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Re: [Xen-devel] [PATCH v11 01/34] ARM: vGIC: avoid rank lock when reading priority
On 12/06/2017 23:34, Stefano Stabellini wrote:
On Mon, 12 Jun 2017, Julien Grall wrote:
Hi Andre,
On 09/06/17 18:41, Andre Przywara wrote:
When reading the priority value of a virtual interrupt, we were taking
the respective rank lock so far.
However for forwarded interrupts (Dom0 only so far) this may lead to a
deadlock with the following call chain:
- MMIO access to change the IRQ affinity, calling the ITARGETSR handler
- this handler takes the appropriate rank lock and calls
vgic_store_itargetsr()
- vgic_store_itargetsr() will eventually call vgic_migrate_irq()
- if this IRQ is already in-flight, it will remove it from the old
VCPU and inject it into the new one, by calling vgic_vcpu_inject_irq()
- vgic_vcpu_inject_irq will call vgic_get_virq_priority()
- vgic_get_virq_priority() tries to take the rank lock - again!
It seems like this code path has never been exercised before.
Fix this by avoiding taking the lock in vgic_get_virq_priority() (like we
do in vgic_get_target_vcpu()).
Actually we are just reading one byte, and priority changes while
interrupts are handled are a benign race that can happen on real hardware
too. So it is safe to just prevent the compiler from reading from the
struct more than once.
Signed-off-by: Andre Przywara <andre.przywara@xxxxxxx>
---
xen/arch/arm/vgic-v2.c | 13 ++++++++-----
xen/arch/arm/vgic-v3.c | 11 +++++++----
xen/arch/arm/vgic.c | 8 +-------
3 files changed, 16 insertions(+), 16 deletions(-)
diff --git a/xen/arch/arm/vgic-v2.c b/xen/arch/arm/vgic-v2.c
index dc9f95b..5370020 100644
--- a/xen/arch/arm/vgic-v2.c
+++ b/xen/arch/arm/vgic-v2.c
@@ -258,9 +258,9 @@ static int vgic_v2_distr_mmio_read(struct vcpu *v,
mmio_info_t *info,
if ( rank == NULL ) goto read_as_zero;
vgic_lock_rank(v, rank, flags);
- ipriorityr = rank->ipriorityr[REG_RANK_INDEX(8,
- gicd_reg -
GICD_IPRIORITYR,
- DABT_WORD)];
+ ipriorityr = ACCESS_ONCE(rank->ipriorityr[REG_RANK_INDEX(8,
+ gicd_reg - GICD_IPRIORITYR,
+ DABT_WORD)]);
The indentation is a bit odd. Can you introduce a temporary variable here?
vgic_unlock_rank(v, rank, flags);
*r = vgic_reg32_extract(ipriorityr, info);
@@ -499,7 +499,7 @@ static int vgic_v2_distr_mmio_write(struct vcpu *v,
mmio_info_t *info,
case VRANGE32(GICD_IPRIORITYR, GICD_IPRIORITYRN):
{
- uint32_t *ipriorityr;
+ uint32_t *ipriorityr, priority;
if ( dabt.size != DABT_BYTE && dabt.size != DABT_WORD ) goto
bad_width;
rank = vgic_rank_offset(v, 8, gicd_reg - GICD_IPRIORITYR,
DABT_WORD);
@@ -508,7 +508,10 @@ static int vgic_v2_distr_mmio_write(struct vcpu *v,
mmio_info_t *info,
ipriorityr = &rank->ipriorityr[REG_RANK_INDEX(8,
gicd_reg -
GICD_IPRIORITYR,
DABT_WORD)];
- vgic_reg32_update(ipriorityr, r, info);
+ priority = ACCESS_ONCE(*ipriorityr);
+ vgic_reg32_update(&priority, r, info);
+ ACCESS_ONCE(*ipriorityr) = priority;
This is a bit odd to read because of the dereferencing. I admit that I would
prefer if you use read_atomic/write_atomic which are easier to understand
(though the naming is confusing).
Let see what Stefano thinks here.
I also prefer *_atomic, especially given what Jan wrote about
ACCESS_ONCE:
Plus ACCESS_ONCE() doesn't enforce a single instruction to be used in
the resulting assembly).
I don't buy this argument. There are quite a few places we rely on
assignment to be atomic (see PV protocols for instance). Furthermore
implementation of atomic_read/atomic_write in Linux (both ARM and x86)
is based on WRITE_ONCE/READ_ONCE, on Xen it is a simple assignment.
In any case, all those macros does not prevent re-ordering at the
processor level nor read/write atomicity if the variable is misaligned.
Cheers,
--
Julien Grall
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