On Mon, Jul 26, 2021, Ben Gardon wrote:
> On Mon, Jul 26, 2021 at 12:52 AM Dan Carpenter <dan.carpenter@xxxxxxxxxx> wrote:
> >
> > [ This is not the correct patch to blame, but there is something going
> > on here which I don't understand so this email is more about me
> > learning rather than reporting bugs. - dan ]
> >
> > Hello Ben Gardon,
> >
> > The patch 531810caa9f4: "KVM: x86/mmu: Use an rwlock for the x86 MMU"
> > from Feb 2, 2021, leads to the following static checker warning:
> >
> > arch/x86/kvm/mmu/mmu.c:5769 kvm_mmu_zap_all()
> > warn: sleeping in atomic context
> >
> > arch/x86/kvm/mmu/mmu.c
> > 5756 void kvm_mmu_zap_all(struct kvm *kvm)
> > 5757 {
> > 5758 struct kvm_mmu_page *sp, *node;
> > 5759 LIST_HEAD(invalid_list);
> > 5760 int ign;
> > 5761
> > 5762 write_lock(&kvm->mmu_lock);
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^
> > This line bumps the preempt count.
> >
> > 5763 restart:
> > 5764 list_for_each_entry_safe(sp, node, &kvm->arch.active_mmu_pages, link) {
> > 5765 if (WARN_ON(sp->role.invalid))
> > 5766 continue;
> > 5767 if (__kvm_mmu_prepare_zap_page(kvm, sp, &invalid_list, &ign))
> > 5768 goto restart;
> > --> 5769 if (cond_resched_rwlock_write(&kvm->mmu_lock))
> > ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > This line triggers a sleeping in atomic warning. What's going on here
> > that I'm not understanding?
>
>
> Hi Dan,
>
> Thanks for sending this. I'm confused by this sequence too. I'm not
> sure how this could sleep in an atomic context.
> My first thought was that there might be something going on with the
> qrwlock's wait_lock, but since this thread already acquired the
> rwlock, it can't be holding / waiting on the wait_lock.
>
> Then I thought the __might_sleep could be in the wrong place, but it's
> in the same place for a regular spinlock, so I think that's fine.
The PREEMPT_LOCK_OFFSET parameter to __might_sleep()
__might_sleep(__FILE__, __LINE__, PREEMPT_LOCK_OFFSET); \
effectively tells it to exempt a single preemption count via preempt_count_equals()
void ___might_sleep(const char *file, int line, int preempt_offset)
{
...
if ((preempt_count_equals(preempt_offset) && !irqs_disabled() &&
!is_idle_task(current) && !current->non_block_count) ||
system_state == SYSTEM_BOOTING || system_state > SYSTEM_RUNNING ||
oops_in_progress)
return;
...
}
which returns true if the preempt count equals the passed in offset.
PREEMPT_LOCK_OFFSET is just the vanilla preempt_disable() offset, which is why
there's no special preemption call in the lock/unlock paths.
#define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET
Dan, is this coming from Smatch? If so, is this by chance a new, in-progress
warning that has special code to handle cond_resched_lock()? I couldn't find
any matches on "sleeping in atomic context" in Smatch. The rwlock variants,
cond_resched_rwlock_{read,write}() were added specifically for KVM's TDP MMU,
maybe they snuck in after a waiver for cond_resched_lock() was added?
