Re: [PATCH v2 0/9] Remove spin_unlock_wait()

From: Manfred Spraul
Date: Mon Jul 10 2017 - 13:22:33 EST

Hi Alan,

On 07/08/2017 06:21 PM, Alan Stern wrote:
Pardon me for barging in, but I found this whole interchange extremely

On Sat, 8 Jul 2017, Ingo Molnar wrote:

* Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:

On Sat, Jul 08, 2017 at 10:35:43AM +0200, Ingo Molnar wrote:
* Manfred Spraul <manfred@xxxxxxxxxxxxxxxx> wrote:

Hi Ingo,

On 07/07/2017 10:31 AM, Ingo Molnar wrote:
There's another, probably just as significant advantage: queued_spin_unlock_wait()
is 'read-only', while spin_lock()+spin_unlock() dirties the lock cache line. On
any bigger system this should make a very measurable difference - if
spin_unlock_wait() is ever used in a performance critical code path.
At least for ipc/sem:
Dirtying the cacheline (in the slow path) allows to remove a smp_mb() in the
hot path.
So for sem_lock(), I either need a primitive that dirties the cacheline or
sem_lock() must continue to use spin_lock()/spin_unlock().
This statement doesn't seem to make sense. Did Manfred mean to write
"smp_mb()" instead of "spin_lock()/spin_unlock()"?
Option 1:
smp_mb(); [[1]]
slow path:
global_flag = 1;

Option 2:
slow path:
global_flag = 1;

The ACQUIRE from spin_lock is at the read of local_lock, not at the write.
i.e.: Without the smp_mb() at [[1]], the CPU can do:
read local_lock;
read global_flag;
write local_lock;
For Option 2, the smp_mb() is not required, because fast path and slow path acquire the same lock.

Technically you could use spin_trylock()+spin_unlock() and avoid the lock acquire
spinning on spin_unlock() and get very close to the slow path performance of a
pure cacheline-dirtying behavior.
This is even more confusing. Did Ingo mean to suggest using
"spin_trylock()+spin_unlock()" in place of "spin_lock()+spin_unlock()"
could provide the desired ordering guarantee without delaying other
CPUs that may try to acquire the lock? That seems highly questionable.
I agree :-)