FreeBSD 6.3 deadlock (vm_map?) with DDB output

James Gritton jamie at
Thu Jun 19 16:18:34 UTC 2008

John Baldwin wrote:
> On Sunday 15 June 2008 07:23:19 am Stef Walter wrote:
>> I've been trying to track down a deadlock on some newish production
>> servers running FreeBSD 6.3-RELEASE-p2. The deadlock occurs on a
>> specific (although mundane) hardware configuration, and each of several
>> servers running this hardware deadlock about once per week.
>> Although I suspect that this is not hardware related, from a (naive)
>> perusal of the attached stack traces.
>> Forgive me if my interpretation of this is all wrong, but I'm pretty
>> desperate for help. So here's my basic understanding of the deadlock:
>> These processes seem to be waiting on the page queue mutex:
>>  sendmail (in vm_mmap > vm_map_find > vm_map_insert > vm_map_pmap_enter)
>>  bsnmpd (in malloc, uma_large_malloc > page_alloc > kmem_malloc)
>>  httpd (in trap > trap_pfault > vm_fault)
>>  [g_up] (in g_vfs_done > bufdone)
>> The page queue mutex is held by rsync process:
>>  rsync (in trap > trap_pfault > vm_fault > pmap_enter)
>> Rsync kernel process (in pmap_enter) was interrupted while holding the
>> page queue lock?
>> Giant is enabled in loader.conf due to the needs of the pf firewall when
>> dealing with user credentials lookups. I do not believe that Giant plays
>> into this deadlock. Kernel config attached.
>> Any and all help or info is welcome. Thanks in advance.
> Try this change:
> jhb         2007-10-27 22:07:40 UTC
>   FreeBSD src repository
>   Modified files:
>     sys/kern             sched_4bsd.c
>   Log:
>   Change the roundrobin implementation in the 4BSD scheduler to trigger a
>   userland preemption directly from hardclock() via sched_clock() when a
>   thread uses up a full quantum instead of using a periodic timeout to cause
>   a userland preemption every so often.  This fixes a potential deadlock
>   when IPI_PREEMPTION isn't enabled where softclock blocks on a lock held
>   by a thread pinned or bound to another CPU.  The current thread on that
>   CPU will never be preempted while softclock is blocked.
>   Note that ULE already drives its round-robin userland preemption from
>   sched_clock() as well and always enables IPI_PREEMPT.
>   MFC after:      1 week
>   Revision  Changes    Path
>   1.108     +8 -29     src/sys/kern/sched_4bsd.c
> We use it at work on 6.x.  W/o this fix, round-robin stops working on 4BSD 
> when softclock() (swi4: clock) blocks on a lock like Giant.

I've been seeing similar troubles on 6.2 and I'll have to give this a 
try as we upgrade to 6.3.  I notice "MFC after: 1 week" in the log; it's 
been a week - any chance of seeing this fix rolled into 6.x?

- Jamie

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