Re: deadlock in synchronize_srcu() in debugfs?

From	Johannes Berg <johannes@sipsolutions.net>
Newsgroups	linux.kernel
Subject	Re: deadlock in synchronize_srcu() in debugfs?
Date	2017-03-27 13:20 +0200
Message-ID	<tpAQN-79D-7@gated-at.bofh.it> (permalink)
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On Fri, 2017-03-24 at 13:20 -0700, Paul E. McKenney wrote:
> 
> And I cannot resist adding this one:
> 
> 	CPU 1				CPU 2
> 	i = srcu_read_lock(&s1);	mutex_lock(&l1);
> 	mutex_lock(&l1);		synchronize_srcu(&s2);
> 	mutex_unlock(&l1);		mutex_unlock(&l1);
> 	srcu_read_unlock(&s1, i);
> 
> 	CPU 3				CPU 4
> 	i = srcu_read_lock(&s2);	mutex_lock(&l2);
> 	mutex_lock(&l2);		synchronize_srcu(&s1);
> 	mutex_unlock(&l2);		mutex_unlock(&l2);
> 	srcu_read_unlock(&s2, i);
> 
> Removing the SRCU statements from any of these CPU would break the
> deadlock.  This can be easily extended to a deadlock cycle involving
> any number of srcu_struct structures.
> 
> But this would still be a cycle involving an srcu_read_lock() and a
> synchronize_srcu() on the same srcu_struct, which is reassuring.

Right, you can cycle this indefinitely. lockdep has some kind of
maximum chain length I think. :)

johannes

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deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-23 16:00 +0100
  Re: deadlock in synchronize_srcu() in debugfs? Nicolai Stange <nicstange@gmail.com> - 2017-03-23 16:40 +0100
    Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-23 16:50 +0100
    Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-27 14:00 +0200
      Re: deadlock in synchronize_srcu() in debugfs? Nicolai Stange <nicstange@gmail.com> - 2017-03-30 09:40 +0200
        Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-30 10:00 +0200
          Re: deadlock in synchronize_srcu() in debugfs? Nicolai Stange <nicstange@gmail.com> - 2017-03-30 12:30 +0200
            Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-30 13:20 +0200
              Re: deadlock in synchronize_srcu() in debugfs? Nicolai Stange <nicstange@gmail.com> - 2017-03-31 11:10 +0200
                Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-31 11:50 +0200
  Re: deadlock in synchronize_srcu() in debugfs? "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-03-23 16:40 +0100
    Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-23 16:50 +0100
  Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-23 16:40 +0100
    Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-24 10:00 +0100
      Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-24 10:30 +0100
        Re: deadlock in synchronize_srcu() in debugfs? "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-03-24 18:50 +0100
          Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-24 20:00 +0100
            Re: deadlock in synchronize_srcu() in debugfs? "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-03-24 20:40 +0100
              Re: deadlock in synchronize_srcu() in debugfs? "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-03-24 21:30 +0100
                Re: deadlock in synchronize_srcu() in debugfs? Johannes Berg <johannes@sipsolutions.net> - 2017-03-27 13:20 +0200

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