[PATCH] mm: slub: Ensure that slab_unlock() is atomic

[Christoph Lameter]

On Tue, 8 Mar 2016, Vineet Gupta wrote:

> # set the bit
> 80543b8e:	ld_s       r2,[r13,0] <--- (A) Finds PG_locked is set
> 80543b90:	or         r3,r2,1    <--- (B) other core unlocks right here
> 80543b94:	st_s       r3,[r13,0] <--- (C) sets PG_locked (overwrites unlock)

Duh. Guess you  need to take the spinlock also in the arch specific
implementation of __bit_spin_unlock(). This is certainly not the only case
in which we use the __ op to unlock.

You need a true atomic op or you need to take the "spinlock" in all
cases where you modify the bit. If you take the lock in __bit_spin_unlock
then the race cannot happen.

> Are you convinced now !

Yes, please fix your arch specific code.