Re: [PATCH] locking: Remove an insn from spin and write locks

[Waiman Long <longman@redhat.com>]

On 08/20/2018 11:50 AM, Matthew Wilcox wrote:
> 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.

We will certainly need to keep the _acquire suffix or it will likely
regress performance for arm64.

>> 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.

I think both Sparc and mips are using qlocks now, though these
architectures are not the ones that I am interested in. I do like to
make sure that there will be no regression for arm64. Will should be
able to answer that.

Cheers,
Longman