On Sat, 3 Mar 2012, Anton Vorontsov wrote:
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:
- Modify swap accounting for vmevent (either start grabbing _irqsave variant of swapfile.c's swap_lock, or try to make the accounting atomic);
- Start using kthreads for vmevent_sample().
Signed-off-by: Anton Vorontsov anton.vorontsov@linaro.org
Applied, thanks!