vmalloc: remove BUG_ON due to racy counting of VM_LAZY_FREE

In free_unmap_area_noflush(), va->flags is marked as VM_LAZY_FREE first, and
then vmap_lazy_nr is increased atomically.

But, in __purge_vmap_area_lazy(), while traversing of vmap_are_list, nr
is counted by checking VM_LAZY_FREE is set to va->flags.  After counting
the variable nr, kernel reads vmap_lazy_nr atomically and checks a
BUG_ON condition whether nr is greater than vmap_lazy_nr to prevent
vmap_lazy_nr from being negative.

The problem is that, if interrupted right after marking VM_LAZY_FREE,
increment of vmap_lazy_nr can be delayed.  Consequently, BUG_ON
condition can be met because nr is counted more than vmap_lazy_nr.

It is highly probable when vmalloc/vfree are called frequently.  This
scenario have been verified by adding delay between marking VM_LAZY_FREE
and increasing vmap_lazy_nr in free_unmap_area_noflush().

Even the vmap_lazy_nr is for checking high watermark, it never be the
strict watermark.  Although the BUG_ON condition is to prevent
vmap_lazy_nr from being negative, vmap_lazy_nr is signed variable.  So,
it could go down to negative value temporarily.

Consequently, removing the BUG_ON condition is proper.

A possible BUG_ON message is like the below.

   kernel BUG at mm/vmalloc.c:517!
   invalid opcode: 0000 [#1] SMP
   EIP: 0060:[<c04824a4>] EFLAGS: 00010297 CPU: 3
   EIP is at __purge_vmap_area_lazy+0x144/0x150
   EAX: ee8a8818 EBX: c08e77d4 ECX: e7c7ae40 EDX: c08e77ec
   ESI: 000081fe EDI: e7c7ae60 EBP: e7c7ae64 ESP: e7c7ae3c
   DS: 007b ES: 007b FS: 00d8 GS: 0033 SS: 0068
   Call Trace:
   [<c0482ad9>] free_unmap_vmap_area_noflush+0x69/0x70
   [<c0482b02>] remove_vm_area+0x22/0x70
   [<c0482c15>] __vunmap+0x45/0xe0
   [<c04831ec>] vmalloc+0x2c/0x30
   Code: 8d 59 e0 eb 04 66 90 89 cb 89 d0 e8 87 fe ff ff 8b 43 20 89 da 8d 48 e0 8d 43 20 3b 04 24 75 e7 fe 05 a8 a5 a3 c0 e9 78 ff ff ff <0f> 0b eb fe 90 8d b4 26 00 00 00 00 56 89 c6 b8 ac a5 a3 c0 31
   EIP: [<c04824a4>] __purge_vmap_area_lazy+0x144/0x150 SS:ESP 0068:e7c7ae3c

[ See also http://marc.info/?l=linux-kernel&m=126335856228090&w=2

 ]

Signed-off-by: Yongseok Koh <yongseok.koh@samsung.com>
Reviewed-by: Minchan Kim <minchan.kim@gmail.com>
Cc: Nick Piggin <npiggin@suse.de>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>