Do spinlocks really need DMB?

Question

I'm working with a dual Cortex-A9 system and I've been trying to understand exactly why spinlock functions need to use DMB. It seems that as long as the merging store buffer is flushed the lock value should end up in the L1 on the unlocking core and the SCU should either invalidate or update the value in the L1 of the other core. This is enough to maintain coherency and safe locking right? And doesn't STREX skip the merging store buffer anyway, meaning we don't even need the flush?

DMB appears to be something of a blunt hammer, especially since it defaults to the system domain, which likely means a write all the way to main memory, which can be expensive.

Are the DMBs in the locks there as a workaround for drivers that don't use smp_mb properly?

I'm currently seeing, based on the performance counters, about 5% of my system cycles disappearing in stalls caused by DMB.

Answer 1

I found these articles may answer your question:

• Locks, SWPs and two Smoking Barriers
• Locks, SWPs and two Smoking Barriers (Part 2)

In particular:

You will note the Data Memory Barrier (DMB) instruction that is issued once the lock has been acquired. The DMB guarantees that all memory accesses before the memory barrier will be observed by all of the other CPUs in the system before all memory accesses made after the memory barrier. This makes more sense if you consider that once a lock has been acquired, a program will then access the data structure(s) locked by the lock. The DMB in the lock function above ensures that accesses to the locked data structure are observed after accesses to the lock.

Answer 2

The DMB is needed in the SMP case because the other processor may see the memory accesses happening in a different order without it, i.e. accesses from inside the critical section may happen before the lock is taken from the point-of-view of the second core.

So the second core could see itself holding the lock and also see updates from inside the cricital section running on the other core, breaking consistency.