Overlapping critical sections without the risk of accidentallyĮnabling interrupts while still in a critical section, as long as theĬache coherency in multi-processor systems ¶ Restoring the interrupts states so they can be freely called from Issues it is recommended to use the local_irq_save() and
#Multi processing operating system examples code#
They are usually used in core kernel code (like interruptįor typical cases where we want to avoid interrupts due to concurrency Should only be used when the current state and interrupts is Local_irq_disable() and local_irq_enable() these APIs #define local_irq_disable() \ asm volatile („cli” : : : „memory”) #define local_irq_enable() \ asm volatile („sti” : : : „memory”) #define local_irq_save(flags) \ asm volatile ("pushf pop %0" :"=g" (flags) : /* no input */ : "memory" ) \Īsm volatile ( "cli" : : : "memory" ) #define local_irq_restore(flags) \ asm volatile ("push %0 popf" : /* no output */ : "g" ( flags ) : "memory", "cc" ) Īlthough the interrupts can be explicitly disabled and enable with So they must have similar efficiency as other instructions"), as seenįrom the implementation details, atomic operations are actually They must be more efficient", or because they "are just instructions Synchronization mechanisms (because they "don't require spinning orĬontext switches", or because they "are implemented in hardware so Programmer must retry the operation (both LDREX and STREX) until theĪlthough they are often interpreted as "light" or "efficient" New value but only succeeds if the exclusive monitor has not detected On ARM the LDREX and STREX instructions are used together to guaranteeĪtomic access: LDREX loads a value and signals the exclusive monitor Prefix is used to lock the system bus while executing the prefixed In order to provide atomic operations on SMP systems differentĪrchitectures use different techniques. The load and store operations are interleaved across CPUs, like in theĮxample below where incrementing a value from two processors will Then we can construct race condition scenarios where
To understand why, we need to decompose the atomic operation in memory Multi-core systems, where an atomic operation is not longerĪtomic at the system level (but still atomic at the core level). One complication with atomic operations is encountered in Process and Interrupt Context Synchronization.Cache coherency in multi-processor systems.
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