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

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On 1/17/2018 11:19 AM, Byungchul Park wrote:
On 1/10/2018 10:24 PM, Petr Mladek wrote:
From: Steven Rostedt <rostedt@xxxxxxxxxxx>

From: Steven Rostedt (VMware) <rostedt@xxxxxxxxxxx>

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.

Hello,

I didn't see what you did, at the last version. You were
tring to transfer the semaphore owner and make it taken
over. I see.

But, what I mentioned last time is still valid. See below.

Of course, it's not an important thing but trivial one though.

--
Thanks,
Byungchul

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