Discussion:
LL/SC live lock
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Timothy Hayes
2018-12-05 18:34:16 UTC
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I’m benchmarking some multithreaded ARMv8 codes on Gem5-Ruby and am constantly running into live lock. I’ve started investigating this and am a little confused the with regards to the way load-linked and store-conditional (LDEX/STEX) are treated in Ruby and its protocols.

In all of the coherence protocol implementations that I’ve looked at so far, RubyRequestType::ATOMIC types are treated the same as stores. This means that the LDEX will try to load its address in modified state. This doesn’t appear to play well with the idiomatic way of writing a spinlock in ARMv8, e.g.

spin_mutex_lock(lock):
spinlock_unlock_wait(lock);
spinlock_lock(lock);
spinlock_unlock_wait(lock):
int tmp;
__asm__ volatile(
" sevl\n"
" 1: wfe\n"
" ldaxr %w0, %1\n"
" cbnz %w0, 1b\n"
: "=&r" (tmp)
: "Q" (lock));
spinlock_lock(lock):
int lockval = 1;
int tmp;
__asm__ volatile(
" sevl\n"
" prfm pstl1strm, %1\n"
" 1: wfe\n"
" 2: ldaxr %w0, %2\n"
" cbnz %w0, 1b\n"
" stxr %w0, %w3, %1\n"
" cbnz %w0, 2b\n"
: "=&r" (tmp), "+Q" (lock)
: "Q" (lock), "r" (lockval)
: "memory");
spinlock_unlock(lock):
atomic_store_explicit(&lck->lock, 0, memory_order_release);

The issue I see is that both unlock_wait and lock use LDEX which loads the lock in modified state. If there is contention for the lock, the LDEX will cause the lock to ping-pong in modified state before it is ever taken which can lead to live lock under modest contention.

Could somebody explain the rationale behind this implementation of load linked/store conditional? I’m guessing it was written with another architecture in mind (Alpha?) but I’m unsure how a mutex could be correctly implemented given these characteristics.
--
Timothy Hayes
Senior Research Engineer
Arm Research
Phone: +44-1223405170
***@arm.com


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