|Unreliable Guide To Locking|
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If a bottom half shares data with user context, you have two problems. Firstly, the current user context can be interrupted by a bottom half, and secondly, the critical region could be entered from another CPU. This is where spin_lock_bh() (include/linux/spinlock.h) is used. It disables bottom halves on that CPU, then grabs the lock. spin_unlock_bh() does the reverse.
This works perfectly for UP as well: the spin lock vanishes, and this macro simply becomes local_bh_disable() (include/asm/softirq.h), which protects you from the bottom half being run.