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From: Waiman Long <waiman.long@hp.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com,
	paolo.bonzini@gmail.com, konrad.wilk@oracle.com,
	boris.ostrovsky@oracle.com, paulmck@linux.vnet.ibm.com,
	riel@redhat.com, torvalds@linux-foundation.org,
	raghavendra.kt@linux.vnet.ibm.com, david.vrabel@citrix.com,
	oleg@redhat.com, scott.norton@hp.com, doug.hatch@hp.com,
	linux-arch@vger.kernel.org, x86@kernel.org,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	xen-devel@lists.xenproject.org, kvm@vger.kernel.org,
	luto@amacapital.net
Subject: Re: [PATCH 9/9] qspinlock,x86,kvm: Implement KVM support for paravirt qspinlock
Date: Thu, 19 Mar 2015 17:08:31 -0400	[thread overview]
Message-ID: <550B3ACF.4050908@hp.com> (raw)
In-Reply-To: <20150319100121.GL21418@twins.programming.kicks-ass.net>

On 03/19/2015 06:01 AM, Peter Zijlstra wrote:
> On Wed, Mar 18, 2015 at 10:45:55PM -0400, Waiman Long wrote:
>> On 03/16/2015 09:16 AM, Peter Zijlstra wrote:
>> I do have some concern about this call site patching mechanism as the
>> modification is not atomic. The spin_unlock() calls are in many places in
>> the kernel. There is a possibility that a thread is calling a certain
>> spin_unlock call site while it is being patched by another one with the
>> alternative() function call.
>>
>> So far, I don't see any problem with bare metal where paravirt_patch_insns()
>> is used to patch it to the move instruction. However, in a virtual guest
>> enivornment where paravirt_patch_call() was used, there were situations
>> where the system panic because of page fault on some invalid memory in the
>> kthread. If you look at the paravirt_patch_call(), you will see:
>>
>>      :
>> b->opcode = 0xe8; /* call */
>> b->delta = delta;
>>
>> If another CPU reads the instruction at the call site at the right moment,
>> it will get the modified call instruction, but not the new delta value. It
>> will then jump to a random location. I believe that was causing the system
>> panic that I saw.
>>
>> So I think it is kind of risky to use it here unless we can guarantee that
>> call site patching is atomic wrt other CPUs.
> Just look at where the patching is done:
>
> init/main.c:start_kernel()
>    check_bugs()
>      alternative_instructions()
>        apply_paravirt()
>
> We're UP and not holding any locks, disable IRQs (see text_poke_early())
> and have NMIs 'disabled'.

You are probably right. The initial apply_paravirt() was done before the 
SMP boot. Subsequent ones were at kernel module load time. I put a 
counter in the __native_queue_spin_unlock() and it registered 26949 
unlock calls in a 16-cpu guest before it got patched out.

The panic that I observed before might be due to some coding error of my 
own.

-Longman

  reply	other threads:[~2015-03-19 21:08 UTC|newest]

Thread overview: 43+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-03-16 13:16 [PATCH 0/9] qspinlock stuff -v15 Peter Zijlstra
2015-03-16 13:16 ` [PATCH 1/9] qspinlock: A simple generic 4-byte queue spinlock Peter Zijlstra
2015-03-16 13:16 ` [PATCH 2/9] qspinlock, x86: Enable x86-64 to use " Peter Zijlstra
2015-03-16 13:16 ` [PATCH 3/9] qspinlock: Add pending bit Peter Zijlstra
2015-03-16 13:16 ` [PATCH 4/9] qspinlock: Extract out code snippets for the next patch Peter Zijlstra
2015-03-16 13:16 ` [PATCH 5/9] qspinlock: Optimize for smaller NR_CPUS Peter Zijlstra
2015-03-16 13:16 ` [PATCH 6/9] qspinlock: Use a simple write to grab the lock Peter Zijlstra
2015-03-16 13:16 ` [PATCH 7/9] qspinlock: Revert to test-and-set on hypervisors Peter Zijlstra
2015-03-16 13:16 ` [PATCH 8/9] qspinlock: Generic paravirt support Peter Zijlstra
     [not found]   ` <5509E51D.7040909@hp.com>
2015-03-19 10:12     ` Peter Zijlstra
2015-03-19 12:25       ` Peter Zijlstra
2015-03-19 13:43         ` Peter Zijlstra
2015-03-19 23:25         ` Waiman Long
2015-04-01 16:20         ` Waiman Long
2015-04-01 17:12           ` Peter Zijlstra
2015-04-01 17:42             ` Peter Zijlstra
2015-04-01 18:17               ` Peter Zijlstra
2015-04-01 18:54                 ` Waiman Long
2015-04-01 18:48                   ` Peter Zijlstra
2015-04-01 19:58                     ` Waiman Long
2015-04-01 21:03                       ` Peter Zijlstra
2015-04-02 16:28                         ` Waiman Long
2015-04-02 17:20                           ` Peter Zijlstra
2015-04-02 19:48                             ` Peter Zijlstra
2015-04-03  3:39                               ` Waiman Long
2015-04-03 13:43                               ` Peter Zijlstra
2015-04-01 20:10             ` Waiman Long
2015-03-16 13:16 ` [PATCH 9/9] qspinlock,x86,kvm: Implement KVM support for paravirt qspinlock Peter Zijlstra
     [not found]   ` <550A3863.2060808@hp.com>
2015-03-19 10:01     ` Peter Zijlstra
2015-03-19 21:08       ` Waiman Long [this message]
2015-03-20  7:43         ` Raghavendra K T
2015-03-16 14:08 ` [Xen-devel] [PATCH 0/9] qspinlock stuff -v15 David Vrabel
2015-03-18 20:36 ` Waiman Long
2015-03-19 18:01 ` [Xen-devel] " David Vrabel
2015-03-19 18:32   ` Peter Zijlstra
2015-03-25 19:47 ` Konrad Rzeszutek Wilk
2015-03-26 20:21   ` Peter Zijlstra
2015-03-27 14:07     ` Konrad Rzeszutek Wilk
2015-03-30 16:41       ` Waiman Long
2015-03-30 16:25   ` Waiman Long
2015-03-30 16:29     ` Peter Zijlstra
2015-03-30 16:43       ` Waiman Long
2015-03-27  6:40 ` Raghavendra K T

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