Hi Lina, On Mon, May 18, 2015 at 6:03 PM, Lina Iyer <lina.iyer@xxxxxxxxxx> wrote: > The lock in question is used differently than traditional locks across > processors. This lock helps synchronizes context transition from > non-secure to secure on the same processor. > > The usecase, goes like this. In cpuidle, any core can be the last core > to power down. The last man also holds the responsibility of shutting > down shared resources like caches etc. The way the power down of a core > works is, there are some high level decisions made in Linux and these > decisions (like to flush and invalidate caches) etc gets transferred > over to the the secure layer. The secure layer executes the ARM WFI that > powers down the cpu, but uses these decisions passed into to determine > if the cache needs to be invalidated upon wakeup etc. > > There is a possible race condition between what Linux thinks is the last > core, vs what secure layer thinks is the last core. Lets say, two cores > c0, c1 are going down. c1 is the second last core to go down from Linux > as such, will not carry information about shared resources when making > the SCM call. c1 made the SCM call, but is stuck handling some FIQs. In > the meanwhile c0, goes idle and since its the last core in Linux, > figures out the state of the shared resources. c0 calls into SCM, and > ends up powering down earlier than c1. Per secure layer, the last core > to go down is c1 and the votes of the shared resources are considered > from that core. Things like cache invalidation without flush may happen > as a result of this inconsistency of last man view point. > > The way we have solved it, Linux acquires a hw spinlock for each core, > when calling into SCM and the secure monitor releases the spinlock. At > any given time, only one core can switch the context from Linux to > secure for power down operations. This guarantees the last man is > synchronized between both Linux and secure. Another core may be spinning > waiting for hw mutex, but they all happen serialized. This mutex is held > in an irq disable context in cpuidle. > > There may be another processor spining to wait on hw mutex, but there > isnt much to do otherwise, because the only operation at this time while > holding the lock is to call into SCM and that would unlock the mutex. Just to make sure I understand, is this how your scenario is solved? - c1 goes down - c0 goes down, carries information about shared resources - c1 takes HWLOCK and calls into SCM, stuck handling FIQs - c0 wants to call into SCM but is waiting spinning on HWLOCK - c1 completes handling FIQs, goes idle, HWLOCK is released by secure monitor - c0 takes HWLOCK, calls into SCM, shared resources handled correctly, HWLOCK in this example is a single shared hwspinlock accessible by c0, c1 and secure monitor. Thanks, Ohad. -- To unsubscribe from this list: send the line "unsubscribe linux-arm-msm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
