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Subject: Re: Interrupt for port 19, but apparently not enabled; per-user
 000000004af23acc
To:     Juergen Gross <jgross@suse.com>
Cc:     "xen-devel@lists.xenproject.org" <xen-devel@lists.xenproject.org>,
        linux-kernel@vger.kernel.org, mheyne@amazon.de
References: <6552fc66-ba19-2c77-7928-b0272d3e1622@xen.org>
 <4d8a7ba7-a9f6-2999-8750-bfe2b85f064e@suse.com>
From:   Julien Grall <julien@xen.org>
Message-ID: <9a08bbf2-ba6a-6e49-3bcb-bfe2beb32b99@xen.org>
Date:   Tue, 22 Jun 2021 14:21:18 +0200
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Hi Juergen,

On 22/06/2021 13:04, Juergen Gross wrote:
> On 22.06.21 12:24, Julien Grall wrote:
>> Hi Juergen,
>>
>> As discussed on IRC yesterday, we noticed a couple of splat in 5.13-rc6 
> 
>> (and stable 5.4) in the evtchn driver:
>>
>> [    7.581000] ------------[ cut here ]------------
>> [    7.581899] Interrupt for port 19, but apparently not 
> enabled;
>> per-user 000000004af23acc
>> [    7.583401] WARNING: CPU: 0 PID: 467 at 
>> /home/ANT.AMAZON.COM/jgrall/works/oss/linux/drivers/xen/evtchn.c:169 
>> evtchn_interrupt+0xd5/0x100
>> [    7.585583] Modules linked in:
>> [    7.586188] CPU: 0 PID: 467 Comm: xenstore-read Not tainted 
>> 5.13.0-rc6 #240
>> [    7.587462] Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), 
>> BIOS rel-1.14.0-0-g155821a1990b-prebuilt.qemu.org 04/01/2014
>> [    7.589462] RIP: e030:evtchn_interrupt+0xd5/0x100
>> [    7.590361] Code: 48 8d bb d8 01 00 00 ba 01 00 00 00 
> be 1d 00 00 00
>> e8 5f 72 c4 ff eb b2 8b 75 20 48 89 da 48 c7 c7 a8 03 5f 82 e8 6b 2d 96 
> 
>> ff <0f> 0b e9 4d ff ff ff 41 0f b6 f4 48 c7 c7 80 da a2 82 e8 f0
>> [    7.593662] RSP: e02b:ffffc90040003e60 EFLAGS: 00010082
>> [    7.594636] RAX: 0000000000000000 RBX: ffff888102328c00 RCX: 
>> 0000000000000027
>> [    7.595924] RDX: 0000000000000000 RSI: ffff88817fe18ad0 RDI: 
>> ffff88817fe18ad8
>> [    7.597216] RBP: ffff888108ef8140 R08: 0000000000000000 R09: 
>> 0000000000000001
>> [    7.598522] R10: 0000000000000000 R11: 7075727265746e49 R12: 
>> 0000000000000000
>> [    7.599810] R13: ffffc90040003ec4 R14: ffff8881001b8000 R15: 
>> ffff888109b36f80
>> [    7.601113] FS:  0000000000000000(0000) GS:ffff88817fe00000(0000) 
>> knlGS:0000000000000000
>> [    7.602570] CS:  10000e030 DS: 0000 ES: 0000 CR0: 0000000080050033
>> [    7.603700] CR2: 00007f15b390e368 CR3: 000000010bb04000 CR4: 
>> 0000000000050660
>> [    7.604993] Call Trace:
>> [    7.605501]  <IRQ>
>> [    7.605929]  __handle_irq_event_percpu+0x4c/0x330
>> [    7.606817]  handle_irq_event_percpu+0x32/0xa0
>> [    7.607670]  handle_irq_event+0x3a/0x60
>> [    7.608416]  handle_edge_irq+0x9b/0x1f0
>> [    7.609154]  generic_handle_irq+0x4f/0x60
>> [    7.609918]  __evtchn_fifo_handle_events+0x195/0x3a0
>> [    7.610864]  __xen_evtchn_do_upcall+0x66/0xb0
>> [    7.611693]  __xen_pv_evtchn_do_upcall+0x1d/0x30
>> [    7.612582]  xen_pv_evtchn_do_upcall+0x9d/0xc0
>> [    7.613439]  </IRQ>
>> [    7.613882]  exc_xen_hypervisor_callback+0x8/0x10
>>
>> This is quite similar to the problem I reported a few months ago (see 
>> [1]) but this time this is happening with fifo rather than 2L.
>>
>> I haven't been able to reproduced it reliably so far. But looking at 
>> the code, I think I have found another potential race after commit
>>
>> commit b6622798bc50b625a1e62f82c7190df40c1f5b21
>> Author: Juergen Gross <jgross@suse.com>
>> Date:   Sat Mar 6 17:18:33 2021 +0100
>>     xen/events: avoid handling the same event on two cpus at the same 
>> time
>>     When changing the cpu affinity of an event it can happen today that
>>     (with some unlucky timing) the same event will be handled 
> on the old
>>     and the new cpu at the same time.
>>     Avoid that by adding an "event active" flag to the per-event data and
>>     call the handler only if this flag isn't set.
>>     Cc: stable@vger.kernel.org
>>     Reported-by: Julien Grall <julien@xen.org>
>>     Signed-off-by: Juergen Gross <jgross@suse.com>
>>     Reviewed-by: Julien Grall <jgrall@amazon.com>
>>     Link: https://lore.kernel.org/r/20210306161833.4552-4-jgross@suse.com
>>     Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
>>
>> The evtchn driver will use the lateeoi handlers. So the code to ack 
>> looks like:
>>
>> do_mask(..., EVT_MASK_REASON_EOI_PENDING)
>> smp_store_release(&info->is_active, 0);
>> clear_evtchn(info->evtchn);
>>
>> The code to handle an interrupts look like:
>>
>> clear_link(...)
>> if ( evtchn_fifo_is_pending(port) && !evtchn_fifo_is_mask()) {
>>    if (xchg_acquire(&info->is_active, 1)
>>      return;
>>    generic_handle_irq();
>> }
>>
>> After changing the affinity, an interrupt may be received once on the 
>> previous vCPU. So, I think the following can happen:
>>
>> vCPU0                             | vCPU1
>>                    |
>>   Receive event              |
>>                    | change affinity to vCPU1
>>   clear_link()              |
>>                        |
>>                 /* The interrupt is re-raised */
>>                    | receive event
>>                      |
>>                    | /* The interrupt is not masked */
>>   info->is_active = 1          |
>>   do_mask(...)              |
>>   info->is_active = 0          |
>>                    | info->is_active = 1
>>   clear_evtchn(...)               |
>>                                   | do_mask(...)
>>                                   | info->is_active = 0
>>                    | clear_evtchn(...)
>>
>> Does this look plausible to you?
> 
> Yes, it does.
> 
> Thanks for the analysis.
> 
> So I guess for lateeoi events we need to clear is_active only in
> xen_irq_lateeoi()? At a first glance this should fix the issue.

It should work and would be quite neat. But, I believe clear_evtchn() 
would have to stick in the ack helper to avoid losing interrupts.

Cheers,

-- 
Julien Grall