panic in closef

[David Cecil]

Hi,

I'm debugging a panic with the call trace below.  This is from a 
6.1-based kernel, but as far as I can tell there are no fixes for this 
problem in current, based on where I think the problem lies.

It falls over in the call to VFS_LOCK_GIANT in closef, trying to 
dereference a NULL mount point.  To me, the check in VFS_NEEDSGIANT 
looks flawed; it checks that MP isn't NULL, then dereferences it, but 
I'd expect the vnode interlock to be held (or similar), but it isn't 
based on what I see of the code.

I looks to me like vgonel was probably running around the same time.  It 
calls delmntque and NULLs v_mount.  I can't see how these two threads 
are synchronised to prevent the race condition I describe, but maybe I'm 
missing something.

Any ideas?

Thanks,
Dave

Unread portion of the kernel message buffer:
2
panic: page fault
cpuid = 0
KDB: stack backtrace:
db_trace_self_wrapper(60794d20) at db_trace_self_wrapper+0x25
kdb_backtrace(f8d0eae8,100,67aeb640,28,f8d0eb64,...) at kdb_backtrace+0x29
panic(60774fbe,607bae69,0,fffff,681dc89b,...) at panic+0x230
trap_fatal(f8d0eb64,6f,67aeb640,0,c,...) at trap_fatal+0x2ce
trap_pfault(f8d0eb64,0,6f) at trap_pfault+0x1ef
trap(7ae70008,28,f8d00028,3,8236e104,...) at trap+0x36d
calltrap() at calltrap+0x5
--- trap 0xc, eip = 0x60565e72, esp = 0xf8d0eba4, ebp = 0xf8d0ec00 ---
closef(7d7f19cc,67aeb640) at closef+0x36
fdfree(67aeb640) at fdfree+0x5a7
exit1(67aeb640,100,f8d0ed30,6074c903,67aeb640,...) at exit1+0x4ee
exit1(67aeb640,f8d0ed04) at exit1
syscall(5fbf003b,812003b,5fbf003b,0,0,...) at syscall+0x2b7
Xint0x80_syscall() at Xint0x80_syscall+0x1f