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Date: Thu, 26 Jan 2012 21:32:22 +0530
From: "Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To: Borislav Petkov <bp@...64.org>
CC: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
tony.luck@...el.com, tglx@...utronix.de, mingo@...e.hu,
hpa@...or.com, x86@...nel.org, linux-edac@...r.kernel.org,
linux-kernel@...r.kernel.org, marcos.mage@...il.com,
prasad@...ux.vnet.ibm.com
Subject: Re: [PATCH] x86, mce: Fix rcu splat in drain_mce_log_buffer()
On 01/14/2012 03:14 AM, Paul E. McKenney wrote:
> On Fri, Jan 13, 2012 at 05:58:03PM +0530, Srivatsa S. Bhat wrote:
>> On 01/11/2012 09:41 PM, Paul E. McKenney wrote:
>>
>>> On Wed, Jan 11, 2012 at 07:54:48PM +0530, Srivatsa S. Bhat wrote:
>>>> While booting, the following message is seen:
>>>
>>> Hmmm... This code is interesting. It looks to me that access to
>>> each entry is gated by the ->finished field. If so, there should be
>>> no need for any kind of rcu_dereference() on mcelog.next -- ACCESS_ONCE()
>>> should work just fine.
>>>
>>
>> [snip]
>>
>>>>
>>>> - next = rcu_dereference_check_mce(mcelog.next);
>>>> + next = rcu_access_index(mcelog.next);
>>>
>>> So this is not so much a use of RCU as it is a standard mailbox algorithm,
>>> with the ->finished flag controlling access. So I suggest ACCESS_ONCE()
>>> for the accesses to mcelog.next.
>>>
>>
>>
>> Hi Paul,
>> Thank you for the suggestion. Here is an updated patch:
>
> Looks good to me, but I do need to defer to people who know this code
> better than do I. The key thing that (from what I can see) makes
> rcu_dereference() unnecessary is that the smp_rmb() used in conjunction
> with polling the .finished field takes care of ordering.
>
> Note that this also relies on the fact that mcelog is statically
> allocated. If it was dynamically allocated, then the code would need
> to wait a grace period between allocating it an initializing it.
>
> Thanx, Paul
>
Ping?
Regards,
Srivatsa S. Bhat
>> ------
>> From: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
>> Subject: [PATCH v2] x86, mce: Fix rcu splat in drain_mce_log_buffer()
>>
>> While booting, the following message is seen:
>>
>> [ 21.665087] ===============================
>> [ 21.669439] [ INFO: suspicious RCU usage. ]
>> [ 21.673798] 3.2.0-0.0.0.28.36b5ec9-default #2 Not tainted
>> [ 21.681353] -------------------------------
>> [ 21.685864] arch/x86/kernel/cpu/mcheck/mce.c:194 suspicious rcu_dereference_index_check() usage!
>> [ 21.695013]
>> [ 21.695014] other info that might help us debug this:
>> [ 21.695016]
>> [ 21.703488]
>> [ 21.703489] rcu_scheduler_active = 1, debug_locks = 1
>> [ 21.710426] 3 locks held by modprobe/2139:
>> [ 21.714754] #0: (&__lockdep_no_validate__){......}, at: [<ffffffff8133afd3>] __driver_attach+0x53/0xa0
>> [ 21.725020] #1:
>> [ 21.725323] ioatdma: Intel(R) QuickData Technology Driver 4.00
>> [ 21.733206] (&__lockdep_no_validate__){......}, at: [<ffffffff8133afe1>] __driver_attach+0x61/0xa0
>> [ 21.743015] #2: (i7core_edac_lock){+.+.+.}, at: [<ffffffffa01cfa5f>] i7core_probe+0x1f/0x5c0 [i7core_edac]
>> [ 21.753708]
>> [ 21.753709] stack backtrace:
>> [ 21.758429] Pid: 2139, comm: modprobe Not tainted 3.2.0-0.0.0.28.36b5ec9-default #2
>> [ 21.768253] Call Trace:
>> [ 21.770838] [<ffffffff810977cd>] lockdep_rcu_suspicious+0xcd/0x100
>> [ 21.777366] [<ffffffff8101aa41>] drain_mcelog_buffer+0x191/0x1b0
>> [ 21.783715] [<ffffffff8101aa78>] mce_register_decode_chain+0x18/0x20
>> [ 21.790430] [<ffffffffa01cf8db>] i7core_register_mci+0x2fb/0x3e4 [i7core_edac]
>> [ 21.798003] [<ffffffffa01cfb14>] i7core_probe+0xd4/0x5c0 [i7core_edac]
>> [ 21.804809] [<ffffffff8129566b>] local_pci_probe+0x5b/0xe0
>> [ 21.810631] [<ffffffff812957c9>] __pci_device_probe+0xd9/0xe0
>> [ 21.816650] [<ffffffff813362e4>] ? get_device+0x14/0x20
>> [ 21.822178] [<ffffffff81296916>] pci_device_probe+0x36/0x60
>> [ 21.828061] [<ffffffff8133ac8a>] really_probe+0x7a/0x2b0
>> [ 21.833676] [<ffffffff8133af23>] driver_probe_device+0x63/0xc0
>> [ 21.839868] [<ffffffff8133b01b>] __driver_attach+0x9b/0xa0
>> [ 21.845718] [<ffffffff8133af80>] ? driver_probe_device+0xc0/0xc0
>> [ 21.852027] [<ffffffff81339168>] bus_for_each_dev+0x68/0x90
>> [ 21.857876] [<ffffffff8133aa3c>] driver_attach+0x1c/0x20
>> [ 21.863462] [<ffffffff8133a64d>] bus_add_driver+0x16d/0x2b0
>> [ 21.869377] [<ffffffff8133b6dc>] driver_register+0x7c/0x160
>> [ 21.875220] [<ffffffff81296bda>] __pci_register_driver+0x6a/0xf0
>> [ 21.881494] [<ffffffffa01fe000>] ? 0xffffffffa01fdfff
>> [ 21.886846] [<ffffffffa01fe047>] i7core_init+0x47/0x1000 [i7core_edac]
>> [ 21.893737] [<ffffffff810001ce>] do_one_initcall+0x3e/0x180
>> [ 21.899670] [<ffffffff810a9b95>] sys_init_module+0xc5/0x220
>> [ 21.905542] [<ffffffff8149bc39>] system_call_fastpath+0x16/0x1b
>>
>> Fix this by using ACCESS_ONCE() instead of rcu_dereference_check_mce()
>> over mcelog.next. Since the access to each entry is controlled by the
>> ->finished field, ACCESS_ONCE() should work just fine. An rcu_dereference
>> is unnecessary here.
>>
>> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@...ux.vnet.ibm.com>
>> ---
>>
>> arch/x86/kernel/cpu/mcheck/mce.c | 2 +-
>> 1 files changed, 1 insertions(+), 1 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/mcheck/mce.c b/arch/x86/kernel/cpu/mcheck/mce.c
>> index f22a9f7..f525f99 100644
>> --- a/arch/x86/kernel/cpu/mcheck/mce.c
>> +++ b/arch/x86/kernel/cpu/mcheck/mce.c
>> @@ -191,7 +191,7 @@ static void drain_mcelog_buffer(void)
>> {
>> unsigned int next, i, prev = 0;
>>
>> - next = rcu_dereference_check_mce(mcelog.next);
>> + next = ACCESS_ONCE(mcelog.next);
>>
>> do {
>> struct mce *m;
>>
>>
--
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