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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
This commit is contained in:
Linus Torvalds 2019-05-16 10:54:19 -07:00
parent b2ca74d32b a9e9bcb45b
commit f57d7715d7
1 changed files with 31 additions and 15 deletions
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46
kernel/locking/rwsem-xadd.c
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@ -130,6 +130,7 @@ static void __rwsem_mark_wake(struct rw_semaphore *sem,
{ {
struct rwsem_waiter *waiter, *tmp; struct rwsem_waiter *waiter, *tmp;
long oldcount, woken = 0, adjustment = 0; long oldcount, woken = 0, adjustment = 0;
struct list_head wlist;
/* /*
* Take a peek at the queue head waiter such that we can determine * 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 * 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 * for above. Note we increment the 'active part' of the count by the
* number of readers before waking any processes up. * 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) { list_for_each_entry(waiter, &sem->wait_list, list) {
struct task_struct *tsk;
if (waiter->type == RWSEM_WAITING_FOR_WRITE) if (waiter->type == RWSEM_WAITING_FOR_WRITE)
break; break;
woken++; 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); get_task_struct(tsk);
list_del(&waiter->list);
/* /*
* Ensure calling get_task_struct() before setting the reader * Ensure calling get_task_struct() before setting the reader
* waiter to nil such that rwsem_down_read_failed() cannot * 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); 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);
} }
/* /*