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Critical Section Tools. (HW interface) locks implemented in ISA T&S, CAS (O.S. )Semaphores Counting and Binary ( a.k.a a mutex lock) (Augmented O.S.) Monitors & conditions Java, C#, or make your own! Need only atomic lock ops at the ISA level, implement the rest yourself.
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Critical Section Tools • (HW interface) locks implemented in ISA • T&S, CAS • (O.S. )Semaphores • Counting and Binary (a.k.a a mutex lock) • (Augmented O.S.) Monitors & conditions • Java, C#, or make your own! • Need only atomic lock ops at the ISA level, implement the rest yourself. • But, as more tools are offered by the kernel, the OS will expand its role here, offering easier-to-use or optimized solutions • Win and Pulse example
Critical Section • A section of code that shares data with other concurrent threads • Sharing of data makes a section critical • Specifically, write contention • In a kernel, many shared data structures • Need to synchronize these to guard against race conditions • OR, make kernel thread scheduling non-preemptive
How to Make an Atomic Exchange • Try: MOVE $s3, $a1 ;move swap val into $s3 • LL $s2, 0($s1) ; load linked • SC $s3,0($s1) ; store conditional • BEQZ $s3, try ;sc returns success in $s3 • MOVE $v1, $s2 ;return load result • ;simple atomic exchange • ; if the contents of $s1 change between the LL and the SC? SC fails and returns 0. • Similar in spirit to return address verification • ; note: context switch between LL and SC? SC fails and returns 0 in $s3 then
Another Lock: fetch-and-increment • Try: LL $t2, 0($t1) • DADDUI $t3, $t2, 1 ;our increment • SC $t3, 0($t1) • BEQZ $t3, try ;branch if store fails
