Re: [RFC PATCH] semaphore: fix uninitialized list_head vs list_force_poison

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 01 Mar 2016 11:55:04 -0800 Dan Williams <dan.j.williams@intel.com> wrote:

> list_force_poison is a debug mechanism to make sure that ZONE_DEVICE
> pages never appear on an lru.  Those pages only exist for enabling DMA
> to device discovered memory ranges and are not suitable for general
> purpose allocations.  list_force_poison() explicitly initializes a
> list_head with a poison value that list_add() can use to detect mistaken
> use of page->lru.
> 
> Unfortunately, it seems calling list_add() leads to the poison value
> leaking on to the stack and occasionally cause stack-allocated
> list_heads to be inadvertently "force poisoned".
> 
>  list_add attempted on force-poisoned entry
>  WARNING: at lib/list_debug.c:34
>  [..]
>  NIP [c00000000043c390] __list_add+0xb0/0x150
>  LR [c00000000043c38c] __list_add+0xac/0x150
>  Call Trace:
>  [c000000fb5fc3320] [c00000000043c38c] __list_add+0xac/0x150 (unreliable)
>  [c000000fb5fc33a0] [c00000000081b454] __down+0x4c/0xf8
>  [c000000fb5fc3410] [c00000000010b6f8] down+0x68/0x70
>  [c000000fb5fc3450] [d0000000201ebf4c] xfs_buf_lock+0x4c/0x150 [xfs]
> 
>  list_add attempted on force-poisoned entry(0000000000000500),
>   new->next == d0000000059ecdb0, new->prev == 0000000000000500
>  WARNING: at lib/list_debug.c:33
>  [..]
>  NIP [c00000000042db78] __list_add+0xa8/0x140
>  LR [c00000000042db74] __list_add+0xa4/0x140
>  Call Trace:
>  [c0000004c749f620] [c00000000042db74] __list_add+0xa4/0x140 (unreliable)
>  [c0000004c749f6b0] [c0000000008010ec] rwsem_down_read_failed+0x6c/0x1a0
>  [c0000004c749f760] [c000000000800828] down_read+0x58/0x60
>  [c0000004c749f7e0] [d000000005a1a6bc] xfs_log_commit_cil+0x7c/0x600 [xfs]
> 
> We can squash these uninitialized list_heads as they pop-up as this
> patch does, or maybe need to rethink how to implement the
> list_force_poison() safety mechanism.

Yes, problem.

>  kernel/locking/rwsem-xadd.c |    4 +++-
>  kernel/locking/semaphore.c  |    4 +++-

The patch adds slight overhead and there will be other uninitialized
list_heads around the place and more will turn up in the future.

I don't see how list_force_poison is fixable, really - we're relying
upon some uninitialized word of memory not having some particular value.
Good luck with that.

Maybe we simply remove list_force_poison() - it isn't terribly
important?

	/* ZONE_DEVICE pages must never appear on a slab lru */

Can we instead add a check of page_zone(page) into the lru-addition
sites?  There are probably quite a few possible places.  (Why does the
comment say "slab"?).

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