2022-09-01 18:18:29

by Suren Baghdasaryan

Subject: [RFC PATCH RESEND 16/28] kernel/fork: assert no VMA readers during its destruction

Assert there are no holders of VMA lock for reading when it is about to be
destroyed.

Signed-off-by: Suren Baghdasaryan <[email protected]>
---
include/linux/mm.h | 8 ++++++++
kernel/fork.c | 2 ++
2 files changed, 10 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index dc72be923e5b..0d9c1563c354 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -676,6 +676,13 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos)
VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
}

+static inline void vma_assert_no_reader(struct vm_area_struct *vma)
+{
+ VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock) &&
+ vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
+ vma);
+}
+
#else /* CONFIG_PER_VMA_LOCK */

static inline void vma_init_lock(struct vm_area_struct *vma) {}
@@ -685,6 +692,7 @@ static inline bool vma_read_trylock(struct vm_area_struct *vma)
static inline void vma_read_unlock(struct vm_area_struct *vma) {}
static inline void vma_assert_locked(struct vm_area_struct *vma) {}
static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos) {}
+static inline void vma_assert_no_reader(struct vm_area_struct *vma) {}

#endif /* CONFIG_PER_VMA_LOCK */

diff --git a/kernel/fork.c b/kernel/fork.c
index 1872ad549fed..b443ba3a247a 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -487,6 +487,8 @@ static void __vm_area_free(struct rcu_head *head)
{
struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
vm_rcu);
+ /* The vma should either have no lock holders or be write-locked. */
+ vma_assert_no_reader(vma);
kmem_cache_free(vm_area_cachep, vma);
}
#endif
--
2.37.2.789.g6183377224-goog

2022-09-09 14:22:55

by Laurent Dufour

Subject: Re: [RFC PATCH RESEND 16/28] kernel/fork: assert no VMA readers during its destruction

Le 01/09/2022 à 19:35, Suren Baghdasaryan a écrit :
> Assert there are no holders of VMA lock for reading when it is about to be
> destroyed.
>
> Signed-off-by: Suren Baghdasaryan <[email protected]>
> ---
> include/linux/mm.h | 8 ++++++++
> kernel/fork.c | 2 ++
> 2 files changed, 10 insertions(+)
>
> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index dc72be923e5b..0d9c1563c354 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -676,6 +676,13 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos)
> VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
> }
>
> +static inline void vma_assert_no_reader(struct vm_area_struct *vma)
> +{
> + VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock) &&
> + vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
> + vma);
> +}
> +
> #else /* CONFIG_PER_VMA_LOCK */
>
> static inline void vma_init_lock(struct vm_area_struct *vma) {}
> @@ -685,6 +692,7 @@ static inline bool vma_read_trylock(struct vm_area_struct *vma)
> static inline void vma_read_unlock(struct vm_area_struct *vma) {}
> static inline void vma_assert_locked(struct vm_area_struct *vma) {}
> static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos) {}
> +static inline void vma_assert_no_reader(struct vm_area_struct *vma) {}
>
> #endif /* CONFIG_PER_VMA_LOCK */
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 1872ad549fed..b443ba3a247a 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -487,6 +487,8 @@ static void __vm_area_free(struct rcu_head *head)
> {
> struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
> vm_rcu);
> + /* The vma should either have no lock holders or be write-locked. */
> + vma_assert_no_reader(vma);

I'm wondering if this can be hit in the case the thread freeing a VMA is
preempted before incrementing the mm ref count, like this:

VMA is about to be freed
write lock VMA
free vma -> call_rcu
..
<--- thread preempted
rcu handler runs
rcu calls __vm_area_free() <<<<<<
unlock mmap_lock and increase the mm seq count


> kmem_cache_free(vm_area_cachep, vma);
> }
> #endif

2022-09-09 16:42:09

by Suren Baghdasaryan

Subject: Re: [RFC PATCH RESEND 16/28] kernel/fork: assert no VMA readers during its destruction

On Fri, Sep 9, 2022 at 6:56 AM Laurent Dufour <[email protected]> wrote:
>
> Le 01/09/2022 à 19:35, Suren Baghdasaryan a écrit :
> > Assert there are no holders of VMA lock for reading when it is about to be
> > destroyed.
> >
> > Signed-off-by: Suren Baghdasaryan <[email protected]>
> > ---
> > include/linux/mm.h | 8 ++++++++
> > kernel/fork.c | 2 ++
> > 2 files changed, 10 insertions(+)
> >
> > diff --git a/include/linux/mm.h b/include/linux/mm.h
> > index dc72be923e5b..0d9c1563c354 100644
> > --- a/include/linux/mm.h
> > +++ b/include/linux/mm.h
> > @@ -676,6 +676,13 @@ static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos)
> > VM_BUG_ON_VMA(vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq), vma);
> > }
> >
> > +static inline void vma_assert_no_reader(struct vm_area_struct *vma)
> > +{
> > + VM_BUG_ON_VMA(rwsem_is_locked(&vma->lock) &&
> > + vma->vm_lock_seq != READ_ONCE(vma->vm_mm->mm_lock_seq),
> > + vma);
> > +}
> > +
> > #else /* CONFIG_PER_VMA_LOCK */
> >
> > static inline void vma_init_lock(struct vm_area_struct *vma) {}
> > @@ -685,6 +692,7 @@ static inline bool vma_read_trylock(struct vm_area_struct *vma)
> > static inline void vma_read_unlock(struct vm_area_struct *vma) {}
> > static inline void vma_assert_locked(struct vm_area_struct *vma) {}
> > static inline void vma_assert_write_locked(struct vm_area_struct *vma, int pos) {}
> > +static inline void vma_assert_no_reader(struct vm_area_struct *vma) {}
> >
> > #endif /* CONFIG_PER_VMA_LOCK */
> >
> > diff --git a/kernel/fork.c b/kernel/fork.c
> > index 1872ad549fed..b443ba3a247a 100644
> > --- a/kernel/fork.c
> > +++ b/kernel/fork.c
> > @@ -487,6 +487,8 @@ static void __vm_area_free(struct rcu_head *head)
> > {
> > struct vm_area_struct *vma = container_of(head, struct vm_area_struct,
> > vm_rcu);
> > + /* The vma should either have no lock holders or be write-locked. */
> > + vma_assert_no_reader(vma);
>
> I'm wondering if this can be hit in the case the thread freeing a VMA is
> preempted before incrementing the mm ref count, like this:
>
> VMA is about to be freed
> write lock VMA
> free vma -> call_rcu
> ..
> <--- thread preempted
> rcu handler runs
> rcu calls __vm_area_free() <<<<<<

At this point the VMA is still write-locked (mm seq count hasn't been
incremented yet), correct? If so then vma_assert_no_reader() will not
assert because the second condition of VMA being write-locked is
satisfied. Did I miss anything?

> unlock mmap_lock and increase the mm seq count
>
>
> > kmem_cache_free(vm_area_cachep, vma);
> > }
> > #endif
>