Re: xen/evtchn: Dom0 boot hangs using preempt_rt kernel 5.10

[Luca Fancellu <luca.fancellu@xxxxxxx>]

Hi Juergen,

If you are willing to do the patch I think it will be faster to being accepted, 
what about the BUG_ON(…) in evtchn_2l_unmask from events_2l.c file?

Cheers,

Luca

> On 18 Mar 2021, at 07:54, Jürgen Groß <jgross@xxxxxxxx> wrote:
> 
> On 17.03.21 15:32, Luca Fancellu wrote:
>> Hi all,
>> we've been encountering an issue when using the kernel 5.10 with preempt_rt 
>> support for Dom0, the problem is that during the boot of Dom0, it hits a 
>> BUG_ON(!irqs_disabled()) from the function evtchn_fifo_unmask defined in 
>> events_fifo.c.
>> This is the call stack:
>> [   17.817018] ------------[ cut here ]------------
>> [   17.817021] kernel BUG at drivers/xen/events/events_fifo.c:258!
>> [   18.817079] Internal error: Oops - BUG: 0 [#1] PREEMPT_RT SMP
>> [   18.817081] Modules linked in: bridge stp llc ipv6
>> [   18.817086] CPU: 3 PID: 558 Comm: xenstored Not tainted 
>> 5.10.16-rt25-yocto-preempt-rt #1
>> [   18.817089] Hardware name: Arm Neoverse N1 System Development Platform 
>> (DT)
>> [   18.817090] pstate: 60400005 (nZCv daif +PAN -UAO -TCO BTYPE=--)
>> [   18.817092] pc : evtchn_fifo_unmask+0xd4/0xe0
>> [   18.817099] lr : xen_irq_lateeoi_locked+0xec/0x200
>> [   18.817102] sp : ffff8000123f3cc0
>> [   18.817102] x29: ffff8000123f3cc0 x28: ffff0000427b1d80
>> [   18.817104] x27: 0000000000000000 x26: 0000000000000000
>> [   18.817106] x25: 0000000000000001 x24: 0000000000000001
>> [   18.817107] x23: ffff0000412fc900 x22: 0000000000000004
>> [   18.817109] x21: 0000000000000000 x20: ffff000042e06990
>> [   18.817110] x19: ffff0000427b1d80 x18: 0000000000000010
>> [   18.817112] x17: 0000000000000000 x16: 0000000000000000
>> [   18.817113] x15: 0000000000000002 x14: 0000000000000001
>> [   18.817114] x13: 000000000001a7e8 x12: 0000000000000040
>> [   18.817116] x11: ffff000040400248 x10: ffff00004040024a
>> [   18.817117] x9 : ffff800011be5200 x8 : ffff000040400270
>> [   18.817119] x7 : 0000000000000000 x6 : 0000000000000003
>> [   18.817120] x5 : 0000000000000000 x4 : ffff000040400308
>> [   18.817121] x3 : ffff0000408a400c x2 : 0000000000000000
>> [   18.817122] x1 : 0000000000000000 x0 : ffff0000408a4000
>> [   18.817124] Call trace:
>> [   18.817125]  evtchn_fifo_unmask+0xd4/0xe0
>> [   18.817127]  xen_irq_lateeoi_locked+0xec/0x200
>> [   18.817129]  xen_irq_lateeoi+0x48/0x64
>> [   18.817131]  evtchn_write+0x124/0x15c
>> [   18.817134]  vfs_write+0xf0/0x2cc
>> [   18.817137]  ksys_write+0xe0/0x100
>> [   18.817139]  __arm64_sys_write+0x20/0x30
>> [   18.817142]  el0_svc_common.constprop.0+0x78/0x1a0
>> [   18.817145]  do_el0_svc+0x24/0x90
>> [   18.817147]  el0_svc+0x14/0x20
>> [   18.817151]  el0_sync_handler+0x1a4/0x1b0
>> [   18.817153]  el0_sync+0x174/0x180
>> [   18.817156] Code: 52800120 b90023e6 97e6d104 17fffff0 (d4210000)
>> [   18.817158] ---[ end trace 0000000000000002 ]---
>> Our last tested kernel was the 5.4 and our analysis pointed out that the 
>> introduction of the lateeoi framework (xen/events: add a new "late EOI" 
>> evtchn framework) in conjunction with the preempt_rt patches (irqs kept 
>> enabled between spinlock_t/rwlock_t _irqsave///​_irqrestore operations) is 
>> the root cause.
>> Given that many modifications were made to the mask/unmask operations, a big 
>> one from Juergen Gross (xen/events: don't unmask an event channel when an 
>> eoi is pending), is the BUG_ON(...) still needed?
>> With the mentioned commit every call to a mask/unmask operation is protected 
>> by a spinlock, so I would like to have some feedbacks from who has more 
>> experience than me on this part of the code.
> 
> I think this BUG_ON() can be removed.
> 
> Are you planning to send a patch?
> 
> 
> Juergen
> <OpenPGP_0xB0DE9DD628BF132F.asc>