Kernel

Atomic Context Traps

• spin_lock held = cannot sleep — no kmalloc(GFP_KERNEL), no mutex_lock, no copy_from_user
• Interrupt can take same spinlock — must use spin_lock_irqsave, not plain spin_lock
• rcu_read_lock() section cannot sleep — no blocking calls inside RCU read-side
• might_sleep() annotation — add to functions that may sleep, catches bugs with CONFIG_DEBUG_ATOMIC_SLEEP

Allocation Failures

• GFP_ATOMIC can return NULL — always check, don't assume success
• vmalloc memory not physically contiguous — cannot use for DMA
• kzalloc over kmalloc — uninitialized memory leaks kernel info to userspace
• Allocation in loop risks OOM — preallocate or use memory pool

User Pointer Handling

• copy_from_user returns bytes NOT copied — 0 means success, not failure
• Never use %s with user pointer in printk — kernel crash or info leak
• User memory can change during syscall — copy to kernel buffer, validate the copy
• __user annotation is documentation — doesn't enforce anything, you must use copy functions