On Mon, Aug 20, 2018 at 11:14:04AM -0400, Waiman Long wrote:
> On 08/20/2018 11:06 AM, Matthew Wilcox wrote:
> > Both spin locks and write locks currently do:
> >
> > f0 0f b1 17 lock cmpxchg %edx,(%rdi)
> > 85 c0 test %eax,%eax
> > 75 05 jne [slowpath]
> >
> > This 'test' insn is superfluous; the cmpxchg insn sets the Z flag
> > appropriately. Peter pointed out that using atomic_try_cmpxchg()
> > will let the compiler know this is true. Comparing before/after
> > disassemblies show the only effect is to remove this insn.
...
> > static __always_inline int queued_spin_trylock(struct qspinlock *lock)
> > {
> > + u32 val = 0;
> > +
> > if (!atomic_read(&lock->val) &&
> > - (atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL) == 0))
> > + (atomic_try_cmpxchg(&lock->val, &val, _Q_LOCKED_VAL)))
>
> Should you keep the _acquire suffix?
I don't know ;-) Probably. Peter didn't include it as part of his
suggested fix, but on reviewing the documentation, it seems likely that
it should be retained. I put them back in and (as expected) it changes
nothing on x86-64.
> BTW, qspinlock and qrwlock are now also used by AArch64, mips and sparc.
> Have you tried to see what the effect will be for those architecture?
Nope! That's why I cc'd linux-arch, because I don't know who (other
than arm64 and x86) is using q-locks these days.
