- * Interrupts are disabled during allocation and deallocation in order to
- * make the slab allocator safe to use in the context of an irq. In addition
- * interrupts are disabled to ensure that the processor does not change
- * while handling per_cpu slabs, due to kernel preemption.
+ *
+ * cpu_slab->lock local lock
+ *
+ * This locks protect slowpath manipulation of all kmem_cache_cpu fields
+ * except the stat counters. This is a percpu structure manipulated only by
+ * the local cpu, so the lock protects against being preempted or interrupted
+ * by an irq. Fast path operations rely on lockless operations instead.
+ * On PREEMPT_RT, the local lock does not actually disable irqs (and thus
+ * prevent the lockless operations), so fastpath operations also need to take
+ * the lock and are no longer lockless.
+ *
+ * lockless fastpaths
+ *
+ * The fast path allocation (slab_alloc_node()) and freeing (do_slab_free())
+ * are fully lockless when satisfied from the percpu slab (and when
+ * cmpxchg_double is possible to use, otherwise slab_lock is taken).
+ * They also don't disable preemption or migration or irqs. They rely on
+ * the transaction id (tid) field to detect being preempted or moved to
+ * another cpu.
+ *
+ * irq, preemption, migration considerations
+ *
+ * Interrupts are disabled as part of list_lock or local_lock operations, or
+ * around the slab_lock operation, in order to make the slab allocator safe
+ * to use in the context of an irq.
+ *
+ * In addition, preemption (or migration on PREEMPT_RT) is disabled in the
+ * allocation slowpath, bulk allocation, and put_cpu_partial(), so that the
+ * local cpu doesn't change in the process and e.g. the kmem_cache_cpu pointer
+ * doesn't have to be revalidated in each section protected by the local lock.