linux-kernel - Re: deadlock in synchronize

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Date:   Mon, 27 Mar 2017 13:18:22 +0200
From:   Johannes Berg <johannes@...solutions.net>
To:     paulmck@...ux.vnet.ibm.com
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Nicolai Stange <nicstange@...il.com>,
        gregkh <gregkh@...uxfoundation.org>, sharon.dvir@...el.com,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        linux-wireless <linux-wireless@...r.kernel.org>
Subject: Re: deadlock in synchronize_srcu() in debugfs?

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