Re: [PATCH v1.1] xen/commom/dt-overlay: Fix missing lock when remove the device

[Henry Wang <xin.wang2@xxxxxxx>]

Hi Julien,

On 5/3/2024 9:04 PM, Julien Grall wrote:
> Hi Henry,
>
> On 26/04/2024 02:55, Henry Wang wrote:
> > If CONFIG_DEBUG=y, below assertion will be triggered:
> > (XEN) Assertion 'rw_is_locked(&dt_host_lock)' failed at 
> > drivers/passthrough/device_tree.c:146
> > (XEN) ----[ Xen-4.19-unstable  arm64  debug=y  Not tainted ]----
> > (XEN) CPU:    0
> > (XEN) PC:     00000a0000257418 iommu_remove_dt_device+0x8c/0xd4
> > (XEN) LR:     00000a00002573a0
> > (XEN) SP:     00008000fff7fb30
> > (XEN) CPSR:   0000000000000249 MODE:64-bit EL2h (Hypervisor, handler)
> > [...]
> >
> > (XEN) Xen call trace:
> > (XEN)    [<00000a0000257418>] iommu_remove_dt_device+0x8c/0xd4 (PC)
> > (XEN)    [<00000a00002573a0>] iommu_remove_dt_device+0x14/0xd4 (LR)
> > (XEN)    [<00000a000020797c>] 
> > dt-overlay.c#remove_node_resources+0x8c/0x90
> > (XEN)    [<00000a0000207f14>] dt-overlay.c#remove_nodes+0x524/0x648
> > (XEN)    [<00000a0000208460>] dt_overlay_sysctl+0x428/0xc68
> > (XEN)    [<00000a00002707f8>] arch_do_sysctl+0x1c/0x2c
> > (XEN)    [<00000a0000230b40>] do_sysctl+0x96c/0x9ec
> > (XEN)    [<00000a0000271e08>] traps.c#do_trap_hypercall+0x1e8/0x288
> > (XEN)    [<00000a0000273490>] do_trap_guest_sync+0x448/0x63c
> > (XEN)    [<00000a000025c480>] entry.o#guest_sync_slowpath+0xa8/0xd8
> > (XEN)
> > (XEN)
> > (XEN) ****************************************
> > (XEN) Panic on CPU 0:
> > (XEN) Assertion 'rw_is_locked(&dt_host_lock)' failed at 
> > drivers/passthrough/device_tree.c:146
> > (XEN) ****************************************
> >
> > This is because iommu_remove_dt_device() is called without taking the
> > dt_host_lock. Fix the issue by taking and releasing the lock properly.
> >
> > Fixes: 7e5c4a8b86f1 ("xen/arm: Implement device tree node removal 
> > functionalities")
> > Signed-off-by: Henry Wang <xin.wang2@xxxxxxx>
> > ---
> > v1.1:
> > - Move the unlock position before the check of rc.
> > ---
> >   xen/common/dt-overlay.c | 2 ++
> >   1 file changed, 2 insertions(+)
> >
> > diff --git a/xen/common/dt-overlay.c b/xen/common/dt-overlay.c
> > index 1b197381f6..ab8f43aea2 100644
> > --- a/xen/common/dt-overlay.c
> > +++ b/xen/common/dt-overlay.c
> > @@ -381,7 +381,9 @@ static int remove_node_resources(struct 
> > dt_device_node *device_node)
> >       {
> >           if ( dt_device_is_protected(device_node) )
> >           {
> > +            write_lock(&dt_host_lock);
> Looking at the code, we are not modifying the device_node, so 
> shouldn't this be a read_lock()?
Hmm yes, however after seeing your comment...

> That said, even though either fix your issue, I am not entirely 
> convinced this is the correct position for the lock. From my 
> understanding, dt_host_lock is meant to ensure that the DT node will 
> not disappear behind your back. So in theory, shouldn't the lock be 
> taken as soon as you get hold of device_node?
...here. I believe you made a point here so I think I will just move the 
write_lock(&dt_host_lock) as soon as getting  overlay_node, i.e. on top 
of the call to remove_descendant_nodes_resources(). Therefore we can 
solve the assertion issue of this patch together.
Kind regards,
Henry

> Cheers,