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authorLinus Torvalds <torvalds@linux-foundation.org>2019-05-16 10:54:19 -0700
committerLinus Torvalds <torvalds@linux-foundation.org>2019-05-16 10:54:19 -0700
commitf57d7715d7645b7c3d1e7b7cb79ac7690fe2d260 (patch)
tree56ce411d562b92a8a369f74ad71d1bdbcc701b77 /kernel
parentb2ca74d32bba153a1507e6b7e36d3ec8a89311a1 (diff)
parenta9e9bcb45b1525ba7aea26ed9441e8632aeeda58 (diff)
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Merge branch 'locking-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull locking fix from Ingo Molnar: "A single rwsem fix" * 'locking-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: locking/rwsem: Prevent decrement of reader count before increment
Diffstat (limited to 'kernel')
-rw-r--r--kernel/locking/rwsem-xadd.c46
1 files changed, 31 insertions, 15 deletions
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 6b3ee9948bf1..0b1f77957240 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -130,6 +130,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
{
struct rwsem_waiter *waiter, *tmp;
long oldcount, woken = 0, adjustment = 0;
+ struct list_head wlist;
/*
* Take a peek at the queue head waiter such that we can determine
@@ -188,18 +189,43 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
* of the queue. We know that woken will be at least 1 as we accounted
* for above. Note we increment the 'active part' of the count by the
* number of readers before waking any processes up.
+ *
+ * We have to do wakeup in 2 passes to prevent the possibility that
+ * the reader count may be decremented before it is incremented. It
+ * is because the to-be-woken waiter may not have slept yet. So it
+ * may see waiter->task got cleared, finish its critical section and
+ * do an unlock before the reader count increment.
+ *
+ * 1) Collect the read-waiters in a separate list, count them and
+ * fully increment the reader count in rwsem.
+ * 2) For each waiters in the new list, clear waiter->task and
+ * put them into wake_q to be woken up later.
*/
- list_for_each_entry_safe(waiter, tmp, &sem->wait_list, list) {
- struct task_struct *tsk;
-
+ list_for_each_entry(waiter, &sem->wait_list, list) {
if (waiter->type == RWSEM_WAITING_FOR_WRITE)
break;
woken++;
- tsk = waiter->task;
+ }
+ list_cut_before(&wlist, &sem->wait_list, &waiter->list);
+
+ adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
+ lockevent_cond_inc(rwsem_wake_reader, woken);
+ if (list_empty(&sem->wait_list)) {
+ /* hit end of list above */
+ adjustment -= RWSEM_WAITING_BIAS;
+ }
+
+ if (adjustment)
+ atomic_long_add(adjustment, &sem->count);
+
+ /* 2nd pass */
+ list_for_each_entry_safe(waiter, tmp, &wlist, list) {
+ struct task_struct *tsk;
+ tsk = waiter->task;
get_task_struct(tsk);
- list_del(&waiter->list);
+
/*
* Ensure calling get_task_struct() before setting the reader
* waiter to nil such that rwsem_down_read_failed() cannot
@@ -213,16 +239,6 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
*/
wake_q_add_safe(wake_q, tsk);
}
-
- adjustment = woken * RWSEM_ACTIVE_READ_BIAS - adjustment;
- lockevent_cond_inc(rwsem_wake_reader, woken);
- if (list_empty(&sem->wait_list)) {
- /* hit end of list above */
- adjustment -= RWSEM_WAITING_BIAS;
- }
-
- if (adjustment)
- atomic_long_add(adjustment, &sem->count);
}
/*