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SubjectRe: Oops NFS Locking in 2.4.x
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>>>>> " " == Olivier Molteni <olivier@molteni.net> writes:


> Unable to handle kernel NULL pointer dereference at virtual
> address 00000000 c013d6e7 *pde = 00000000 Oops: 0000 CPU: 0
> EIP: 0010:[<c013d6e7>] Using defaults from ksymoops -t
> elf32-i386 -a i386 EFLAGS: 00010296 eax: 00000000 ebx: 00000000
> ecx: 00000000 edx: f7d77960 esi: 00000004 edi: 00000001 ebp:
> bfffdb2c esp: c626ff74 ds: 0018 es: 0018 ss: 0018 Process
> cyrdeliver (pid: 15652, stackpage=c626f000) Stack: f7d77960
> ed00eac0 c013f048 f7d77960 00000000 ecc895c0 00000000 c012e3f5
> ecc895c0 ed00eac0 00000000 ecc895c0 00000000 ecc895c0
> bfffbb2c
> c012e447
> ecc895c0 ed00eac0 c626e000 c0106d2b 00000004 00000004
> bfffcb2c
> bfffbb2c Call Trace: [<c013f048>] [<c012e3f5>] [<c012e447>]
> [<c0106d2b>] Code: 8b 03 89 02 c7 03 00 00 00 00 8b 56 04 8b 43
> 04 89 50 04 89
>>> EIP; c013d6e7 <locks_delete_lock+b/e8> <=====
> Trace; c013f048 <locks_remove_posix+58/6c> Trace; c012e3f5
> <filp_close+55/64> Trace; c012e447 <sys_close+43/54> Trace;
> c0106d2b <system_call+33/38> Code; c013d6e7
> <locks_delete_lock+b/e8> 00000000 <_EIP>: Code; c013d6e7
> <locks_delete_lock+b/e8> <=====
> 0: 8b 03 mov (%ebx),%eax <=====
> Code; c013d6e9 <locks_delete_lock+d/e8>
> 2: 89 02 mov %eax,(%edx)
> Code; c013d6eb <locks_delete_lock+f/e8>
> 4: c7 03 00 00 00 00 movl $0x0,(%ebx)
> Code; c013d6f1 <locks_delete_lock+15/e8>
> a: 8b 56 04 mov 0x4(%esi),%edx
> Code; c013d6f4 <locks_delete_lock+18/e8>
> d: 8b 43 04 mov 0x4(%ebx),%eax
> Code; c013d6f7 <locks_delete_lock+1b/e8>
> 10: 89 50 04 mov %edx,0x4(%eax)
> Code; c013d6fa <locks_delete_lock+1e/e8>
> 13: 89 00 mov %eax,(%eax)

Looks like 2 processes are trying to free the same lock. The problem
is that both processes can call filp_close() at the same
time (by calling sys_close()).

The bug boils down to:

- locks_unlock_delete() assumes that the BKL (kernel_lock()) is
sufficient to protect against *thisfl_p from disappearing
beneath it due to some second process.
BUT
- The call to lock() in locks_unlock_delete() sleeps when the
underlying filesystem is NFS, hence 2 processes can race despite
the BKL assumption.

Cheers,
Trond
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