Messages in this thread | | | Date | Tue, 26 May 2015 14:36:34 -0600 | From | Lina Iyer <> | Subject | Re: [PATCH RFC] hwspinlock: Don't take software spinlock before hwspinlock |
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On Sat, May 23 2015 at 01:36 -0600, Ohad Ben-Cohen wrote: >Hi Lina, > >On Mon, May 18, 2015 at 6:03 PM, Lina Iyer <lina.iyer@linaro.org> 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. > That is correct.
-- Lina
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