NULL pointer dereference at fat_detach

[Stanislaw Gruszka]

Hi

>From time to time Fedora users reports crash at fat_detach.
It happens randomly and seldom. Seems to be related with
unmount operation.

Early reports are from 3.0 and problem still randomly occurs
on recent kernels.

Calltraces looks like below:

BUG: unable to handle kernel NULL pointer dereference at 0000009c
IP: [<c046d983>] do_raw_spin_lock+0xd/0x1e

[<c07fcc94>] _raw_spin_lock+0xd/0xf
[<f8fe03de>] fat_detach+0x20/0x59 [fat]
[<f8fe0874>] fat_evict_inode+0x5b/0x5e [fat]
[<c05058ad>] evict+0x57/0xe9
[<c0505a67>] iput+0xf8/0xfd
[<c051ce0f>] fsnotify_destroy_mark+0xdf/0xf8
[<c051e488>] sys_inotify_rm_watch+0x59/0x79
[<c07fcfbc>] syscall_call+0x7/0xb

what most likely indicate that fat_evict is called with
inode with i_sb->s_fs_info == NULL.

Some more info about this problem can be found here:
https://bugzilla.redhat.com/show_bug.cgi?id=768534

Any insight on this isssue is welcome.

Thanks
Stanislaw 
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