[PATCH v5 1/2] printk: Add console owner and waiter logic to load balance console writes

Petr Mladek
2018-01-10T13:24:43Z

From: Steven Rostedt <rostedt@goodmis.org>

From: Steven Rostedt (VMware) <rostedt@goodmis.org>

This patch implements what I discussed in Kernel Summit. I added
lockdep annotation (hopefully correctly), and it hasn't had any splats
(since I fixed some bugs in the first iterations). It did catch
problems when I had the owner covering too much. But now that the owner
is only set when actively calling the consoles, lockdep has stayed
quiet.

Here's the design again:

I added a "console_owner" which is set to a task that is actively
writing to the consoles. It is *not* the same as the owner of the
console_lock. It is only set when doing the calls to the console
functions. It is protected by a console_owner_lock which is a raw spin
lock.

There is a console_waiter. This is set when there is an active console
owner that is not current, and waiter is not set. This too is protected
by console_owner_lock.

In printk() when it tries to write to the consoles, we have:

if (console_trylock())
console_unlock();

Now I added an else, which will check if there is an active owner, and
no current waiter. If that is the case, then console_waiter is set, and
the task goes into a spin until it is no longer set.

When the active console owner finishes writing the current message to
the consoles, it grabs the console_owner_lock and sees if there is a
waiter, and clears console_owner.

If there is a waiter, then it breaks out of the loop, clears the waiter
flag (because that will release the waiter from its spin), and exits.
Note, it does *not* release the console semaphore. Because it is a
semaphore, there is no owner. Another task may release it. This means
that the waiter is guaranteed to be the new console owner! Which it
becomes.

Then the waiter calls console_unlock() and continues to write to the
consoles.

If another task comes along and does a printk() it too can become the
new waiter, and we wash rinse and repeat!

By Petr Mladek about possible new deadlocks:

The thing is that we move console_sem only to printk() call
that normally calls console_unlock() as well. It means that
the transferred owner should not bring new type of dependencies.
As Steven said somewhere: "If there is a deadlock, it was
there even before."

We could look at it from this side. The possible deadlock would
look like:

CPU0 CPU1

console_unlock()

console_owner = current;

spin_lockA()
printk()
spin = true;
while (...)

call_console_drivers()
spin_lockA()

This would be a deadlock. CPU0 would wait for the lock A.
While CPU1 would own the lockA and would wait for CPU0
to finish calling the console drivers and pass the console_sem
owner.

But if the above is true than the following scenario was
already possible before:

CPU0

spin_lockA()
printk()
console_unlock()
call_console_drivers()
spin_lockA()

By other words, this deadlock was there even before. Such
deadlocks are prevented by using printk_deferred() in
the sections guarded by the lock A.

Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
[pmladek@suse.com: Commit message about possible deadlocks]
---
kernel/printk/printk.c | 108 ++++++++++++++++++++++++++++++++++++++++++++++++-
1 file changed, 107 insertions(+), 1 deletion(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index b9006617710f..7e6459abba43 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -86,8 +86,15 @@ EXPORT_SYMBOL_GPL(console_drivers);
static struct lockdep_map console_lock_dep_map = {
.name = "console_lock"
};
+static struct lockdep_map console_owner_dep_map = {
+ .name = "console_owner"
+};
#endif

+static DEFINE_RAW_SPINLOCK(console_owner_lock);
+static struct task_struct *console_owner;
+static bool console_waiter;
+
enum devkmsg_log_bits {
__DEVKMSG_LOG_BIT_ON = 0,
__DEVKMSG_LOG_BIT_OFF,
@@ -1753,8 +1760,56 @@ asmlinkage int vprintk_emit(int facility, int level,
* semaphore. The release will print out buffers and wake up
* /dev/kmsg and syslog() users.
*/
- if (console_trylock())
+ if (console_trylock()) {
console_unlock();
+ } else {
+ struct task_struct *owner = NULL;
+ bool waiter;
+ bool spin = false;
+
+ printk_safe_enter_irqsave(flags);
+
+ raw_spin_lock(&console_owner_lock);
+ owner = READ_ONCE(console_owner);
+ waiter = READ_ONCE(console_waiter);
+ if (!waiter && owner && owner != current) {
+ WRITE_ONCE(console_waiter, true);
+ spin = true;
+ }
+ raw_spin_unlock(&console_owner_lock);
+
+ /*
+ * If there is an active printk() writing to the
+ * consoles, instead of having it write our data too,
+ * see if we can offload that load from the active
+ * printer, and do some printing ourselves.
+ * Go into a spin only if there isn't already a waiter
+ * spinning, and there is an active printer, and
+ * that active printer isn't us (recursive printk?).
+ */
+ if (spin) {
+ /* We spin waiting for the owner to release us */
+ spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
+ /* Owner will clear console_waiter on hand off */
+ while (READ_ONCE(console_waiter))
+ cpu_relax();
+
+ spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+ printk_safe_exit_irqrestore(flags);
+
+ /*
+ * The owner passed the console lock to us.
+ * Since we did not spin on console lock, annotate
+ * this as a trylock. Otherwise lockdep will
+ * complain.
+ */
+ mutex_acquire(&console_lock_dep_map, 0, 1, _THIS_IP_);
+ console_unlock();
+ printk_safe_enter_irqsave(flags);
+ }
+ printk_safe_exit_irqrestore(flags);
+
+ }
}

return printed_len;
@@ -2141,6 +2196,7 @@ void console_unlock(void)
static u64 seen_seq;
unsigned long flags;
bool wake_klogd = false;
+ bool waiter = false;
bool do_cond_resched, retry;

if (console_suspended) {
@@ -2229,14 +2285,64 @@ void console_unlock(void)
console_seq++;
raw_spin_unlock(&logbuf_lock);

+ /*
+ * While actively printing out messages, if another printk()
+ * were to occur on another CPU, it may wait for this one to
+ * finish. This task can not be preempted if there is a
+ * waiter waiting to take over.
+ */
+ raw_spin_lock(&console_owner_lock);
+ console_owner = current;
+ raw_spin_unlock(&console_owner_lock);
+
+ /* The waiter may spin on us after setting console_owner */
+ spin_acquire(&console_owner_dep_map, 0, 0, _THIS_IP_);
+
stop_critical_timings(); /* don't trace print latency */
call_console_drivers(ext_text, ext_len, text, len);
start_critical_timings();
+
+ raw_spin_lock(&console_owner_lock);
+ waiter = READ_ONCE(console_waiter);
+ console_owner = NULL;
+ raw_spin_unlock(&console_owner_lock);
+
+ /*
+ * If there is a waiter waiting for us, then pass the
+ * rest of the work load over to that waiter.
+ */
+ if (waiter)
+ break;
+
+ /* There was no waiter, and nothing will spin on us here */
+ spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+
printk_safe_exit_irqrestore(flags);

if (do_cond_resched)
cond_resched();
}
+
+ /*
+ * If there is an active waiter waiting on the console_lock.
+ * Pass off the printing to the waiter, and the waiter
+ * will continue printing on its CPU, and when all writing
+ * has finished, the last printer will wake up klogd.
+ */
+ if (waiter) {
+ WRITE_ONCE(console_waiter, false);
+ /* The waiter is now free to continue */
+ spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+ /*
+ * Hand off console_lock to waiter. The waiter will perform
+ * the up(). After this, the waiter is the console_lock owner.
+ */
+ mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
+ printk_safe_exit_irqrestore(flags);
+ /* Note, if waiter is set, logbuf_lock is not held */
+ return;
+ }
+
console_locked = 0;

/* Release the exclusive_console once it is used */
--
2.13.6