Re: [PATCH] x86, mce: Fix rcu splat in drain_mce_log_buffer()
From: Paul E. McKenney
Date: Wed Jan 11 2012 - 11:50:57 EST
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.
> [ 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 rcu_access_index() instead of rcu_dereference_check_mce().
> This is similar to what was done in commit a4dd992 (rcu: create new
> rcu_access_index() and use in mce) for a similar case in mce_poll.
> Link to that discussion: http://thread.gmane.org/gmane.linux.kernel/1058460/
>
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> ---
> I am no MCE expert. Hence requesting a thorough review of the patch.
>
> 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..c191fec 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 = 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.
Thanx, Paul
> do {
> struct mce *m;
>
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