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Repair Windows errors before they cause bigger problemsFix Now →Scan for outdated or missing drivers - takes under a minuteDriver Scan →Clear out junk files and repair common Windows errorsFree Scan →To coordinate shared resources safely across cores, make the ownership-changing operation atomic: the processor must complete the read-modify-write as one indivisible transaction, rather than letting code check a lock and set it in two separately interruptible steps. Then use memory-ordering rules that match the data being protected, and choose synchronization whose scope fits the system.
Why a check-then-set lock fails
Suppose two tasks share a UART and each checks a lock word before writing. If the first task reads “unlocked” and is interrupted before it sets the lock, a second task can read the same value and also claim ownership. Both then write to the UART, so their output can interleave. Raising one task’s priority does not close this window; a higher-priority task or interrupt may be the agent that preempts it.
The fix is not merely to make each individual load and store fast. The entire ownership decision must be indivisible: no competing core or interrupt may observe the lock between the check and the update. Aaron Bauch, a senior field application engineer writing for Embedded.com, describes an atomic operation as one completed in “one uninterrupted sequence,” even when it consists of multiple internal events.
What makes an atomic operation safe across cores?
Use a processor-supported read-modify-write
An atomic read-modify-write (RMW) operation reads a memory location, computes an update, and writes the result without exposing an intermediate state to competing agents. Its exclusivity must be provided by the processor instruction set architecture (ISA) and the memory hardware. A sequence of ordinary load and store instructions is not a substitute: another core can run between them.
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Atomicity prevents competing agents from splitting the update. It does not, by itself, guarantee that other reads and writes to the protected data become visible in the order the program expects. Lock acquisition and release also need appropriate memory ordering. In C11, the language provides atomic types and operations, including acquire and release ordering, but a compiler can only implement those guarantees correctly when the target ISA and hardware support them. Check the implementation and target documentation rather than assuming that a source-level atomic is equally supported on every platform.
Separate mutual exclusion from ordering
A lock answers “which agent owns this resource?” Memory ordering answers “which data operations must be visible before or after that ownership changes?” Both questions matter. An atomic operation on the lock word can stop two cores from claiming ownership simultaneously, while acquire and release semantics ensure the protected work is ordered around the lock. A barrier can order or make memory operations visible within its defined scope, but it does not automatically choose one owner among competing agents.
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How Arm LDADD illustrates hardware acceleration
Arm Version 8.1 and later introduce LDADD and related instructions. LDADD performs an atomic read-add-write: it reads a value from memory, adds a register value, and writes the result back as one atomic operation. The old value is available to software, which can use it as part of an ownership protocol. The hardware enforces the transaction’s exclusivity; it is not equivalent to a software load followed by an add and a store.
LDADD is an example of an ISA feature that can accelerate an atomic primitive, not a universal drop-in lock algorithm. The lock protocol still has to define what the returned value means, how a lock is released, and what ordering applies to the protected data. Choose an instruction or compiler-provided atomic operation that directly matches the protocol you need, and verify support for the specific Arm target.
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Which synchronization approach fits?
| Approach | What it is suited to | Main limitation or trade-off |
|---|---|---|
| Interrupt masking on a single core | Preventing a local interrupt handler from interleaving with a short critical section on that core. | Does not by itself exclude another core. Availability and effects depend on the platform. |
| Atomic instruction or language atomic | Changing a shared ownership word or counter indivisibly across competing agents, when supported by the target implementation. | Contention can still make agents wait; memory ordering and the lock protocol must also be correct. Portability depends on ISA and compiler support. |
| Lock-based ownership | Giving one agent exclusive access to a shared resource such as a UART while it performs a multi-step operation. | Can serialize access and incur waiting under contention. A correct atomic primitive and suitable ordering are still required. |
| Scope-limited barrier | Coordinating ordering or progress among a defined set of GPU invocations, such as a workgroup. | A barrier is not a general exclusive lock, and it cannot synchronize agents outside its defined scope. |
There is no common benchmark in the cited material that supports a universal speedup figure. Actual latency depends on the processor, memory system, contention, and synchronization pattern. An atomic instruction removes the unsafe check-then-set window; it does not make all waiting or contention disappear.
GPU synchronization has explicit scopes
Vulkan defines synchronization scopes that include device, queue family, workgroup, and subgroup. Atomic and barrier operations apply within specified scopes, so choosing one requires knowing which invocations must coordinate. A workgroup-scoped operation cannot be treated as synchronization across an entire device.
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The Khronos Vulkan specification states that invocations on different devices cannot synchronize through SPIR-V alone; synchronization between them requires API synchronization commands. This is an important distinction from a CPU lock: GPU synchronization is governed by API- and shader-defined scopes, and a barrier only coordinates the participants and memory effects its scope covers.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Debug the interaction between cores, not just the code path
A race can disappear when execution is slowed by logging or a breakpoint, so print statements alone may not reveal the failing interleaving. A multicore debugger should let you run, stop, and inspect cores independently, coordinate breakpoints, and trigger related debug events across cores. Arm CoreSight Cross Trigger Interface (CTI) facilities can help coordinate those events. IAR Embedded Workbench is an example of a debugger identified for this kind of multicore control.
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- Inspect the lock word and the relevant protected data on each core, not only the core that first reports an error.
- Set coordinated breakpoints around acquisition and release to see whether two agents enter the critical section concurrently.
- Check the target’s atomic-instruction and memory-ordering support when observed behavior differs from the source-level protocol.
- For GPU work, verify that the barrier or atomic’s scope actually includes every invocation that must coordinate.
Practical decision checklist
- Identify every agent that can access the resource: tasks, interrupt handlers, other cores, or GPU invocations.
- Use an indivisible atomic primitive for shared ownership; do not rely on a separate load and store.
- Choose ordering that makes protected writes visible to the next owner.
- Confirm the relevant ISA, compiler, hardware, or API supports the required behavior on the target.
- Match synchronization scope to the participants, and use API-level synchronization when shader-level scope is insufficient.
- Debug with cross-core visibility and coordinated control so the failing interleaving can be observed.
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