si_meminfo() calls nr_blockdev_pages() that grabs bdev_lock, but it is not safe to grab the lock from the hardirq context (the lock is never taken with an _irqsave variant in block_dev.c). When taken from an inappropriate context it easily causes the following deadlock:

- - - - ================================= [ INFO: inconsistent lock state ] 3.2.0+ #1 --------------------------------- inconsistent {HARDIRQ-ON-W} -> {IN-HARDIRQ-W} usage. swapper/0/0 [HC1[1]:SC0[0]:HE0:SE1] takes: (bdev_lock){?.+...}, at: [<ffffffff810f1017>] nr_blockdev_pages+0x17/0x70 {HARDIRQ-ON-W} state was registered at: [<ffffffff81061b20>] mark_irqflags+0x140/0x1b0 [<ffffffff81062f03>] __lock_acquire+0x4c3/0x9c0 [<ffffffff810639c6>] lock_acquire+0x96/0xc0 [<ffffffff8131c58c>] _raw_spin_lock+0x2c/0x40 [<ffffffff810f1017>] nr_blockdev_pages+0x17/0x70 [<ffffffff81089ba8>] si_meminfo+0x38/0x60 [<ffffffff81675493>] eventpoll_init+0x11/0xa1 [<ffffffff8165eb40>] do_one_initcall+0x7a/0x12e [<ffffffff8165ec8e>] kernel_init+0x9a/0x114 [<ffffffff8131e934>] kernel_thread_helper+0x4/0x10 irq event stamp: 135250 hardirqs last enabled at (135247): [<ffffffff81009897>] default_idle+0x27/0x50 hardirqs last disabled at (135248): [<ffffffff8131e1ab>] apic_timer_interrupt+0x6b/0x80 softirqs last enabled at (135250): [<ffffffff8103814e>] _local_bh_enable+0xe/0x10 softirqs last disabled at (135249): [<ffffffff81038665>] irq_enter+0x65/0x80

other info that might help us debug this: Possible unsafe locking scenario:

CPU0 ---- lock(bdev_lock); <Interrupt> lock(bdev_lock);

*** DEADLOCK ***

no locks held by swapper/0/0. - - - -

The patch fixes the issue by using totalram_pages instead of si_meminfo().

p.s. Note that VMEVENT_EATTR_NR_SWAP_PAGES type calls si_swapinfo(), which has a very similar problem. But there is no easy way to fix it.

Do we have any use case for the VMEVENT_EATTR_NR_SWAP_PAGES event? If not, I'd vote for removing it and thus keeping things simple.

Otherwise we would have two options:

1. Modify swap accounting for vmevent (either start grabbing _irqsave variant of swapfile.c's swap_lock, or try to make the accounting atomic); 2. Start using kthreads for vmevent_sample().

Signed-off-by: Anton Vorontsov anton.vorontsov@linaro.org ---

The patch is for git://github.com/penberg/linux.git vmevent/core.

mm/vmevent.c | 6 +++--- 1 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/mm/vmevent.c b/mm/vmevent.c index 2342752..1375f9d 100644 --- a/mm/vmevent.c +++ b/mm/vmevent.c @@ -51,18 +51,18 @@ static bool vmevent_match(struct vmevent_watch *watch, static void vmevent_sample(struct vmevent_watch *watch) { struct vmevent_watch_event event; - struct sysinfo si; int n = 0;

memset(&event, 0, sizeof(event));

event.nr_free_pages = global_page_state(NR_FREE_PAGES);

- si_meminfo(&si); - event.nr_avail_pages = si.totalram; + event.nr_avail_pages = totalram_pages;

#ifdef CONFIG_SWAP if (watch->config.event_attrs & VMEVENT_EATTR_NR_SWAP_PAGES) { + struct sysinfo si; + si_swapinfo(&si); event.nr_swap_pages = si.totalswap; }