Re: [PATCH] iommu/iova: wait 'fq_timer' handler to finish before destroying 'fq'

[Robin Murphy]

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? 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,
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@xxxxxxxxxx>
> ---
>   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);
>