This is a note to let you know that I've just added the patch titled
printk: add missing memory barrier to wake_up_klogd()
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
printk-add-missing-memory-barrier-to-wake_up_klogd.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
commit 39e083b74c2bb939faa917e6e19a7956ab01d668
Author: John Ogness <john.ogness@xxxxxxxxxxxxx>
Date: Thu Apr 21 23:28:38 2022 +0206
printk: add missing memory barrier to wake_up_klogd()
[ Upstream commit 1f5d783094cf28b4905f51cad846eb5d1db6673e ]
It is important that any new records are visible to preparing
waiters before the waker checks if the wait queue is empty.
Otherwise it is possible that:
- there are new records available
- the waker sees an empty wait queue and does not wake
- the preparing waiter sees no new records and begins to wait
This is exactly the problem that the function description of
waitqueue_active() warns about.
Use wq_has_sleeper() instead of waitqueue_active() because it
includes the necessary full memory barrier.
Signed-off-by: John Ogness <john.ogness@xxxxxxxxxxxxx>
Reviewed-by: Petr Mladek <pmladek@xxxxxxxx>
Signed-off-by: Petr Mladek <pmladek@xxxxxxxx>
Link: https://lore.kernel.org/r/20220421212250.565456-4-john.ogness@xxxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index e6a815a1cd76..dc074fb12b05 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -735,8 +735,19 @@ static ssize_t devkmsg_read(struct file *file, char __user *buf,
goto out;
}
+ /*
+ * Guarantee this task is visible on the waitqueue before
+ * checking the wake condition.
+ *
+ * The full memory barrier within set_current_state() of
+ * prepare_to_wait_event() pairs with the full memory barrier
+ * within wq_has_sleeper().
+ *
+ * This pairs with wake_up_klogd:A.
+ */
ret = wait_event_interruptible(log_wait,
- prb_read_valid(prb, atomic64_read(&user->seq), r));
+ prb_read_valid(prb,
+ atomic64_read(&user->seq), r)); /* LMM(devkmsg_read:A) */
if (ret)
goto out;
}
@@ -1502,7 +1513,18 @@ static int syslog_print(char __user *buf, int size)
seq = syslog_seq;
mutex_unlock(&syslog_lock);
- len = wait_event_interruptible(log_wait, prb_read_valid(prb, seq, NULL));
+ /*
+ * Guarantee this task is visible on the waitqueue before
+ * checking the wake condition.
+ *
+ * The full memory barrier within set_current_state() of
+ * prepare_to_wait_event() pairs with the full memory barrier
+ * within wq_has_sleeper().
+ *
+ * This pairs with wake_up_klogd:A.
+ */
+ len = wait_event_interruptible(log_wait,
+ prb_read_valid(prb, seq, NULL)); /* LMM(syslog_print:A) */
mutex_lock(&syslog_lock);
if (len)
@@ -3236,7 +3258,18 @@ void wake_up_klogd(void)
return;
preempt_disable();
- if (waitqueue_active(&log_wait)) {
+ /*
+ * Guarantee any new records can be seen by tasks preparing to wait
+ * before this context checks if the wait queue is empty.
+ *
+ * The full memory barrier within wq_has_sleeper() pairs with the full
+ * memory barrier within set_current_state() of
+ * prepare_to_wait_event(), which is called after ___wait_event() adds
+ * the waiter but before it has checked the wait condition.
+ *
+ * This pairs with devkmsg_read:A and syslog_print:A.
+ */
+ if (wq_has_sleeper(&log_wait)) { /* LMM(wake_up_klogd:A) */
this_cpu_or(printk_pending, PRINTK_PENDING_WAKEUP);
irq_work_queue(this_cpu_ptr(&wake_up_klogd_work));
}
