linux-kernel - Re: [PATCH vfio 3/3] pds/vfio: Fix possible sleep while in atomic context

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Message-ID: <20230919185938.GU13795@ziepe.ca>
Date:   Tue, 19 Sep 2023 15:59:38 -0300
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     Brett Creeley <brett.creeley@....com>, yishaih@...dia.com,
        shameerali.kolothum.thodi@...wei.com, kevin.tian@...el.com,
        dan.carpenter@...aro.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, shannon.nelson@....com
Subject: Re: [PATCH vfio 3/3] pds/vfio: Fix possible sleep while in atomic
 context

On Thu, Sep 14, 2023 at 04:38:37PM -0600, Alex Williamson wrote:
> On Thu, 14 Sep 2023 12:15:40 -0700
> Brett Creeley <brett.creeley@....com> wrote:
> 
> > The driver could possibly sleep while in atomic context resulting
> > in the following call trace while CONFIG_DEBUG_ATOMIC_SLEEP=y is
> > set:
> > 
> > [  227.229806] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:283
> > [  227.229818] in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 2817, name: bash
> > [  227.229824] preempt_count: 1, expected: 0
> > [  227.229827] RCU nest depth: 0, expected: 0
> > [  227.229832] CPU: 5 PID: 2817 Comm: bash Tainted: G S         OE      6.6.0-rc1-next-20230911 #1
> > [  227.229839] Hardware name: HPE ProLiant DL360 Gen10/ProLiant DL360 Gen10, BIOS U32 01/23/2021
> > [  227.229843] Call Trace:
> > [  227.229848]  <TASK>
> > [  227.229853]  dump_stack_lvl+0x36/0x50
> > [  227.229865]  __might_resched+0x123/0x170
> > [  227.229877]  mutex_lock+0x1e/0x50
> > [  227.229891]  pds_vfio_put_lm_file+0x1e/0xa0 [pds_vfio_pci]
> > [  227.229909]  pds_vfio_put_save_file+0x19/0x30 [pds_vfio_pci]
> > [  227.229923]  pds_vfio_state_mutex_unlock+0x2e/0x80 [pds_vfio_pci]
> > [  227.229937]  pci_reset_function+0x4b/0x70
> > [  227.229948]  reset_store+0x5b/0xa0
> > [  227.229959]  kernfs_fop_write_iter+0x137/0x1d0
> > [  227.229972]  vfs_write+0x2de/0x410
> > [  227.229986]  ksys_write+0x5d/0xd0
> > [  227.229996]  do_syscall_64+0x3b/0x90
> > [  227.230004]  entry_SYSCALL_64_after_hwframe+0x6e/0xd8
> > [  227.230017] RIP: 0033:0x7fb202b1fa28
> > [  227.230023] Code: 89 02 48 c7 c0 ff ff ff ff eb b3 0f 1f 80 00 00 00 00 f3 0f 1e fa 48 8d 05 15 4d 2a 00 8b 00 85 c0 75 17 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 58 c3 0f 1f 80 00 00 00 00 41 54 49 89 d4 55
> > [  227.230028] RSP: 002b:00007fff6915fbd8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
> > [  227.230036] RAX: ffffffffffffffda RBX: 0000000000000002 RCX: 00007fb202b1fa28
> > [  227.230040] RDX: 0000000000000002 RSI: 000055f3834d5aa0 RDI: 0000000000000001
> > [  227.230044] RBP: 000055f3834d5aa0 R08: 000000000000000a R09: 00007fb202b7fae0
> > [  227.230047] R10: 000000000000000a R11: 0000000000000246 R12: 00007fb202dc06e0
> > [  227.230050] R13: 0000000000000002 R14: 00007fb202dbb860 R15: 0000000000000002
> > [  227.230056]  </TASK>

I usually encourage people to trim the oops, remove the time stamp at least.
> > 
> > This can happen if pds_vfio_put_restore_file() and/or
> > pds_vfio_put_save_file() grab the mutex_lock(&lm_file->lock)
> > while the spin_lock(&pds_vfio->reset_lock) is held, which can
> > happen during while calling pds_vfio_state_mutex_unlock().
> > 
> > Fix this by releasing the spin_unlock(&pds_vfio->reset_lock) before
> > calling pds_vfio_put_restore_file() and pds_vfio_put_save_file() and
> > re-acquiring spin_lock(&pds_vfio->reset_lock) after the previously
> > mentioned functions are called to protect setting the subsequent
> > state/deferred reset settings.
> > 
> > The only possible concerns are other threads that may call
> > pds_vfio_put_restore_file() and/or pds_vfio_put_save_file(). However,
> > those paths are already protected by the state mutex_lock().
> 
> Is there another viable solution to change reset_lock to a mutex?
> 
> I think this is the origin of this algorithm:
> 
> https://lore.kernel.org/all/20211019191025.GA4072278@nvidia.com/
> 
> But it's not clear to me why Jason chose an example with a spinlock and
> if some subtlety here requires it.  Thanks,

I think there was no specific reason it must be a spinlock

Certainly I'm not feeling comfortable just unlocking and relocking
like that. It would need a big explanation why it is safe in a
comment.

Jason