Re: Preemption-related bug in propagate_priority(): it's OK to hold Giant but not be running

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 _at_
> 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.

-- 

John Baldwin <jhb_at_FreeBSD.org>  <><  http://www.FreeBSD.org/~jhb/
"Power Users Use the Power to Serve!"  -  http://www.FreeBSD.org/