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

From: Christoph Lameter
Date: Tue Mar 08 2016 - 15:41:07 EST

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.