Re: [PATCH] lockdep: Print proper scenario if cross deadlock detected at acquisition time

[Boqun Feng <boqun.feng@gmail.com>]

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On Tue, Sep 19, 2017 at 08:52:06PM +0800, Boqun Feng wrote:
> For a potential deadlock about CROSSRELEASE as follow:
> 
> 	P1		P2
> 	===========	=============
> 	lock(A)
> 	lock(X)
> 			lock(A)
> 			commit(X)
> 
> 	A: normal lock, X: cross lock
> 
> , we could detect it at two places:
> 
> 1. commit time:
> 
> 	We have run P1 first, and have dependency A --> X in graph, and
> 	then we run P2, and find the deadlock.
> 
> 2. acquisition time:
> 
> 	We have run P2 first, and have dependency X --> A, in
> 	graph(because another P3 may run previously and is acquiring for
> 	lock X), and then we run P1 and find the deadlock.
> 
> In current print_circular_lock_scenario(), for 1) we could print the
> right scenario and note that's a deadlock related to CROSSRELEASE,
> however for 2) we print the scenario as a normal lockdep deadlock,
> instead we print something like:

Hmm... this sentence is redundant.. the paragraph should be:

... for 2) we currenlty print the scenario as a normal lockdep deadlock:

Apologies for this.

Regards,
Boqun

> 
> | [   35.310179] ======================================================
> | [   35.310749] WARNING: possible circular locking dependency detected
> | [   35.310749] 4.13.0-rc4+ #1 Not tainted
> | [   35.310749] ------------------------------------------------------
> | [   35.310749] torture_onoff/766 is trying to acquire lock:
> | [   35.313943]  ((complete)&st->done){+.+.}, at: [<ffffffffb905f5a6>] takedown_cpu+0x86/0xf0
> | [   35.313943] 
> | [   35.313943] but task is already holding lock:
> | [   35.313943]  (sparse_irq_lock){+.+.}, at: [<ffffffffb90c5e42>] irq_lock_sparse+0x12/0x20
> | [   35.313943] 
> | [   35.313943] which lock already depends on the new lock.
> ...
> | [   35.313943] other info that might help us debug this:
> | [   35.313943] 
> | [   35.313943]  Possible unsafe locking scenario:
> | [   35.313943] 
> | [   35.313943]        CPU0                    CPU1
> | [   35.313943]        ----                    ----
> | [   35.313943]   lock(sparse_irq_lock);
> | [   35.313943]                                lock((complete)&st->done);
> | [   35.313943]                                lock(sparse_irq_lock);
> | [   35.313943]   lock((complete)&st->done);
> | [   35.313943] 
> | [   35.313943]  *** DEADLOCK ***
> 
> It's better to print a proper scenario related to CROSSRELEASE to help
> users find their bugs more easily, so improve this.
> 
> Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> Cc: Byungchul Park <byungchul.park@lge.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Signed-off-by: Boqun Feng <boqun.feng@gmail.com>
> ---
> The sample of print_circular_lock_scenario() is from Paul Mckenney.
> 
>  kernel/locking/lockdep.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index 44c8d0d17170..67a407bcc814 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -1156,6 +1156,23 @@ print_circular_lock_scenario(struct held_lock *src,
>  		__print_lock_name(target);
>  		printk(KERN_CONT ");\n");
>  		printk("\n *** DEADLOCK ***\n\n");
> +	} else if (cross_lock(src->instance)) {
> +		printk(" Possible unsafe locking scenario by crosslock:\n\n");
> +		printk("       CPU0                    CPU1\n");
> +		printk("       ----                    ----\n");
> +		printk("  lock(");
> +		__print_lock_name(target);
> +		printk(KERN_CONT ");\n");
> +		printk("  lock(");
> +		__print_lock_name(source);
> +		printk(KERN_CONT ");\n");
> +		printk("                               lock(");
> +		__print_lock_name(parent == source ? target : parent);
> +		printk(KERN_CONT ");\n");
> +		printk("                               unlock(");
> +		__print_lock_name(source);
> +		printk(KERN_CONT ");\n");
> +		printk("\n *** DEADLOCK ***\n\n");
>  	} else {
>  		printk(" Possible unsafe locking scenario:\n\n");
>  		printk("       CPU0                    CPU1\n");
> -- 
> 2.14.1
> 

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