kernel-concurrency

name	kernel-concurrency
description	Kernel concurrency skill for Linux locking and synchronization. Use when choosing spinlocks vs mutexes, using RCU, seqlocks, completions, or applying memory barriers in kernel code. Activates on queries about kernel spinlock, mutex, RCU, seqlock, memory barrier, or PREEMPT_RT locking.

Kernel Concurrency

Purpose

Guide agents through synchronization in the Linux kernel: spinlocks, mutexes, semaphores, RCU, seqlocks, completions, and memory ordering rules — critical for correct drivers and subsystem patches.

When to Use

• IRQ handler shares data with process context
• Read-mostly data structures needing RCU
• Choosing lock type for probe vs ioctl paths
• Debugging deadlocks or scheduling while atomic

Workflow

1. Lock selection tree

Context can sleep?
├── No (IRQ, spinlock held, preempt disabled)
│   └── spin_lock_irqsave() / atomic_t
└── Yes
    ├── Exclusive long-held → mutex
    ├── Reader/writer → rw_semaphore or RCU (read-mostly)
    └── One-shot signal → completion

Never sleep while holding a spinlock (kmalloc(GFP_KERNEL), mutex_lock).

2. Spinlock + IRQ

spinlock_t lock;
unsigned long flags;

spin_lock_irqsave(&lock, flags);
/* critical section — no blocking */
spin_unlock_irqrestore(&lock, flags);

Use spin_lock_bh when softirq/tasklet sharing is the concern.

3. Mutex in process context

struct mutex m;
mutex_lock(&m);
/* may allocate, may sleep */
mutex_unlock(&m);

4. RCU (read-copy update)

/* Readers — no lock */
rcu_read_lock();
p = rcu_dereference(ptr);
/* use p */
rcu_read_unlock();

/* Writer */
new = kmalloc(...);
rcu_assign_pointer(ptr, new);
synchronize_rcu();
kfree(old);

RCU readers must not block indefinitely. Grace period completes after all CPUs quiescent.

5. Seqlock (jiffies, timestamps)

unsigned seq;
do {
    seq = read_seqbegin(&seqlock);
    /* read shared data */
} while (read_seqretry(&seqlock, seq));

Writer uses write_seqlock / write_sequnlock.

6. Completions

DECLARE_COMPLETION(done);
/* waiter */
wait_for_completion(&done);
/* signaller */
complete(&done);

7. Memory barriers

Kernel provides smp_mb(), smp_wmb(), smp_rmb(). Device MMIO uses readl/writel (ordered on most arches). See skills/low-level-programming/memory-model for userspace analogies.

8. Agent usage

/kernel-concurrency Protect shared ring buffer between IRQ and read() syscall

Common Problems

Symptom	Cause	Fix
scheduling while atomic	Sleep under spinlock	Use GFP_ATOMIC or defer work
Deadlock	AB-BA mutex order	Global lock ordering
RCU stall	Reader blocked too long	rcu_read_lock section minimal
Lost wake	complete before wait	Use INIT_COMPLETION each cycle
Corrupt counter	Non-atomic RMW in IRQ	atomic_t or lock

Related Skills

• skills/kernel/device-drivers — threaded IRQ pattern
• skills/low-level-programming/memory-model — C11 atomics vs kernel
• skills/debuggers/concurrency-debugging — userspace TSan
• skills/kernel/kernel-internals — scheduler preemption
• skills/profilers/linux-perf — lock contention profiling