linux-kernel - Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning

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Date:	Thu, 8 Jan 2009 13:03:09 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Chris Mason <chris.mason@...cle.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...e.hu>, paulmck@...ux.vnet.ibm.com,
	Gregory Haskins <ghaskins@...ell.com>,
	Matthew Wilcox <matthew@....cx>,
	Andi Kleen <andi@...stfloor.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	linux-btrfs <linux-btrfs@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Nick Piggin <npiggin@...e.de>,
	Peter Morreale <pmorreale@...ell.com>,
	Sven Dietrich <SDietrich@...ell.com>
Subject: Re: [PATCH -v7][RFC]: mutex: implement adaptive spinning


On Thu, 8 Jan 2009, Linus Torvalds wrote:
> 
> And I don't even believe that is the bug. I suspect the bug is simpler. 
> 
> I think the "need_resched()" needs to go in the outer loop, or at least 
> happen in the "!owner" case. Because at least with preemption, what can 
> happen otherwise is
> 
>  - process A gets the lock, but gets preempted before it sets lock->owner.
> 
>    End result: count = 0, owner = NULL.
> 
>  - processes B/C goes into the spin loop, filling up all CPU's (assuming 
>    dual-core here), and will now both loop forever if they hold the kernel 
>    lock (or have some other preemption disabling thing over their down()).
> 
> And all the while, process A would _happily_ set ->owner, and eventually 
> release the mutex, but it never gets to run to do either of them so.
> 
> In fact, you might not even need a process C: all you need is for B to be 
> on the same runqueue as A, and having enough load on the other CPU's that 
> A never gets migrated away. So "C" might be in user space.
> 
> I dunno. There are probably variations on the above.

Ouch! I think you are on to something:

        for (;;) {
                struct thread_info *owner;

                old_val = atomic_cmpxchg(&lock->count, 1, 0);
                if (old_val == 1) {
                        lock_acquired(&lock->dep_map, ip);
                        mutex_set_owner(lock);
                        return 0;
                }

                if (old_val < 0 && !list_empty(&lock->wait_list))
                        break;

                /* See who owns it, and spin on him if anybody */
                owner = ACCESS_ONCE(lock->owner);

The owner was preempted before assigning lock->owner (as you stated).

                if (owner && !spin_on_owner(lock, owner))
                        break;

We just spin :-(

I think adding the:

+		if (need_resched())
+			break;

would solve the problem.

Thanks,

-- Steve


                cpu_relax();
        }

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