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SubjectRe: [PATCH v4] printk: Add console owner and waiter logic to load balance console writes
On Fri 2017-11-24 16:54:16, Petr Mladek wrote:
> On Wed 2017-11-08 10:27:23, Steven Rostedt wrote:
> > 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.
>
> > Index: linux-trace.git/kernel/printk/printk.c
> > ===================================================================
> > --- linux-trace.git.orig/kernel/printk/printk.c
> > +++ linux-trace.git/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
> > * 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_);
>
> I am not sure that this correctly imitates the real lock
> dependency. The trylock flag means that we are able to skip
> this section when the lock is taken elsewhere. But it is not
> the whole truth. In fact, we are blocked in this code path
> when console_sem is taken by another process.
>
> The use of console_owner_lock is not enough. The other
> console_sem calls a lot of code outside the section
> guarded by console_owner_lock.
>
> I think that we have actually entered the cross-release bunch
> of problems, see https://lwn.net/Articles/709849/
>
> IMHO, we need to use struct lockdep_map_cross for
> console_lock_dep_map. Also we need to put somewhere
> lock_commit_crosslock().
>
> I am going to play with it. Also I add Byungchul Park into CC.
> This is why I keep most of the context in this reply (I am sorry
> for it).

See my first attempt below. I do not get any lockdep
warning but it is possible that I just did it wrong.


From 0345785d4767f853ff2d733b565084c3339f9fe0 Mon Sep 17 00:00:00 2001
From: Petr Mladek <pmladek@suse.com>
Date: Fri, 24 Nov 2017 16:50:25 +0100
Subject: [PATCH] printk: Try to describe real console_sem dependecies using
the crosslock feature

console_sem might be newly taken and released by different
processes. This is an attempt to check the crossrelease
dependencies.
---
kernel/printk/printk.c | 25 +++++++++----------------
1 file changed, 9 insertions(+), 16 deletions(-)

diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c
index 040fb948924e..bda25feae0d5 100644
--- a/kernel/printk/printk.c
+++ b/kernel/printk/printk.c
@@ -83,9 +83,9 @@ struct console *console_drivers;
EXPORT_SYMBOL_GPL(console_drivers);

#ifdef CONFIG_LOCKDEP
-static struct lockdep_map console_lock_dep_map = {
- .name = "console_lock"
-};
+static struct lockdep_map_cross console_lock_dep_map =
+ STATIC_CROSS_LOCKDEP_MAP_INIT("console_lock", &console_sem);
+
static struct lockdep_map console_owner_dep_map = {
.name = "console_owner"
};
@@ -218,7 +218,7 @@ static int nr_ext_console_drivers;
*/
#define down_console_sem() do { \
down(&console_sem);\
- mutex_acquire(&console_lock_dep_map, 0, 0, _RET_IP_);\
+ mutex_acquire((struct lockdep_map *)&console_lock_dep_map, 0, 0, _RET_IP_);\
} while (0)

static int __down_trylock_console_sem(unsigned long ip)
@@ -237,7 +237,7 @@ static int __down_trylock_console_sem(unsigned long ip)

if (lock_failed)
return 1;
- mutex_acquire(&console_lock_dep_map, 0, 1, ip);
+ mutex_acquire((struct lockdep_map *)&console_lock_dep_map, 0, 1, ip);
return 0;
}
#define down_trylock_console_sem() __down_trylock_console_sem(_RET_IP_)
@@ -246,7 +246,7 @@ static void __up_console_sem(unsigned long ip)
{
unsigned long flags;

- mutex_release(&console_lock_dep_map, 1, ip);
+ mutex_release((struct lockdep_map *)&console_lock_dep_map, 1, ip);

printk_safe_enter_irqsave(flags);
up(&console_sem);
@@ -1797,13 +1797,6 @@ asmlinkage int vprintk_emit(int facility, int level,
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);
}
@@ -2334,10 +2327,10 @@ void console_unlock(void)
/* 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.
+ * Hand off console_lock to waiter. After this, the waiter
+ * is the console_lock owner.
*/
- mutex_release(&console_lock_dep_map, 1, _THIS_IP_);
+ lock_commit_crosslock((struct lockdep_map *)&console_lock_dep_map);
printk_safe_exit_irqrestore(flags);
/* Note, if waiter is set, logbuf_lock is not held */
return;
--
2.13.6
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