From mboxrd@z Thu Jan  1 00:00:00 1970
From: Peter Zijlstra <peterz@infradead.org>
Subject: Re: mmu_notifiers: turn off lockdep around mm_take_all_locks
Date: Tue, 07 Jul 2009 20:15:05 +0200
Message-ID: <1246990505.5197.2.camel@laptop>
References: <20090707180630.GA8008@amt.cnet>
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Cc: Andrea Arcangeli <aarcange@redhat.com>,
	Peter Zijlstra <peter@programming.kicks-ass.net>,
	kvm <kvm@vger.kernel.org>, Avi Kivity <avi@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>
Return-path: <kvm-owner@vger.kernel.org>
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On Tue, 2009-07-07 at 15:06 -0300, Marcelo Tosatti wrote:
> KVM guests with CONFIG_LOCKDEP=y trigger the following warning:
> 
> BUG: MAX_LOCK_DEPTH too low!
> turning off the locking correctness validator.
> Pid: 4624, comm: qemu-system-x86 Not tainted 2.6.31-rc2-03981-g3abaf21
> #32
> Call Trace:
>  [<ffffffff81068bab>] __lock_acquire+0x1559/0x15fc
>  [<ffffffff810be4d9>] ? mm_take_all_locks+0x99/0x109
>  [<ffffffff810be4d9>] ? mm_take_all_locks+0x99/0x109
>  [<ffffffff81068d3c>] lock_acquire+0xee/0x112
>  [<ffffffff810be516>] ? mm_take_all_locks+0xd6/0x109
>  [<ffffffff81402596>] ? _spin_lock_nest_lock+0x20/0x50
>  [<ffffffff814025b7>] _spin_lock_nest_lock+0x41/0x50
>  [<ffffffff810be516>] ? mm_take_all_locks+0xd6/0x109
>  [<ffffffff810be516>] mm_take_all_locks+0xd6/0x109
>  [<ffffffff810d0f76>] do_mmu_notifier_register+0xd4/0x199
>  [<ffffffff810d1060>] mmu_notifier_register+0x13/0x15
>  [<ffffffffa0107f16>] kvm_dev_ioctl+0x13f/0x30e [kvm]
>  [<ffffffff810e6a3a>] vfs_ioctl+0x2f/0x7d
>  [<ffffffff810e6fb7>] do_vfs_ioctl+0x4af/0x4ec
>  [<ffffffff814030b4>] ? error_exit+0x94/0xb0
>  [<ffffffff81401f92>] ? trace_hardirqs_off_thunk+0x3a/0x3c
>  [<ffffffff8100bc2d>] ? retint_swapgs+0xe/0x13
>  [<ffffffff810e703b>] sys_ioctl+0x47/0x6a
>  [<ffffffff811d849c>] ? __up_read+0x1a/0x85
>  [<ffffffff8100b1db>] system_call_fastpath+0x16/0x1b
> 
> Since mm_take_all_locks takes a gazillion locks.
> 
> Is there any way around this other than completly shutting down lockdep?

When we created this the promise was that kvm would only do this on a
fresh mm with only a few vmas, has that changed?