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SubjectRe: [PATCH 2/2] drm: mm always protect change to unused_nodes with unused_lock spinlock
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On Mon, 2009-11-16 at 17:23 +0100, Thomas Hellstrom wrote:
> Dave Airlie wrote:
> > On Sat, Nov 14, 2009 at 5:56 AM, Jerome Glisse <jglisse@redhat.com> wrote:
> >
> >> unused_nodes modification needs to be protected by unused_lock spinlock.
> >> Here is an example of an usage where there is no such protection without
> >> this patch.
> >>
> >> Process 1: 1-drm_mm_pre_get(this function modify unused_nodes list)
> >> 2-spin_lock(spinlock protecting mm struct)
> >> 3-drm_mm_put_block(this function might modify unused_nodes
> >> list but doesn't protect modification with unused_lock)
> >> 4-spin_unlock(spinlock protecting mm struct)
> >> Process2: 1-drm_mm_pre_get(this function modify unused_nodes list)
> >> At this point Process1 & Process2 might both be doing modification to
> >> unused_nodes list. This patch add unused_lock protection into
> >> drm_mm_put_block to avoid such issue.
> >>
> >
> > Have we got a bug number or reproducer for this?
> >
> > I've cc'ed Thomas and Chris who were last ppl to touch drm_mm.c for some
> > sort of acks.
> >
> > Dave.
> >
> >
> >> Signed-off-by: Jerome Glisse <jglisse@redhat.com>
> >> ---
> >> drivers/gpu/drm/drm_mm.c | 9 +++++++++
> >> 1 files changed, 9 insertions(+), 0 deletions(-)
> >>
> >> diff --git a/drivers/gpu/drm/drm_mm.c b/drivers/gpu/drm/drm_mm.c
> >> index c861d80..97dc5a4 100644
> >> --- a/drivers/gpu/drm/drm_mm.c
> >> +++ b/drivers/gpu/drm/drm_mm.c
> >> @@ -103,6 +103,11 @@ static struct drm_mm_node *drm_mm_kmalloc(struct drm_mm *mm, int atomic)
> >> return child;
> >> }
> >>
> >> +/* drm_mm_pre_get() - pre allocate drm_mm_node structure
> >> + * drm_mm: memory manager struct we are pre-allocating for
> >> + *
> >> + * Returns 0 on success or -ENOMEM if allocation fails.
> >> + */
> >> int drm_mm_pre_get(struct drm_mm *mm)
> >> {
> >> struct drm_mm_node *node;
> >> @@ -253,12 +258,14 @@ void drm_mm_put_block(struct drm_mm_node *cur)
> >> prev_node->size += next_node->size;
> >> list_del(&next_node->ml_entry);
> >> list_del(&next_node->fl_entry);
> >> + spin_lock(&mm->unused_lock);
> >> if (mm->num_unused < MM_UNUSED_TARGET) {
> >> list_add(&next_node->fl_entry,
> >> &mm->unused_nodes);
> >> ++mm->num_unused;
> >> } else
> >> kfree(next_node);
> >> + spin_unlock(&mm->unused_lock);
> >> } else {
> >> next_node->size += cur->size;
> >> next_node->start = cur->start;
> >> @@ -271,11 +278,13 @@ void drm_mm_put_block(struct drm_mm_node *cur)
> >> list_add(&cur->fl_entry, &mm->fl_entry);
> >> } else {
> >> list_del(&cur->ml_entry);
> >> + spin_lock(&mm->unused_lock);
> >> if (mm->num_unused < MM_UNUSED_TARGET) {
> >> list_add(&cur->fl_entry, &mm->unused_nodes);
> >> ++mm->num_unused;
> >> } else
> >> kfree(cur);
> >> + spin_unlock(&mm->unused_lock);
> >> }
> >> }
> >>
> >> --
> >> 1.6.5.2
> >>
> >>
> >>
> Hmm. Ouch. The patch looks correct, although I'm not 100% sure it's OK
> to kfree() within a spinlocked region? Perhaps better to take it out.

Would kfree() possibly sleep? I wouldn't think so, if not it should be
safe.

robert.

> /Thomas
>
>
>
>
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