On Thu, Jun 13, 2019 at 09:44:50PM -0300, Jason Gunthorpe wrote:
> From: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
>
> If the trylock on the hmm->mirrors_sem fails the function will return
> without decrementing the notifiers that were previously incremented. Since
> the caller will not call invalidate_range_end() on EAGAIN this will result
> in notifiers becoming permanently incremented and deadlock.
>
> If the sync_cpu_device_pagetables() required blocking the function will
> not return EAGAIN even though the device continues to touch the
> pages. This is a violation of the mmu notifier contract.
>
> Switch, and rename, the ranges_lock to a spin lock so we can reliably
> obtain it without blocking during error unwind.
>
> The error unwind is necessary since the notifiers count must be held
> incremented across the call to sync_cpu_device_pagetables() as we cannot
> allow the range to become marked valid by a parallel
> invalidate_start/end() pair while doing sync_cpu_device_pagetables().
>
> Signed-off-by: Jason Gunthorpe <jgg@xxxxxxxxxxxx>
> Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx>
> Tested-by: Philip Yang <Philip.Yang@xxxxxxx>
> ---
> include/linux/hmm.h | 2 +-
> mm/hmm.c | 77 +++++++++++++++++++++++++++------------------
> 2 files changed, 48 insertions(+), 31 deletions(-)
>
> diff --git a/include/linux/hmm.h b/include/linux/hmm.h
> index bf013e96525771..0fa8ea34ccef6d 100644
> --- a/include/linux/hmm.h
> +++ b/include/linux/hmm.h
> @@ -86,7 +86,7 @@
> struct hmm {
> struct mm_struct *mm;
> struct kref kref;
> - struct mutex lock;
> + spinlock_t ranges_lock;
> struct list_head ranges;
> struct list_head mirrors;
> struct mmu_notifier mmu_notifier;
> diff --git a/mm/hmm.c b/mm/hmm.c
> index c0d43302fd6b2f..1172a4f0206963 100644
> --- a/mm/hmm.c
> +++ b/mm/hmm.c
> @@ -67,7 +67,7 @@ static struct hmm *hmm_get_or_create(struct mm_struct *mm)
> init_rwsem(&hmm->mirrors_sem);
> hmm->mmu_notifier.ops = NULL;
> INIT_LIST_HEAD(&hmm->ranges);
> - mutex_init(&hmm->lock);
> + spin_lock_init(&hmm->ranges_lock);
> kref_init(&hmm->kref);
> hmm->notifiers = 0;
> hmm->mm = mm;
> @@ -124,18 +124,19 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
> {
> struct hmm *hmm = container_of(mn, struct hmm, mmu_notifier);
> struct hmm_mirror *mirror;
> + unsigned long flags;
>
> /* Bail out if hmm is in the process of being freed */
> if (!kref_get_unless_zero(&hmm->kref))
> return;
>
> - mutex_lock(&hmm->lock);
> + spin_lock_irqsave(&hmm->ranges_lock, flags);
> /*
> * Since hmm_range_register() holds the mmget() lock hmm_release() is
> * prevented as long as a range exists.
> */
> WARN_ON(!list_empty(&hmm->ranges));
> - mutex_unlock(&hmm->lock);
> + spin_unlock_irqrestore(&hmm->ranges_lock, flags);
>
> down_read(&hmm->mirrors_sem);
> list_for_each_entry(mirror, &hmm->mirrors, list) {
> @@ -151,6 +152,23 @@ static void hmm_release(struct mmu_notifier *mn, struct mm_struct *mm)
> hmm_put(hmm);
> }
>
> +static void notifiers_decrement(struct hmm *hmm)
> +{
> + lockdep_assert_held(&hmm->ranges_lock);
> +
> + hmm->notifiers--;
> + if (!hmm->notifiers) {
Nitpick, when doing dec and test or inc and test ops I find it much
easier to read if they are merged into one line, i.e.
if (!--hmm->notifiers) {
Otherwise this looks fine:
Reviewed-by: Christoph Hellwig <hch@xxxxxx>
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