Benjamin Herrenschmidt <b...@kernel.crashing.org> writes: > On Wed, 2016-09-07 at 10:17 +0530, Nikunj A Dadhania wrote: >> > David Gibson <da...@gibson.dropbear.id.au> writes: >> >> > >> > [ Unknown signature status ] >> > On Fri, Sep 02, 2016 at 12:02:55PM +0530, Nikunj A Dadhania wrote: >> > > >> > > > > > Signed-off-by: Nikunj A Dadhania <nik...@linux.vnet.ibm.com> >> > >> > This really needs a comment indicating that this implementation isn't >> > strictly correct (although probably good enough in practice). >> >> Sure. And it also does not help if someone uses any store other than >> store conditional, that isn't taken care. >> >> Assumption here is the locking primitives use load with reservation and >> store conditional. And no other ld/st variant touch this memory. > > This is an incorrect assumption. spin_unlock for example uses a normal > store. > > That being said, you will observe the difference in value which should > hopefully make things work... > > I *hope* we don't have anything that relies on a normal store of the same > value as the atomic breaking the reservation, I *think* we might get away > with it, but it is indeed fishy.
In arch/powerpc/include/asm/spinlock.h: We have __arch_spin_trylock() which uses lwarx and "stwcx." instructions to acquire lock. And later during the unlock, a normal store will do. IMHO, that is the reason for it work. Regards, Nikunj
