Re: xen/evtchn and forced threaded irq

[Boris Ostrovsky]

On Tue, Feb 19, 2019 at 05:31:10PM +0000, Julien Grall wrote:
> Hi all,
> 
> I have been looking at using Linux RT in Dom0. Once the guest is started,
> the console is ending to have a lot of warning (see trace below).
> 
> After some investigation, this is because the irq handler will now be threaded.
> I can reproduce the same error with the vanilla Linux when passing the option
> 'threadirqs' on the command line (the trace below is from 5.0.0-rc7 that has
> not RT support).
> 
> FWIW, the interrupt for port 6 is used to for the guest to communicate with
> xenstore.
> 
> From my understanding, this is happening because the interrupt handler is now
> run in a thread. So we can have the following happening.
> 
>    Interrupt context            |     Interrupt thread
>                                 |  
>    receive interrupt port 6     |
>    clear the evtchn port        |
>    set IRQF_RUNTHREAD	        |
>    kick interrupt thread        |
>                                 |    clear IRQF_RUNTHREAD
>                                 |    call evtchn_interrupt
>    receive interrupt port 6     |
>    clear the evtchn port        |
>    set IRQF_RUNTHREAD           |
>    kick interrupt thread        |
>                                 |    disable interrupt port 6
>                                 |    evtchn->enabled = false
>                                 |    [....]
>                                 |
>                                 |    *** Handling the second interrupt ***
>                                 |    clear IRQF_RUNTHREAD
>                                 |    call evtchn_interrupt
>                                 |    WARN(...)
> 
> I am not entirely sure how to fix this. I have two solutions in mind:
> 
> 1) Prevent the interrupt handler to be threaded. We would also need to
> switch from spin_lock to raw_spin_lock as the former may sleep on RT-Linux.
> 
> 2) Remove the warning

I think access to evtchn->enabled is racy so (with or without the warning) we can't use it reliably.

Another alternative could be to queue the irq if !evtchn->enabled and handle it in evtchn_write() (which is where irq is supposed to be re-enabled).


-boris


> 
> None of them are ideals. Do you have an opionion/better suggestion?
> 
> [  127.192087] Interrupt for port 6, but apparently not enabled; per-user 0000000078d39c7f
> [  127.200333] WARNING: CPU: 0 PID: 2553 at drivers/xen/evtchn.c:167 evtchn_interrupt+0xfc/0x120
> [  127.208799] Modules linked in:
> [  127.211939] CPU: 0 PID: 2553 Comm: irq/52-evtchn:x Tainted: G        W
>   5.0.0-rc7-00023-g2a3d41623699 #1257
> [  127.222374] Hardware name: ARM Juno development board (r2) (DT)
> [  127.228381] pstate: 40000005 (nZcv daif -PAN -UAO)
> [  127.233256] pc : evtchn_interrupt+0xfc/0x120
> [  127.237607] lr : evtchn_interrupt+0xfc/0x120
> [  127.241952] sp : ffff000012d2bd60
> [  127.245347] x29: ffff000012d2bd60 x28: ffff00001015d608
> [  127.250741] x27: ffff8008b39de400 x26: ffff00001015d330
> [  127.256137] x25: ffff00001015d2d4 x24: 0000000000000001
> [  127.261532] x23: ffff00001015d570 x22: 0000000000000034
> [  127.266926] x21: 0000000000000000 x20: ffff8008b7f02400
> [  127.272322] x19: ffff8008b3aba000 x18: 0000000000000037
> [  127.277717] x17: 0000000000000000 x16: 0000000000000000
> [  127.283112] x15: 00000000fffffff0 x14: 0000000000000000
> [  127.288507] x13: 3030303030207265 x12: 000000000000000c
> [  127.293902] x11: ffff000010ea0a88 x10: 0000000000000000
> [  127.299297] x9 : 00000000fffb9fff x8 : 0000000000000000
> [  127.304701] x7 : 0000000000000001 x6 : ffff8008bac5c240
> [  127.310087] x5 : ffff8008bac5c240 x4 : 0000000000000000
> [  127.315483] x3 : ffff8008bac64708 x2 : ffff8008b39de400
> [  127.320877] x1 : 893ac9b38837b800 x0 : 0000000000000000
> [  127.326272] Call trace:
> [  127.328802]  evtchn_interrupt+0xfc/0x120
> [  127.332804]  irq_forced_thread_fn+0x38/0x98
> [  127.337066]  irq_thread+0x190/0x238
> [  127.340636]  kthread+0x134/0x138
> [  127.343942]  ret_from_fork+0x10/0x1c
> [  127.347593] ---[ end trace 1d3fa385877cc18b ]---
> 
> 
> Cheers,
> 
> -- 
> Julien Grall