IB/umem: Fix possible hang on process exit

If ib_umem_release() is called after ib_uverbs_close() sets context->closing,
then a process can get stuck in a D state, because the code boils down to

	if (down_write_trylock(&mm->mmap_sem))
		down_write(&mm->mmap_sem);

which is obviously a stupid instant deadlock.  Fix the code so that we
only try to take the lock once.

This bug was introduced in commit f7c6a7b5 ("IB/uverbs: Export
ib_umem_get()/ib_umem_release() to modules") which fortunately never
made it into a release, and was reported by Pete Wyckoff <pw@osc.edu>.

Signed-off-by: Roland Dreier <rolandd@cisco.com>

1 files changed, 9 insertions, 7 deletions

diff --git a/drivers/infiniband/core/umem.c b/drivers/infiniband/core/umem.c
index b4aec5103c99..d40652a80151 100644
--- a/drivers/infiniband/core/umem.c
+++ b/drivers/infiniband/core/umem.c
@@ -225,13 +225,15 @@ void ib_umem_release(struct ib_umem *umem)
          * up here and not be able to take the mmap_sem.  In that case
          * we defer the vm_locked accounting to the system workqueue.
          */
-        if (context->closing && !down_write_trylock(&mm->mmap_sem)) {
-                INIT_WORK(&umem->work, ib_umem_account);
-                umem->mm   = mm;
-                umem->diff = diff;
-
-                schedule_work(&umem->work);
-                return;
+        if (context->closing) {
+                if (!down_write_trylock(&mm->mmap_sem)) {
+                        INIT_WORK(&umem->work, ib_umem_account);
+                        umem->mm   = mm;
+                        umem->diff = diff;
+
+                        schedule_work(&umem->work);
+                        return;
+                }
         } else
                 down_write(&mm->mmap_sem);