Hi Linux hackers ! I'm reading some code in net/core/dev.c and something puzzles me : somewhere in __dev_queue_xmit, we have : int cpu = smp_processor_id(); /* ok because BHs are off */ and, indeed, a few lines up there is : /* Disable soft irqs for various locks below. Also * stops preemption for RCU. */ rcu_read_lock_bh(); Now I'm wondering : is this really ok ?
From what I understand, this code can very well run in user context, with hard IRQs ON, and in fact it should, according to the following comment :
* When calling this method, interrupts MUST be enabled. This is because * the BH enable code must have IRQs enabled so that it will not deadlock. * --BLG Then I guess it could be preempted at any time, especially with aggressive versions of preemptions ? And if so, are we not at risk that our thread is migrated to an other CPU just after smp_processor_id returned ? Cheers, V Ray