Duplicate free

[David Cecil]

A little more information that I hope might trigger some thoughts.

I have also seen a trace very similar to the one in my original mail, 
but instead of bio_done calling g_disk_done, it calls g_mirror_done.  
This time it panics because it tries to dereference the bio_from field.  
The common thread is that in both cases, the bio is freed prior to the 
done handler referencing it (well at least the bio_from field is NULL in 
the g_mirror_case, so I'm assuming it was freed).  I wonder if these 
handlers are not playing nicely together?  Some locking required between 
them that is missing maybe?


Unread portion of the kernel message buffer:
ad4: TIMEOUT - WRITE_DMA retrying (1 retry left) LBA=46231338

Fatal trap 12: page fault while in kernel mode
cpuid = 1; apic id = 01
fault virtual address   = 0x0
fault code              = supervisor read, page not present
instruction pointer     = 0x20:0x8053318b
stack pointer           = 0x28:0xe1466c4c
frame pointer           = 0x28:0xe1466c54
code segment            = base 0x0, limit 0xfffff, type 0x1b
                        = DPL 0, pres 1, def32 1, gran 1
processor eflags        = interrupt enabled, resume, IOPL = 0
current process         = 19 (swi6: task queue)
trap number             = 12
panic: page fault
cpuid = 1
KDB: stack backtrace:
db_trace_self_wrapper(8075f7be) at db_trace_self_wrapper+0x25
kdb_backtrace(100,863ba190,28,e1466c0c,c,...) at kdb_backtrace+0x29
panic(807403d4,8077f4a5,0,fffff,863be29b,...) at panic+0x124
trap_fatal(e1466c0c,0,863ba190,0,c,...) at trap_fatal+0x2ce
trap_pfault(e1466c0c,0,0) at trap_pfault+0x1e7
trap(e1460008,28,28,8a0cb840,89b1a6b4,...) at trap+0x36d
calltrap() at calltrap+0x5
--- trap 0xc, eip = 0x8053318b, esp = 0xe1466c4c, ebp = 0xe1466c54 ---
g_mirror_done(89b1a6b4) at g_mirror_done+0xb
biodone(89b1a6b4) at biodone+0x8b
ad_done(8a0cb840) at ad_done+0x2a
ata_completed(8a0cb840,0) at ata_completed+0x534
taskqueue_run(8644bac0,e1466cec,80566279,0,0,...) at taskqueue_run+0xbd
taskqueue_swi_run(0) at taskqueue_swi_run+0xe
ithread_execute_handlers(863be200,86433000) at 
ithread_execute_handlers+0x139
ithread_loop(86468460,e1466d38) at ithread_loop+0x64
fork_exit(805662ec,86468460,e1466d38) at fork_exit+0x71
fork_trampoline() at fork_trampoline+0x8
--- trap 0x1, eip = 0, esp = 0xe1466d6c, ebp = 0 ---



ext David Cecil wrote:
> Hi,
>
> I've encountered a duplicate free in 6.1-RELEASE-based code.  I have 
> noticed that the same stack trace was reported about two years ago on 
> a number of occasions, and some code was added to try and help debug 
> the situation.  However, I don't see any resolution.  Does anyone have 
> any more information on this before I try and debug it further?
>
> Any hints for trying to find who freed it first?  Maybe I should add 
> the KTR debug that went into 1.66 of geom_io.c.
>
> db> bt
> Tracing pid 17 tid 100016 td 0x86badaf0
> kdb_enter(80750631) at kdb_enter+0x2b
> panic(807786cd,8a778d80,81856780,80747d25,807786b1,...) at panic+0x137
> uma_dbg_free(81856780,0,8a778d80) at uma_dbg_free+0x110
> uma_zfree_arg(81856780,8a778d80,0) at uma_zfree_arg+0x66
> g_destroy_bio(8a778d80,805319b4,8a778d80,e1ca3c60,805c9620,...) at 
> g_destroy_bio+0x13
> g_disk_done(8a778d80) at g_disk_done+0x62
> biodone(8a778d80) at biodone+0x58
> ad_done(88042840) at ad_done+0x2a
> ata_completed(88042840,0,86c38cdc,0,80753b6d,...) at ata_completed+0x504
> taskqueue_run(86c38cc0,e1ca3cec,8056999a,0,0,...) at taskqueue_run+0x86
> taskqueue_swi_run(0) at taskqueue_swi_run+0xe
> ithread_execute_handlers(86c14000,86c29280) at 
> ithread_execute_handlers+0xfa
> ithread_loop(86c534f0,e1ca3d38,86c534f0,80569a10,0,...) at 
> ithread_loop+0x76
> fork_exit(80569a10,86c534f0,e1ca3d38) at fork_exit+0xa0
> fork_trampoline() at fork_trampoline+0x8
> --- trap 0x1, eip = 0, esp = 0xe1ca3d6c, ebp = 0 ---
>
> The panic string is:
> Duplicate free of item 0x8a778d80 from zone 0x81856780(g_bio)
>
> Thanks,
> Dave
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