misc/sgi-gru: use mmu_notifier_get/put for struct gru_mm_struct

GRU is already using almost the same algorithm as get/put, it even
helpfully has a 10 year old comment to make this algorithm common, which
is finally happening.

There are a few differences and fixes from this conversion:
- GRU used rcu not srcu to read the hlist
- Unclear how the locking worked to prevent gru_register_mmu_notifier()
  from running concurrently with gru_drop_mmu_notifier() - this version is
  safe
- GRU had a release function which only set a variable without any locking
  that skiped the synchronize_srcu during unregister which looks racey,
  but this makes it reliable via the integrated call_srcu().
- It is unclear if the mmap_sem is actually held when
  __mmu_notifier_register() was called, lockdep will now warn if this is
  wrong

Link: https://lore.kernel.org/r/20190806231548.25242-5-jgg@ziepe.ca
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dimitri Sivanich <sivanich@hpe.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/drivers/misc/sgi-gru/grufile.c b/drivers/misc/sgi-gru/grufile.c
index a2a142ae087b..9d042310214f 100644
--- a/drivers/misc/sgi-gru/grufile.c
+++ b/drivers/misc/sgi-gru/grufile.c
@@ -573,6 +573,7 @@ static void __exit gru_exit(void)
 	gru_free_tables();
 	misc_deregister(&gru_miscdev);
 	gru_proc_exit();
+	mmu_notifier_synchronize();
 }
 
 static const struct file_operations gru_fops = {