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Linux Seqlock & Sequential Memory Consistency Studio
Simulate Linux kernel-level sequence locking (seqlock_t). Model optimistic
read_seqbegin() traversals, torn-read detection via read_seqretry(),
zero reader cacheline mutation, and smp_rmb() hardware memory barriers.
seqlock_t Kernel
0-Lock Readers
Torn-Read Safe
Protocol ensuring reader-writer memory consistency
Number of CPU cores concurrently reading state
Frequency of write_seqlock() executions
Memory footprint vulnerable to split multi-word torn reads
⏱️ Sequence Counter Transitions (Even/Odd) & Reader Retry Loop
Sequence Counter: Even (42) • Zero Torn ReadsReader Lookup Latency
0.4 ns
Zero atomic lock writes
Torn Read Rate
0.000%
Filtered by read_seqretry()
Reader Retry Rate
0.012%
Interleaved writer collision
Cacheline Invalidation Rate
0 / sec
Readers are purely read-only
🔬 Deep Kernel Synchronization & Memory Barrier Analysis
Calculating sequence counter transitions and memory barrier verification...
⚠️ 5 Fatal Traps in Linux Kernel Seqlock Usage
1. Protecting Pointers with Seqlocks (Kernel Panic Hazard):
Because seqlock readers run concurrently with writers, a reader may observe an inconsistent, partially updated pointer. If the reader dereferences this pointer before calling
read_seqretry(), the CPU encounters an invalid memory address and crashes with a kernel page fault. Seqlocks must ONLY protect plain values, not pointer topologies (use RCU for pointers).
2. Writer Starvation of Readers under High Write Frequencies:
If a writer updates a seqlock 50,000 times per second, readers will continually detect sequence changes in
read_seqretry() and loop infinitely, starving reader threads. Seqlocks are strictly designed for read-heavy, low-write frequency data.
3. Omitting Memory Barriers (smp_rmb / smp_wmb):
Relying on raw integer increments without compiler and hardware memory barriers allows the CPU to speculatively execute data reads before checking the sequence counter.
read_seqbegin() and read_seqretry() enforce essential CPU barriers.
4. Forgetting write_seqlock_irqsave() in Interrupt Contexts:
If an interrupt handler reads a seqlock that was acquired by a writer on the same CPU without disabling interrupts, the interrupt handler will spin waiting for the counter to become even. Because the writer is preempted by the interrupt, the CPU enters an unrecoverable deadlock. Always use
write_seqlock_irqsave() if accessed from IRQ handlers.
5. Side Effects Inside read_seqbegin() Loop:
Any operation inside the read-side loop (such as memory allocations, queue pushes, or printing logs) will be repeated multiple times if a retry occurs. Critical sections within
read_seqretry() must be completely idempotent and side-effect free.
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