Re: sem_lock() vs qspinlocks

[Manfred Spraul <manfred@colorfullife.com>]

Hi Peter,


On 05/20/2016 06:04 PM, Peter Zijlstra wrote:
> On Fri, May 20, 2016 at 05:21:49PM +0200, Peter Zijlstra wrote:
>
>> Let me write a patch..
> OK, something like the below then.. lemme go build that and verify that
> too fixes things.
>
> ---
> Subject: locking,qspinlock: Fix spin_is_locked() and spin_unlock_wait()
>
> Similar to commits:
>
>    51d7d5205d33 ("powerpc: Add smp_mb() to arch_spin_is_locked()")
>    d86b8da04dfa ("arm64: spinlock: serialise spin_unlock_wait against concurrent lockers")
>
> qspinlock suffers from the fact that the _Q_LOCKED_VAL store is
> unordered inside the ACQUIRE of the lock.
>
> And while this is not a problem for the regular mutual exclusive
> critical section usage of spinlocks, it breaks creative locking like:
>
> 	spin_lock(A)			spin_lock(B)
> 	spin_unlock_wait(B)		if (!spin_is_locked(A))
> 	do_something()			  do_something()
>
> In that both CPUs can end up running do_something at the same time,
> because our _Q_LOCKED_VAL store can drop past the spin_unlock_wait()
> spin_is_locked() loads (even on x86!!).
How would we handle mixed spin_lock()/mutex_lock() code?
For the IPC code, I would like to replace the outer lock with a mutex.
The code only uses spinlocks, because at the time it was written, the 
mutex code didn't contain a busy wait.
With a mutex, the code would become simpler (all the 
lock/unlock/kmalloc/relock parts could be removed).

The result would be something like:

	mutex_lock(A)			spin_lock(B)
	spin_unlock_wait(B)		if (!mutex_is_locked(A))
	do_something()			  do_something()

--
     Manfred