Re: [PATCH v5 2/2] printk: Hide console waiter logic into helpers

Petr Mladek
2018-01-12T16:08:46Z

On Fri 2018-01-12 10:37:54, Steven Rostedt wrote:
> On Thu, 11 Jan 2018 13:03:41 +0100
> Petr Mladek <pmladek@suse.com> wrote:
> > All the other changes look good to me. I will use them in the next version.
>
> Great.

Please, find below the updated version. If I get Ack at least from
Steven and no nack's, I will put it into linux-next next week.

>From f67f70d910d9cf310a7bc73e97bf14097d31b059 Mon Sep 17 00:00:00 2001
From: Petr Mladek <pmladek@suse.com>
Date: Fri, 22 Dec 2017 18:58:46 +0100
Subject: [PATCH v6 2/4] printk: Hide console waiter logic into helpers

The commit ("printk: Add console owner and waiter logic to load balance
console writes") made vprintk_emit() and console_unlock() even more
complicated.

This patch extracts the new code into 3 helper functions. They should
help to keep it rather self-contained. It will be easier to use and
maintain.

This patch just shuffles the existing code. It does not change
the functionality.

Signed-off-by: Petr Mladek <pmladek@suse.com>
---
Changes against v5:

+ updated some comments (Steven)
+ do console_trylock() in console_trylock_spinning() (Steven)

kernel/printk/printk.c | 245 +++++++++++++++++++++++++++++--------------------
1 file changed, 148 insertions(+), 97 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 7e6459abba43..3057dbc69b4f 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -86,15 +86,8 @@ 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,
@@ -1551,6 +1544,146 @@ SYSCALL_DEFINE3(syslog, int, type, char __user *, buf, int, len)
}

/*
+ * Special console_lock variants that help to reduce the risk of soft-lockups.
+ * They allow to pass console_lock to another printk() call using a busy wait.
+ */
+
+#ifdef CONFIG_LOCKDEP
+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;
+
+/**
+ * console_lock_spinning_enable - mark beginning of code where another
+ * thread might safely busy wait
+ *
+ * This basically converts console_lock into a spinlock. This marks
+ * the section where the console_lock owner can not sleep, because
+ * there may be a waiter spinning (like a spinlock). Also it must be
+ * ready to hand over the lock at the end of the section.
+ */
+static void console_lock_spinning_enable(void)
+{
+ 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_);
+}
+
+/**
+ * console_lock_spinning_disable_and_check - mark end of code where another
+ * thread was able to busy wait and check if there is a waiter
+ *
+ * This is called at the end of the section where spinning is allowed.
+ * It has two functions. First, it is a signal that it is not longer
+ * safe to start busy waiting for the lock. Second, it checks if
+ * there is a busy waiter and passes the lock rights to her.
+ *
+ * Important: Callers lose the lock if there was the busy waiter.
+ * They must not touch items synchronized by console_lock
+ * in this case.
+ *
+ * Return: 1 if the lock rights were passed, 0 otherwise.
+ */
+static int console_lock_spinning_disable_and_check(void)
+{
+ int waiter;
+
+ raw_spin_lock(&console_owner_lock);
+ waiter = READ_ONCE(console_waiter);
+ console_owner = NULL;
+ raw_spin_unlock(&console_owner_lock);
+
+ if (!waiter) {
+ spin_release(&console_owner_dep_map, 1, _THIS_IP_);
+ return 0;
+ }
+
+ /* The waiter is now free to continue */
+ WRITE_ONCE(console_waiter, false);
+
+ 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_);
+ return 1;
+}
+
+/**
+ * console_trylock_spinning - try to get console_lock by busy waiting
+ *
+ * This allows to busy wait for the console_lock when the current
+ * owner is running in a special marked sections. It means that
+ * the current owner is running and cannot reschedule until it
+ * is ready to loose the lock.
+ *
+ * Return: 1 if we got the lock, 0 othrewise
+ */
+static int console_trylock_spinning(void)
+{
+ struct task_struct *owner = NULL;
+ bool waiter;
+ bool spin = false;
+ unsigned long flags;
+
+ if (console_trylock())
+ return 1;
+
+ 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) {
+ printk_safe_exit_irqrestore(flags);
+ return 0;
+ }
+
+ /* 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_);
+
+ return 1;
+}
+
+/*
* Call the console drivers, asking them to write out
* log_buf[start] to log_buf[end - 1].
* The console_lock must be held.
@@ -1760,56 +1893,8 @@ 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_spinning())
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;
@@ -1910,6 +1995,8 @@ static ssize_t msg_print_ext_header(char *buf, size_t size,
static ssize_t msg_print_ext_body(char *buf, size_t size,
char *dict, size_t dict_len,
char *text, size_t text_len) { return 0; }
+static void console_lock_spinning_enable(void) { }
+static int console_lock_spinning_disable_and_check(void) { return 0; }
static void call_console_drivers(const char *ext_text, size_t ext_len,
const char *text, size_t len) {}
static size_t msg_print_text(const struct printk_log *msg,
@@ -2196,7 +2283,6 @@ 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) {
@@ -2291,31 +2377,16 @@ void console_unlock(void)
* 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_);
+ console_lock_spinning_enable();

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_);
+ if (console_lock_spinning_disable_and_check()) {
+ printk_safe_exit_irqrestore(flags);
+ return;
+ }

printk_safe_exit_irqrestore(flags);

@@ -2323,26 +2394,6 @@ void console_unlock(void)
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