex today, but I didn't get the real panic message >>>because of what looks like one or two other bugs. Here's the high-level: >>> >>>sodealloc() tries to destroy the mutex on a socket more than once, >>>causing WITNESS to panic. The soft clock ithread preempts the serial >>>output in panic() when a critical section is released, resulting in a >>>context switch to that ithread. That ithread then attempts to lock a >>>mutex, and is horrified to find out that Giant is held by the thread it >>>preempted, but that that thread isn't blocked: >>> >>>tiger-2# ppanic: process 15955(tcpconnect):2 holds Giant but isn't >>>blocked on a lock >>> >>> % rwatson -- note that double p is from the initiating panic >>> >>>cpuid = 0 >>>KDB: enter: panic >>>[thread 100071] >>>Stopped at kdb_enter+0x2b: nop >>>db> trace >>>kdb_enter(c07fc101) at kdb_enter+0x2b >>>panic(c07ff1ec,3e53,c5f90588,2,c080c957) at panic+0x127 >>>propagate_priority(c547b180,c08c47b8,c5f91780,c08bdbc0,0) at >>>propagate_priority+ >>>0x139 >>>turnstile_wait(0,c08bdbc0,c5f91780,c08bdbc0,2,c07fb4a5,217) at >>>turnstile_wait+0x >>>2de >>>_mtx_lock_sleep(c08bdbc0,c547b180,0,c07fd124,f7) at _mtx_lock_sleep+0x15f >>>_mtx_lock_flags(c08bdbc0,0,c07fd124,f7,0) at _mtx_lock_flags+0x85 >>>softclock(0) at softclock+0x16a >>>ithread_loop(c5436e80,e942fd48,c5436e80,c05f7c80,0) at ithread_loop+0x124 >>>fork_exit(c05f7c80,c5436e80,e942fd48) at fork_exit+0xa4 >>>fork_trampoline() at fork_trampoline+0x8 >>>--- trap 0x1, eip = 0, esp = 0xe942fd7c, ebp = 0 --- >>>db> show locks >>>db> show pcpu >>>cpuid = 0 >>>curthread = 0xc547b180: pid 87 "swi4: clock sio" >>>curpcb = 0xe942fda0 >>>fpcurthread = none >>>idlethread = 0xc53e4a80: pid 14 "idle: cpu0" >>>APIC ID = 0 >>>currentldt = 0x28 >>>spin locks held: >>>db> ps >>> pid proc uarea uid ppid pgrp flag stat wmesg wchan >>>cmd 15955 c5f90400 ebbb1000 0 525 15955 0004002 [Can run] tcpconnect >>>... >>> 87 c548b400 e946e000 0 0 0 000020c [LOCK Giant c5432d00] >>>swi4: clock sio >>>db> trace 15955 >>>sched_switch(c5f91780,c5451d80,6) at sched_switch+0x16f >>>mi_switch(6,c5451d80,c5451ed0,c5451d80,ebba492c) at mi_switch+0x264 >>>maybe_preempt(c5451d80) at maybe_preempt+0x156 >>>sched_add(c5451d80,4,c53e2d80,c5451d80,c5450800) at sched_add+0x153 >>>setrunqueue(c5451d80,4) at setrunqueue+0xab >>>ithread_schedule(c53e2d80,1f,c5f91780,c5f91780,2580) at >>>ithread_schedule+0xb3 >>>intr_execute_handlers(c53dc0d8,ebba49a8,1f,ebba49f0,c0780ad3) at >>>intr_execute_ha >>>ndlers+0xf5 >>>lapic_handle_intr(4f) at lapic_handle_intr+0x2e >>>Xapic_isr2() at Xapic_isr2+0x33 >>>--- interrupt, eip = 0xc0619446, esp = 0xebba49ec, ebp = 0xebba49f0 --- >>>critical_exit(3f8,ebba4a30,c076f731,c08fa000,0) at critical_exit+0x92 >>>_mtx_unlock_spin_flags(c08fa000,0,c08174fc,be4) at >>>_mtx_unlock_spin_flags+0x8d >>>siocnputc(c08987c0,70) at siocnputc+0xb9 >>>cnputc(70) at cnputc+0x4d >>>putchar(70,ebba4b44) at putchar+0x52 >>>kvprintf(c07fc2ed,c06219b8,ebba4b44,a,ebba4b64) at kvprintf+0x77 >>>printf(c07fc2ed,c08bf060,c08bf060,100,c07ff91e,ebba4b98) at printf+0x43 >>>panic(c07ff91e,c07b69b1,c0813aa8,c07ff6af,c5a23ebc) at panic+0xcb >>>witness_destroy(c5a23ebc,c5a23dec,ebba4bd0,c063f15f,c5a23ebc) at >>>witness_destroy >>>+0x40 >>>mtx_destroy(c5a23ebc,c5e94e00,c5a23ebc,ebba4be8,c063f4fd) at >>>mtx_destroy+0x7a >>>sodealloc(c5a23dec,c5a23eb0,c5832bb0,c5a23dec,ebba4c00) at sodealloc+0xf3 >>>sofree(c5a23dec) at sofree+0x281 >>>soclose(c5a23dec,c5832bb0,0,ebba4c2c,c05efad0) at soclose+0x24d >>>soo_close(c5832bb0,c5f91780) at soo_close+0x4b >>>fdrop_locked(c5832bb0,c5f91780,c53ccf40,0,c07f90b1) at fdrop_locked+0x84 >>>fdrop(c5832bb0,c5f91780,ebba4c78,c0628c5c,c07f90ba) at fdrop+0x24 >>>closef(c5832bb0,c5f91780,c5ebc628,0,c07f90b1) at closef+0x1db >>>close(c5f91780,ebba4d14,1,14b,292) at close+0xe6 >>>syscall(2f,bfbf002f,bfbf002f,3,bfbfedf7) at syscall+0x213 >>>Xint0x80_syscall() at Xint0x80_syscall+0x1f >>>--- syscall (6, FreeBSD ELF32, close), eip = 0x280c154b, esp = >>>0xbfbfec0c, ebp = >>> 0xbfbfec48 --- >>>db> show locks 15955 >>>exclusive sleep mutex Giant r = 0 (0xc08bdbc0) locked @ >>>kern/kern_descrip.c:967 >>> >>>So on face value, it looks like propagate_priority() is unhappy about the >>>tcpconnect thread (that was preempted) still holding Giant. However, >>>given that preemption is supposed to preempt but leave a thread runnable, >>>this would seem not to be a panic-worthy condition. >>> >>> >>Yeah, pp doesn't like a thread in TD_CAN_RUN(). That state can only happen >>if you have already panic'd though. I can fix it to allow TD_CAN_RUN if >>the kernel has already panic'd though. >> >> > >Actually, this won't work. Here's the problem: the reason you got to pp() is >that you blocked on Giant, right. So your ithread is going to block after >pp() returns. Since the thread at the end of the chain is TD_CAN_RUN and due >to the weird TDF_INPANIC stuff we have in choosethread(), the thread holding >the lock that the ithread needs is _never_ going to run, so your ithread will >never resume. The end result is that if you made pp() just return rather >than panic'ing in this case, you would get a single 'p' character on your >output, and then the machine would deadlock. At least this way you get into >ddb. :-P > it would be relatively simple to put those 'discarded' threads onto a single list in the kernel and 'c' could put them back on the run queue :-) > > >
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?41758849.5080500>