Load-link/store-conditional

In computer science, load-linked/store-conditional[1] (LL/SC), sometimes known as load-reserved/store-conditional[2] (LR/SC), are a pair of instructions used in multithreading to achieve synchronization. Load-link returns the current value of a memory location, while a subsequent store-conditional to the same memory location will store a new value only if no updates have occurred to that location since the load-link. Together, this implements a lock-free atomic read-modify-write operation.

"Load-linked" is also known as load-link, load-reserved,[2] and load-locked.

LL/SC was originally proposed by Jensen, Hagensen, and Broughton for the S-1 AAP multiprocessor[1] at Lawrence Livermore National Laboratory.

Comparison of LL/SC and compare-and-swap

If any updates have occurred, the store-conditional is guaranteed to fail, even if the value read by the load-link has since been restored. As such, an LL/SC pair is stronger than a read followed by a compare-and-swap (CAS), which will not detect updates if the old value has been restored (see ABA problem).

Real implementations of LL/SC do not always succeed even if there are no concurrent updates to the memory location in question. Any exceptional events between the two operations, such as a context switch, another load-link, or even (on many platforms) another load or store operation, will cause the store-conditional to spuriously fail. Older implementations will fail if there are any updates broadcast over the memory bus. This is often called weak LL/SC by researchers, as it breaks many theoretical LL/SC algorithms. Weakness is relative, and some weak implementations can be used for some algorithms.

LL/SC is more difficult to emulate than CAS. Additionally, stopping running code between paired LL/SC instructions, such as when single-stepping through code, can prevent forward progress, making debugging tricky.[3]

Nevertheless, LL/SC is equivalent to CAS in the sense that either primitive can be implemented in terms of the other, in O(1) and in a wait-free manner.[4]

Implementations

LL/SC instructions are supported by:

Some CPUs require the address being accessed exclusively to be configured in write-through mode.

Typically, CPUs track the load-linked address at a cache-line or other granularity, such that any modification to any portion of the cache line (whether via another core's store-conditional or merely by an ordinary store) is sufficient to cause the store-conditional to fail.

All of these platforms provide weak LL/SC. The PowerPC implementation allows an LL/SC pair to wrap loads and even stores to other cache lines (although this approach is vulnerable to false cache line sharing). This allows it to implement, for example, lock-free reference counting in the face of changing object graphs with arbitrary counter reuse (which otherwise requires double compare-and-swap, DCAS). RISC-V provides an architectural guarantee of eventual progress for LL/SC sequences of limited length.

Some ARM implementations define platform dependent blocks, ranging from 8 bytes to 2048 bytes, and an LL/SC attempt in any given block fails if there is between the LL and SC a normal memory access inside the same block. Other ARM implementations fail if there is a modification anywhere in the whole address space. The former implementation is the stronger and most practical.

LL/SC has two advantages over CAS when designing a load-store architecture: reads and writes are separate instructions, as required by the design philosophy (and pipeline architecture); and both instructions can be performed using only two registers (address and value), fitting naturally into common 2-operand ISAs. CAS, on the other hand, requires three registers (address, old value, new value) and a dependency between the value read and the value written. x86, being a CISC architecture, does not have this constraint; though modern chips may well translate a CAS instruction into separate LL/SC micro-operations internally.

Extensions

Hardware LL/SC implementations typically do not allow nesting of LL/SC pairs.[5] A nesting LL/SC mechanism can be used to provide a MCAS primitive (multi-word CAS, where the words can be scattered).[6] In 2013, Trevor Brown, Faith Ellen, and Eric Ruppert have implemented in software a multi-address LL/SC extension (which they call LLX/SCX) that relies on automated code generation;[7] they have used it to implement one of the best performing concurrent binary search tree (actually a chromatic tree), slightly beating the JDK CAS-based skip list implementation.[8]

See also

References
  1. "S-1 project". Stanford Computer Science wiki. 2018-11-30.
  2. Andrew Waterman; Krste Asanović, eds. (2017-05-07). "7.2 Load-Reserved/Store-Conditional Instructions". The RISC-V Instruction Set Manual, Volume 1: User-Level ISA, Version 2.2 (PDF).
  3. Keno Fischer (2020-05-02). "Julia 1.5 Feature Preview: Time Traveling (Linux) Bug Reporting". Retrieved 2020-05-14.
  4. James H. Anderson; Mark Moir (1995). "Universal constructions for multi-object operations". PODC '95 Proceedings of the fourteenth annual ACM symposium on Principles of distributed computing. ACM. pp. 184–193. doi:10.1145/224964.224985. ISBN 0-89791-710-3. See their Table 1, Figures 1 & 2 and Section 2 in particular.
  5. James R. Larus; Ravi Rajwar (2007). Transactional Memory. Morgan & Claypool. p. 55. ISBN 978-1-59829-124-7.
  6. Keir Fraser (February 2004). Practical lock-freedom (PDF) (Technical report). University of Cambridge Computer Laboratory. p. 20. UCAM-CL-TR-579.
  7. Brown, Trevor; Ellen, Faith; Ruppert, Eric (2013). "Pragmatic primitives for non-blocking data structures" (PDF). PODC '13 Proceedings of the 2013 ACM symposium on Principles of distributed computing. ACM. pp. 13–22. doi:10.1145/2484239.2484273. ISBN 978-1-4503-2065-8. See also slides
  8. Trevor Brown; Faith Ellen; Eric Ruppert (2014). "A general technique for non-blocking trees" (PDF). PPoPP '14 ACM SIGPLAN Symposium on Principles and Practice of Parallel Programming. ACM. pp. 329–342. doi:10.1145/2555243.2555267. ISBN 978-1-4503-2656-8.
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