[PATCH] mm/hmm: potential deadlock in nonblocking code

[Dan Carpenter <dan.carpenter@oracle.com>]

There is a deadlock bug when these functions are used in nonblocking
mode.  The else side is only meant to be taken in when the code is
used in blocking mode.  But the way it's written now, if we manage to
take the lock without blocking then we try to take it a second time in
the else statement which leads to a deadlock.

Fixes: a3402cb621c1 ("mm/hmm: improve driver API to work and wait over a range")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
---
 mm/hmm.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/mm/hmm.c b/mm/hmm.c
index e14e0aa4d2cb..3b97bb087b28 100644
--- a/mm/hmm.c
+++ b/mm/hmm.c
@@ -207,9 +207,11 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn,
 	update.event = HMM_UPDATE_INVALIDATE;
 	update.blockable = nrange->blockable;
 
-	if (!nrange->blockable && !mutex_trylock(&hmm->lock)) {
-		ret = -EAGAIN;
-		goto out;
+	if (!nrange->blockable) {
+		if (!mutex_trylock(&hmm->lock)) {
+			ret = -EAGAIN;
+			goto out;
+		}
 	} else
 		mutex_lock(&hmm->lock);
 	hmm->notifiers++;
@@ -222,9 +224,11 @@ static int hmm_invalidate_range_start(struct mmu_notifier *mn,
 	mutex_unlock(&hmm->lock);
 
 
-	if (!nrange->blockable && !down_read_trylock(&hmm->mirrors_sem)) {
-		ret = -EAGAIN;
-		goto out;
+	if (!nrange->blockable) {
+		if (!down_read_trylock(&hmm->mirrors_sem)) {
+			ret = -EAGAIN;
+			goto out;
+		}
 	} else
 		down_read(&hmm->mirrors_sem);
 	list_for_each_entry(mirror, &hmm->mirrors, list) {
-- 
2.17.1