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

Byungchul Park
2018-01-17T02:20:01Z

On 1/10/2018 10:24 PM, Petr Mladek wrote:
> 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.

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.

> 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_);

Why don't you move this over "while (READ_ONCE(console_waiter))" and
right after acquire()?

As I said last time, only acquisitions between acquire() and release()
are meaningful. Are you taking care of acquisitions within cpu_relax()?
If so, leave it.

> + 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_);

Why don't you move this over "if (waiter)"?

> +
> 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_);

Why don't you remove this release() after relocating the upper one?

> + /*
> + * 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 */
>

--
Thanks,
Byungchul