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

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Date:   Thu, 14 Sep 2023 16:38:37 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Brett Creeley <brett.creeley@....com>
Cc:     <jgg@...pe.ca>, <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, 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>
> 
> 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,

Alex

> Reported-by: Dan Carpenter <dan.carpenter@...aro.org>
> Closes: https://lore.kernel.org/kvm/1f9bc27b-3de9-4891-9687-ba2820c1b390@moroto.mountain/
> Signed-off-by: Brett Creeley <brett.creeley@....com>
> Reviewed-by: Shannon Nelson <shannon.nelson@....com>
> ---
>  drivers/vfio/pci/pds/vfio_dev.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/drivers/vfio/pci/pds/vfio_dev.c b/drivers/vfio/pci/pds/vfio_dev.c
> index 9db5f2c8f1ea..6e664cb05dd1 100644
> --- a/drivers/vfio/pci/pds/vfio_dev.c
> +++ b/drivers/vfio/pci/pds/vfio_dev.c
> @@ -33,8 +33,10 @@ void pds_vfio_state_mutex_unlock(struct pds_vfio_pci_device *pds_vfio)
>  	if (pds_vfio->deferred_reset) {
>  		pds_vfio->deferred_reset = false;
>  		if (pds_vfio->state == VFIO_DEVICE_STATE_ERROR) {
> +			spin_unlock(&pds_vfio->reset_lock);
>  			pds_vfio_put_restore_file(pds_vfio);
>  			pds_vfio_put_save_file(pds_vfio);
> +			spin_lock(&pds_vfio->reset_lock);
>  			pds_vfio_dirty_disable(pds_vfio, false);
>  		}
>  		pds_vfio->state = pds_vfio->deferred_reset_state;