Subject: Re: [PATCH] iommu/iova: wait 'fq_timer' handler to finish before
 destroying 'fq'
To:     Robin Murphy <robin.murphy@arm.com>, <joro@8bytes.org>,
        <iommu@lists.linux-foundation.org>, <linux-kernel@vger.kernel.org>
CC:     <yaohongbo@huawei.com>, <huawei.libin@huawei.com>
References: <1564219269-14346-1-git-send-email-wangxiongfeng2@huawei.com>
 <ef2c9b27-a644-928d-5bae-1ae4d2f2c099@arm.com>
From:   Xiongfeng Wang <wangxiongfeng2@huawei.com>
Message-ID: <5a3a3b14-7837-8a6d-b3ce-ed44bb98359d@huawei.com>
Date:   Fri, 10 Dec 2021 09:46:09 +0800
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On 2021/12/9 21:17, Robin Murphy wrote:
> Sorry I missed this before...
> 
> On 2019-07-27 10:21, Xiongfeng Wang wrote:
>> Fix following crash that occurs when 'fq_flush_timeout()' access
>> 'fq->lock' while 'iovad->fq' has been cleared. This happens when the
>> 'fq_timer' handler is being executed and we call
>> 'free_iova_flush_queue()'. When the timer handler is being executed,
>> its pending state is cleared and it is detached. This patch use
>> 'del_timer_sync()' to wait for the timer handler 'fq_flush_timeout()' to
>> finish before destroying the flush queue.
> 
> So if I understand correctly, you shut down the device - which naturally frees
> some DMA mappings into the FQ - then hotplug it out, such that tearing down its
> group and default domain can end up racing with the timeout firing on a
> different CPU? 

Yes, it is.

> It would help if the commit message actually explained that -
> I've just reverse-engineered it from the given symptom - rather than focusing on
> details that aren't really important. fq->lock is hardly significant, since
> *any* access to the FQ while it's being destroyed is fundamentally unsound. I
> also spent way too long trying to understand the significance of the full stack
> trace below before realising that it is in fact just irrelevant - there's only
> one way fq_flush_timeout() ever gets called, and it's the obvious one.
> 
> The fix itself seems reasonable - the kerneldoc for del_timer_sync() is slightly
> scary, but since free_iova_flush_queue() doesn't touch any of the locks and
> definitely shouldn't run in IRQ context I believe we're OK.
> 
> This will affect my IOVA refactoring series a little, so I'm happy to help
> improve the writeup if you like - provided that my understanding is actually
> correct - and include it in a v2 of that.

Thanks a lot. Appreciate it.

Thanks,
Xiongfeng

> 
> Thanks,
> Robin.
> 
>> [ 9052.361840] Unable to handle kernel paging request at virtual address
>> 0000a02fd6c66008
>> [ 9052.361843] Mem abort info:
>> [ 9052.361845]   ESR = 0x96000004
>> [ 9052.361847]   Exception class = DABT (current EL), IL = 32 bits
>> [ 9052.361849]   SET = 0, FnV = 0
>> [ 9052.361850]   EA = 0, S1PTW = 0
>> [ 9052.361852] Data abort info:
>> [ 9052.361853]   ISV = 0, ISS = 0x00000004
>> [ 9052.361855]   CM = 0, WnR = 0
>> [ 9052.361860] user pgtable: 4k pages, 48-bit VAs, pgdp = 000000009b665b91
>> [ 9052.361863] [0000a02fd6c66008] pgd=0000000000000000
>> [ 9052.361870] Internal error: Oops: 96000004 [#1] SMP
>> [ 9052.361873] Process rmmod (pid: 51122, stack limit = 0x000000003f5524f7)
>> [ 9052.361881] CPU: 69 PID: 51122 Comm: rmmod Kdump: loaded Tainted:
>> G           OE     4.19.36-vhulk1906.3.0.h356.eulerosv2r8.aarch64 #1
>> [ 9052.361882] Hardware name: Huawei TaiShan 2280 V2/BC82AMDC, BIOS 0.81
>> 07/10/2019
>> [ 9052.361885] pstate: 80400089 (Nzcv daIf +PAN -UAO)
>> [ 9052.361902] pc : fq_flush_timeout+0x9c/0x110
>> [ 9052.361904] lr :           (null)
>> [ 9052.361906] sp : ffff00000965bd80
>> [ 9052.361907] x29: ffff00000965bd80 x28: 0000000000000202
>> [ 9052.361912] x27: 0000000000000000 x26: 0000000000000053
>> [ 9052.361915] x25: ffffa026ed805008 x24: ffff000009119810
>> [ 9052.361919] x23: ffff00000911b938 x22: ffff00000911bc04
>> [ 9052.361922] x21: ffffa026ed804f28 x20: 0000a02fd6c66008
>> [ 9052.361926] x19: 0000a02fd6c64000 x18: ffff000009117000
>> [ 9052.361929] x17: 0000000000000008 x16: 0000000000000000
>> [ 9052.361933] x15: ffff000009119708 x14: 0000000000000115
>> [ 9052.361936] x13: ffff0000092f09d7 x12: 0000000000000000
>> [ 9052.361940] x11: 0000000000000001 x10: ffff00000965be98
>> [ 9052.361943] x9 : 0000000000000000 x8 : 0000000000000007
>> [ 9052.361947] x7 : 0000000000000010 x6 : 000000d658b784ef
>> [ 9052.361950] x5 : 00ffffffffffffff x4 : 00000000ffffffff
>> [ 9052.361954] x3 : 0000000000000013 x2 : 0000000000000001
>> [ 9052.361957] x1 : 0000000000000000 x0 : 0000a02fd6c66008
>> [ 9052.361961] Call trace:
>> [ 9052.361967]  fq_flush_timeout+0x9c/0x110
>> [ 9052.361976]  call_timer_fn+0x34/0x178
>> [ 9052.361980]  expire_timers+0xec/0x158
>> [ 9052.361983]  run_timer_softirq+0xc0/0x1f8
>> [ 9052.361987]  __do_softirq+0x120/0x324
>> [ 9052.361995]  irq_exit+0x11c/0x140
>> [ 9052.362003]  __handle_domain_irq+0x6c/0xc0
>> [ 9052.362005]  gic_handle_irq+0x6c/0x150
>> [ 9052.362008]  el1_irq+0xb8/0x140
>> [ 9052.362010]  vprintk_emit+0x2b4/0x320
>> [ 9052.362013]  vprintk_default+0x54/0x90
>> [ 9052.362016]  vprintk_func+0xa0/0x150
>> [ 9052.362019]  printk+0x74/0x94
>> [ 9052.362034]  nvme_get_smart+0x200/0x220 [nvme]
>> [ 9052.362041]  nvme_remove+0x38/0x250 [nvme]
>> [ 9052.362051]  pci_device_remove+0x48/0xd8
>> [ 9052.362065]  device_release_driver_internal+0x1b4/0x250
>> [ 9052.362068]  driver_detach+0x64/0xe8
>> [ 9052.362072]  bus_remove_driver+0x64/0x118
>> [ 9052.362074]  driver_unregister+0x34/0x60
>> [ 9052.362077]  pci_unregister_driver+0x24/0xd8
>> [ 9052.362083]  nvme_exit+0x24/0x1754 [nvme]
>> [ 9052.362094]  __arm64_sys_delete_module+0x19c/0x2a0
>> [ 9052.362102]  el0_svc_common+0x78/0x130
>> [ 9052.362106]  el0_svc_handler+0x38/0x78
>> [ 9052.362108]  el0_svc+0x8/0xc
>>
>> Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
>> ---
>>   drivers/iommu/iova.c | 3 +--
>>   1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/drivers/iommu/iova.c b/drivers/iommu/iova.c
>> index 3e1a8a6..90e8035 100644
>> --- a/drivers/iommu/iova.c
>> +++ b/drivers/iommu/iova.c
>> @@ -64,8 +64,7 @@ static void free_iova_flush_queue(struct iova_domain *iovad)
>>       if (!has_iova_flush_queue(iovad))
>>           return;
>>   -    if (timer_pending(&iovad->fq_timer))
>> -        del_timer(&iovad->fq_timer);
>> +    del_timer_sync(&iovad->fq_timer);
>>         fq_destroy_all_entries(iovad);
>>  
> .