Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 1 Nov 2017 09:34:47 +0900 | From | Stafford Horne <> | Subject | Re: [PATCH v4 13/13] openrisc: add tick timer multi-core sync logic |
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On Wed, Nov 01, 2017 at 08:17:59AM +0900, Stafford Horne wrote: > On Tue, Oct 31, 2017 at 02:06:21PM +0000, Matt Redfearn wrote: > > Hi, > > > > > > On 29/10/17 23:11, Stafford Horne wrote: > > > In case timers are not in sync when cpus start (i.e. hot plug / offset > > > resets) we need to synchronize the secondary cpus internal timer with > > > the main cpu. This is needed as in OpenRISC SMP there is only one > > > clocksource registered which reads from the same ttcr register on each > > > cpu. > > > > > > This synchronization routine heavily borrows from mips implementation that > > > does something similar. > [..] > > > diff --git a/arch/openrisc/kernel/smp.c b/arch/openrisc/kernel/smp.c > > > index 4763b8b9161e..4d80ce6fa045 100644 > > > --- a/arch/openrisc/kernel/smp.c > > > +++ b/arch/openrisc/kernel/smp.c > > > @@ -100,6 +100,7 @@ int __cpu_up(unsigned int cpu, struct task_struct *idle) > > > pr_crit("CPU%u: failed to start\n", cpu); > > > return -EIO; > > > } > > > + synchronise_count_master(cpu); > > > return 0; > > > } > > > @@ -129,6 +130,8 @@ asmlinkage __init void secondary_start_kernel(void) > > > set_cpu_online(cpu, true); > > > complete(&cpu_running); > > > + synchronise_count_slave(cpu); > > > + > > > > > > Note that until 8f46cca1e6c06a058374816887059bcc017b382f, the MIPS timer > > synchronization code contained the possibility of deadlock. If you mark a > > CPU online before it goes into the synchronize loop, then the boot CPU can > > schedule a different thread and send IPIs to all "online" CPUs. It gets > > stuck waiting for the secondary to ack it's IPI, since this secondary CPU > > has not enabled IRQs yet, and is stuck waiting for the master to synchronise > > with it. The system then deadlocks. > > Commit 8f46cca1e6c06a058374816887059bcc017b382f fixed this for MIPS and you > > might want to similarly move the > > > > set_cpu_online(cpu, true); > > > > after counters are synchronized. > > Thank you for the heads up. I do remember having interim issues with the timer > syncing but I havent seen it for a while. I think I fixed it by also moving > synchronise_count_slave. > > Let me double check. Also, I see your patch 8f46cca1e6c06a0583748168 was merged > last year?
Hello,
I should have read a bit more closely, definitely this could be an issue if the boot cpu has other things running.
However, looking at mainline I can see the clock sync comes after set_cpu_online again after this patch in mips.
6f542ebeaee0 MIPS: Fix race on setting and getting cpu_online_mask Author: Matija Glavinic Pecotic <matija.glavinic-pecotic.ext@nokia.com>
Is this deadlock an issue in mips again?
-Stafford
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