Hey Joe- Thanks for the writeup. On Fri, Mar 23, 2018 at 11:09:59AM -0400, joe.korty@xxxxxxxxxxxxxxxxx wrote: > I see the below kernel splat in 4.9-rt when I run a test program that > continually changes the affinity of some set of running pids: > > do not call blocking ops when !TASK_RUNNING; state=2 set at ... > ... > stop_one_cpu+0x60/0x80 > migrate_enable+0x21f/0x3e0 > rt_spin_unlock+0x2f/0x40 > prepare_to_wait+0x5c/0x80 > ... This is clearly a problem. > The reason is that spin_unlock, write_unlock, and read_unlock call > migrate_enable, and since 4.4-rt, migrate_enable will sleep if it discovers > that a migration is in order. But sleeping in the unlock services is not > expected by most kernel developers, I don't buy this, see below: > and where that counts most is in code sequences like the following: > > set_current_state(TASK_UNINTERRUPIBLE); > spin_unlock(&s); > schedule(); The analog in mainline is CONFIG_PREEMPT and the implicit preempt_enable() in spin_unlock(). In this configuration, a kernel developer should _absolutely_ expect their task to be suspended (and potentially migrated), _regardless of the task state_ if there is a preemption event on the CPU on which this task is executing. Similarly, on RT, there is nothing _conceptually_ wrong on RT with migrating on migrate_enable(), regardless of task state, if there is a pending migration event. It's clear, however, that the mechanism used here is broken ... Julia -- To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
