Re: [PATCH v2] hugetlbfs: Disable softIRQ when taking hugetlb_lock

[Mike Kravetz]

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.
-- 
Mike Kravetz