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System Architecture and Programming Implications 5–7
This load-locked/store-conditional paradigm may be used whenever an atomic update of a
shared aligned quadword is desired, including getting the effect of atomic byte writes.
5.5.3 Atomic Update of Data Structures
Before accessing shared writable data structures (those that are not a single aligned longword
or quadword), the programmer can acquire control of the data structure by using an atomic
update to set a software lock variable. Such a software lock can be cleared with an ordinary
store instruction.
A software-critical section, therefore, may look like the sequence:
stq_c_loop:
spin_loop:
LDQ R1,lock_variable ; This optional spin-loop code
BLBS R1,already_set ; should be used unless the
; lock is known to be low-contention.
LDQ_L R1,lock_variable ; \
BLBS R1,already_set ; \
OR R1,#1,R2 ; > Set lock bit
STQ_C R2,lock_variable ; /
BEQ R2,stq_c_fail ; /
MB
<critical section: updates various data structures>
MB ; Second MB
STQ R31,lock_variable ; Clear lock bit
:
:
already_set:
<code to block or reschedule or test for too many iterations>
BR spin_loop
stq_c_fail:
<code to test for too many iterations>
BR stq_c_loop
This code has a number of subtleties:
• If the lock_variable is already set, the spin loop is done without doing any stores. This
avoidance of stores improves memory subsystem performance and avoids the deadlock
described below. The loop uses an ordinary load. This code sequence is preferred unless
the lock is known to be low-contention, because the sequence increases the probability
that the LDQ_L hits in the cache and the LDQ_L/STQ_C sequence complete quickly
and successfully.
• If the lock_variable is actually being changed from 0 to 1, and the STQ_C fails (due to
an interrupt, or because another processor simultaneously changed lock_variable), the
entire process starts over by reading the lock_variable again.
• Only the fall-through path of the BLBS instructions does a STx_C; some implementa-
tions may not allow a successful STx_C after a branch-taken.
• Only register-to-register operate instructions are used to do the modify.