• Steven Rostedt (VMware)'s avatar
    printk: Add console owner and waiter logic to load balance console writes · 59423114
    Steven Rostedt (VMware) authored
    [ Upstream commit dbdda842 ]
    
    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.
    
    By Steven Rostedt:
    
    To demonstrate the issue, this module has been shown to lock up a
    system with 4 CPUs and a slow console (like a serial console). It is
    also able to lock up a 8 CPU system with only a fast (VGA) console, by
    passing in "loops=100". The changes in this commit prevent this module
    from locking up the system.
    
     #include <linux/module.h>
     #include <linux/delay.h>
     #include <linux/sched.h>
     #include <linux/mutex.h>
     #include <linux/workqueue.h>
     #include <linux/hrtimer.h>
    
     static bool stop_testing;
     static unsigned int loops = 1;
    
     static void preempt_printk_workfn(struct work_struct *work)
     {
     	int i;
    
     	while (!READ_ONCE(stop_testing)) {
     		for (i = 0; i < loops && !READ_ONCE(stop_testing); i++) {
     			preempt_disable();
     			pr_emerg("%5d%-75s\n", smp_processor_id(),
     				 " XXX NOPREEMPT");
     			preempt_enable();
     		}
     		msleep(1);
     	}
     }
    
     static struct work_struct __percpu *works;
    
     static void finish(void)
     {
     	int cpu;
    
     	WRITE_ONCE(stop_testing, true);
     	for_each_online_cpu(cpu)
     		flush_work(per_cpu_ptr(works, cpu));
     	free_percpu(works);
     }
    
     static int __init test_init(void)
     {
     	int cpu;
    
     	works = alloc_percpu(struct work_struct);
     	if (!works)
     		return -ENOMEM;
    
     	/*
     	 * This is just a test module. This will break if you
     	 * do any CPU hot plugging between loading and
     	 * unloading the module.
     	 */
    
     	for_each_online_cpu(cpu) {
     		struct work_struct *work = per_cpu_ptr(works, cpu);
    
     		INIT_WORK(work, &preempt_printk_workfn);
     		schedule_work_on(cpu, work);
     	}
    
     	return 0;
     }
    
     static void __exit test_exit(void)
     {
     	finish();
     }
    
     module_param(loops, uint, 0);
     module_init(test_init);
     module_exit(test_exit);
     MODULE_LICENSE("GPL");
    
    Link: http://lkml.kernel.org/r/20180110132418.7080-2-pmladek@suse.com
    
    
    Cc: akpm@linux-foundation.org
    Cc: linux-mm@kvack.org
    Cc: Cong Wang <xiyou.wangcong@gmail.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Mel Gorman <mgorman@suse.de>
    Cc: Michal Hocko <mhocko@kernel.org>
    Cc: Vlastimil Babka <vbabka@suse.cz>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Jan Kara <jack@suse.cz>
    Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
    Cc: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Cc: Byungchul Park <byungchul.park@lge.com>
    Cc: Tejun Heo <tj@kernel.org>
    Cc: Pavel Machek <pavel@ucw.cz>
    Cc: linux-kernel@vger.kernel.org
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    [pmladek@suse.com: Commit message about possible deadlocks]
    Acked-by: default avatarSergey Senozhatsky <sergey.senozhatsky@gmail.com>
    Signed-off-by: default avatarPetr Mladek <pmladek@suse.com>
    Signed-off-by: default avatarSasha Levin <sashal@kernel.org>
    59423114
printk.c 82.7 KB