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SubjectRe: [PATCH v2] hugetlbfs: Disable softIRQ when taking hugetlb_lock
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On 12/11/19 2:19 PM, Waiman Long wrote:
> On 12/11/19 5:04 PM, Mike Kravetz wrote:
>> Cc: Michal
>>
>> Sorry for the late reply on this effort.
>>
>> On 12/11/19 11:46 AM, Waiman Long wrote:
>>> The following lockdep splat was observed when a certain hugetlbfs test
>>> was run:
>>>
>>> [ 612.388273] ================================
>>> [ 612.411273] WARNING: inconsistent lock state
>>> [ 612.432273] 4.18.0-159.el8.x86_64+debug #1 Tainted: G W --------- - -
>>> [ 612.469273] --------------------------------
>>> [ 612.489273] inconsistent {SOFTIRQ-ON-W} -> {IN-SOFTIRQ-W} usage.
>>> [ 612.517273] swapper/30/0 [HC0[0]:SC1[1]:HE1:SE0] takes:
>>> [ 612.541273] ffffffff9acdc038 (hugetlb_lock){+.?.}, at: free_huge_page+0x36f/0xaa0
>>> [ 612.576273] {SOFTIRQ-ON-W} state was registered at:
>>> [ 612.598273] lock_acquire+0x14f/0x3b0
>>> [ 612.616273] _raw_spin_lock+0x30/0x70
>>> [ 612.634273] __nr_hugepages_store_common+0x11b/0xb30
>>> [ 612.657273] hugetlb_sysctl_handler_common+0x209/0x2d0
>>> [ 612.681273] proc_sys_call_handler+0x37f/0x450
>>> [ 612.703273] vfs_write+0x157/0x460
>>> [ 612.719273] ksys_write+0xb8/0x170
>>> [ 612.736273] do_syscall_64+0xa5/0x4d0
>>> [ 612.753273] entry_SYSCALL_64_after_hwframe+0x6a/0xdf
>>> [ 612.777273] irq event stamp: 691296
>>> [ 612.794273] hardirqs last enabled at (691296): [<ffffffff99bb034b>] _raw_spin_unlock_irqrestore+0x4b/0x60
>>> [ 612.839273] hardirqs last disabled at (691295): [<ffffffff99bb0ad2>] _raw_spin_lock_irqsave+0x22/0x81
>>> [ 612.882273] softirqs last enabled at (691284): [<ffffffff97ff0c63>] irq_enter+0xc3/0xe0
>>> [ 612.922273] softirqs last disabled at (691285): [<ffffffff97ff0ebe>] irq_exit+0x23e/0x2b0
>>> [ 612.962273]
>>> [ 612.962273] other info that might help us debug this:
>>> [ 612.993273] Possible unsafe locking scenario:
>>> [ 612.993273]
>>> [ 613.020273] CPU0
>>> [ 613.031273] ----
>>> [ 613.042273] lock(hugetlb_lock);
>>> [ 613.057273] <Interrupt>
>>> [ 613.069273] lock(hugetlb_lock);
>>> [ 613.085273]
>>> [ 613.085273] *** DEADLOCK ***
>>> :
>>> [ 613.245273] Call Trace:
>>> [ 613.256273] <IRQ>
>>> [ 613.265273] dump_stack+0x9a/0xf0
>>> [ 613.281273] mark_lock+0xd0c/0x12f0
>>> [ 613.297273] ? print_shortest_lock_dependencies+0x80/0x80
>>> [ 613.322273] ? sched_clock_cpu+0x18/0x1e0
>>> [ 613.341273] __lock_acquire+0x146b/0x48c0
>>> [ 613.360273] ? trace_hardirqs_on+0x10/0x10
>>> [ 613.379273] ? trace_hardirqs_on_caller+0x27b/0x580
>>> [ 613.401273] lock_acquire+0x14f/0x3b0
>>> [ 613.419273] ? free_huge_page+0x36f/0xaa0
>>> [ 613.440273] _raw_spin_lock+0x30/0x70
>>> [ 613.458273] ? free_huge_page+0x36f/0xaa0
>>> [ 613.477273] free_huge_page+0x36f/0xaa0
>>> [ 613.495273] bio_check_pages_dirty+0x2fc/0x5c0
>>> [ 613.516273] clone_endio+0x17f/0x670 [dm_mod]
>>> [ 613.536273] ? disable_discard+0x90/0x90 [dm_mod]
>>> [ 613.558273] ? bio_endio+0x4ba/0x930
>>> [ 613.575273] ? blk_account_io_completion+0x400/0x530
>>> [ 613.598273] blk_update_request+0x276/0xe50
>>> [ 613.617273] scsi_end_request+0x7b/0x6a0
>>> [ 613.636273] ? lock_downgrade+0x6f0/0x6f0
>>> [ 613.654273] scsi_io_completion+0x1c6/0x1570
>>> [ 613.674273] ? sd_completed_bytes+0x3a0/0x3a0 [sd_mod]
>>> [ 613.698273] ? scsi_mq_requeue_cmd+0xc0/0xc0
>>> [ 613.718273] blk_done_softirq+0x22e/0x350
>>> [ 613.737273] ? blk_softirq_cpu_dead+0x230/0x230
>>> [ 613.758273] __do_softirq+0x23d/0xad8
>>> [ 613.776273] irq_exit+0x23e/0x2b0
>>> [ 613.792273] do_IRQ+0x11a/0x200
>>> [ 613.806273] common_interrupt+0xf/0xf
>>> [ 613.823273] </IRQ>
>> This is interesting. I'm trying to wrap my head around how we ended up
>> with a BIO pointing to a hugetlbfs page. My 'guess' is that user space
>> code passed an address to some system call or driver. And, that system
>> call or driver set up the IO. For the purpose of addressing this issue,
>> it does not matter. I am just a little confused/curious.
>>
>>> Since hugetlb_lock can be taken from both process and softIRQ contexts,
>>> we need to protect the lock from nested locking by disabling softIRQ
>>> using spin_lock_bh() before taking it.
>>>
>>> Currently, only free_huge_page() is known to be called from softIRQ
>>> context.
>> We discussed this exact same issue more than a year ago. See,
>> https://lkml.org/lkml/2018/9/5/398
>>
>> At that time, the only 'known' caller of put_page for a hugetlbfs page from
>> softirq context was in powerpc specific code. IIRC, Aneesh addressed the
>> issue last year by modifying the powerpc specific code. The more general
>> issue in the hugetlbfs code was never addressed. :(
>>
>> As part of the discussion in the previous e-mail thread, the issue of
>> whether we should address put_page for hugetlbfs pages for only softirq
>> or extend to hardirq context was discussed. The conclusion (or at least
>> suggestion from Andrew and Michal) was that we should modify code to allow
>> for calls from hardirq context. The reasoning IIRC, was that put_page of
>> other pages was allowed from hardirq context, so hugetlbfs pages should be
>> no different.
>>
>> Matthew, do you think that reasoning from last year is still valid? Should
>> we be targeting soft or hard irq calls?
>>
>> One other thing. free_huge_page may also take a subpool specific lock via
>> spin_lock(). See hugepage_subpool_put_pages. This would also need to take
>> irq context into account.
>
> Thanks for the background information.
>
> We will need to use spin_lock_irq() or spin_lock_irqsave() for allowing
> hardirq context calls like what is in the v1 patch. I will look further
> into the subpool specific lock also.

Sorry,

I did not fully read all of Matthew's comments/suggestions on the original
patch. His initial suggestion was for a workqueue approach that you did
start implementing, but thought was too complex. Andi also suggested this
approach.

The workqueue approach would address both soft and hard irq context issues.
As a result, I too think this is the approach we should explore. Since there
is more than one lock involved, this also is reason for a work queue approach.

I'll take a look at initial workqueue implementation. However, I have not
dealt with workqueues in some time so it may take few days to evaluate.
--
Mike Kravetz

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