Date: Thu, 20 Jun 2024 16:57:55 +0800
From: Jiwei Sun <sunjw10@...look.com>
To: Bjorn Helgaas <helgaas@...nel.org>
Cc: nirmal.patel@...ux.intel.com, jonathan.derrick@...ux.dev,
 lpieralisi@...nel.org, kw@...ux.com, robh@...nel.org, bhelgaas@...gle.com,
 linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org, sunjw10@...ovo.com,
 ahuang12@...ovo.com, Thomas Gleixner <tglx@...utronix.de>,
 Keith Busch <kbusch@...nel.org>
Subject: Re: [PATCH] PCI: vmd: Use raw spinlock for cfg_lock



On 6/20/24 04:00, Bjorn Helgaas wrote:
> [+cc Thomas in case he has msi_lock comment, Keith in case he has
> cfg_lock comment]
> 
> On Wed, Jun 19, 2024 at 07:27:59PM +0800, Jiwei Sun wrote:
>> From: Jiwei Sun <sunjw10@...ovo.com>
>>
>> If the kernel is built with the following configurations and booting
>>   CONFIG_VMD=y
>>   CONFIG_DEBUG_LOCKDEP=y
>>   CONFIG_DEBUG_SPINLOCK=y
>>   CONFIG_PROVE_LOCKING=y
>>   CONFIG_PROVE_RAW_LOCK_NESTING=y
>>
>> The following log appears,
>>
>> =============================
>> [ BUG: Invalid wait context ]
>> 6.10.0-rc4 #80 Not tainted
>> -----------------------------
>> kworker/18:2/633 is trying to lock:
>> ffff888c474e5648 (&vmd->cfg_lock){....}-{3:3}, at: vmd_pci_write+0x185/0x2a0
>> other info that might help us debug this:
>> context-{5:5}
>> 4 locks held by kworker/18:2/633:
>>  #0: ffff888100108958 ((wq_completion)events){+.+.}-{0:0}, at: process_one_work+0xf78/0x1920
>>  #1: ffffc9000ae1fd90 ((work_completion)(&wfc.work)){+.+.}-{0:0}, at: process_one_work+0x7fe/0x1920
>>  #2: ffff888c483508a8 (&md->mutex){+.+.}-{4:4}, at: __pci_enable_msi_range+0x208/0x800
>>  #3: ffff888c48329bd8 (&dev->msi_lock){....}-{2:2}, at: pci_msi_update_mask+0x91/0x170
>> stack backtrace:
>> CPU: 18 PID: 633 Comm: kworker/18:2 Not tainted 6.10.0-rc4 #80 7c0f2526417bfbb7579e3c3442683c5961773c75
>> Hardware name: Lenovo ThinkSystem SR630/-[7X01RCZ000]-, BIOS IVEL60O-2.71 09/28/2020
>> Workqueue: events work_for_cpu_fn
>> Call Trace:
>>  <TASK>
>>  dump_stack_lvl+0x7c/0xc0
>>  __lock_acquire+0x9e5/0x1ed0
>>  lock_acquire+0x194/0x490
>>  _raw_spin_lock_irqsave+0x42/0x90
>>  vmd_pci_write+0x185/0x2a0
>>  pci_msi_update_mask+0x10c/0x170
>>  __pci_enable_msi_range+0x291/0x800
>>  pci_alloc_irq_vectors_affinity+0x13e/0x1d0
>>  pcie_portdrv_probe+0x570/0xe60
>>  local_pci_probe+0xdc/0x190
>>  work_for_cpu_fn+0x4e/0xa0
>>  process_one_work+0x86d/0x1920
>>  process_scheduled_works+0xd7/0x140
>>  worker_thread+0x3e9/0xb90
>>  kthread+0x2e9/0x3d0
>>  ret_from_fork+0x2d/0x60
>>  ret_from_fork_asm+0x1a/0x30
>>  </TASK>
>>
>> The root cause is that the dev->msi_lock is a raw spinlock, but
>> vmd->cfg_lock is a spinlock.
> 
> Can you expand this a little bit?  This isn't enough unless one
> already knows the difference between raw_spinlock_t and spinlock_t,
> which I didn't.
> 
> Documentation/locking/locktypes.rst says they are the same except when
> CONFIG_PREEMPT_RT is set (might be worth mentioning with the config
> list above?), but that with CONFIG_PREEMPT_RT, spinlock_t is based on
> rt_mutex.
> 
> And I guess there's a rule that you can't acquire rt_mutex while
> holding a raw_spinlock.

Thanks for your review and comments. Sorry for not explaining this clearly.
Yes, you are right, if CONFIG_PREEMPT_RT is not set, the spinlock_t is 
based on raw_spinlock, there is no any question in the above call trace.

But as you mentioned, if CONFIG_PREEMPT_RT is set, the spinlock_t is based
on rt_mutex, a task will be scheduled when waiting for rt_mutex. For example, 
there are two threads are trying to hold a rt_mutex lock, if A hold the
lock firstly, and B will be scheduled in rtlock_slowlock_locked() waiting
for A to release the lock. The raw_spinlock is a real spinning lock, which
is not allowed the task of the raw_spinlock owner is scheduled in its
critical region. In other words, we should not try to acquire rt_mutex lock
in the critical region of the raw_spinlock when CONFIG_PREEMPT_RT is set.

CONFIG_PROVE_LOCKING and CONFIG_PROVE_RAW_LOCK_NESTING options are
used to detect the invalid lock nesting (the raw_spinlock vs. spinlock
nesting checks) [1]. Here is the call path:

  pci_msi_update_mask  ---> hold raw_spinlock dev->msi_lock
    pci_write_config_dword
     pci_bus_write_config_dword
       vmd_pci_write   ---> hold spinlock_t vmd->cfg_lock

The above call path is the invalid lock nesting becuase the vmd driver
tries to acquire the vmd->cfg_lock spinlock within the raw_spinlock
region (dev->msi_lock). That's why the message "BUG: Invalid wait contex"
is shown. 

[1] https://lore.kernel.org/lkml/YBBA81osV7cHN2fb@hirez.programming.kicks-ass.net/

Thanks,
Regards,
Jiwei

> 
> The dev->msi_lock was added by 77e89afc25f3 ("PCI/MSI: Protect
> msi_desc::masked for multi-MSI") and only used in
> pci_msi_update_mask():
> 
>   raw_spin_lock_irqsave(lock, flags);
>   desc->pci.msi_mask &= ~clear;
>   desc->pci.msi_mask |= set;
>   pci_write_config_dword(msi_desc_to_pci_dev(desc), desc->pci.mask_pos,
> 			 desc->pci.msi_mask);
>   raw_spin_unlock_irqrestore(lock, flags);
> 
> The vmd->cfg_lock was added by 185a383ada2e ("x86/PCI: Add driver for
> Intel Volume Management Device (VMD)") and is only used around VMD
> config accesses, e.g.,
> 
>   * CPU may deadlock if config space is not serialized on some versions of this
>   * hardware, so all config space access is done under a spinlock.
> 
>   static int vmd_pci_read(...)
>   {
>     spin_lock_irqsave(&vmd->cfg_lock, flags);
>     switch (len) {
>     case 1:
> 	    *value = readb(addr);
> 	    break;
>     case 2:
> 	    *value = readw(addr);
> 	    break;
>     case 4:
> 	    *value = readl(addr);
> 	    break;
>     default:
> 	    ret = -EINVAL;
> 	    break;
>     }
>     spin_unlock_irqrestore(&vmd->cfg_lock, flags);
>   }
> 
> IIUC those reads turn into single PCIe MMIO reads, so I wouldn't
> expect any concurrency issues there that need locking.
> 
> But apparently there's something weird that can deadlock the CPU.
> 
>> Signed-off-by: Jiwei Sun<sunjw10@...ovo.com>
>> Suggested-by: Adrian Huang <ahuang12@...ovo.com>
>> ---
>>  drivers/pci/controller/vmd.c | 12 ++++++------
>>  1 file changed, 6 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/pci/controller/vmd.c b/drivers/pci/controller/vmd.c
>> index 87b7856f375a..45d0ebf96adc 100644
>> --- a/drivers/pci/controller/vmd.c
>> +++ b/drivers/pci/controller/vmd.c
>> @@ -125,7 +125,7 @@ struct vmd_irq_list {
>>  struct vmd_dev {
>>  	struct pci_dev		*dev;
>>  
>> -	spinlock_t		cfg_lock;
>> +	raw_spinlock_t		cfg_lock;
>>  	void __iomem		*cfgbar;
>>  
>>  	int msix_count;
>> @@ -402,7 +402,7 @@ static int vmd_pci_read(struct pci_bus *bus, unsigned int devfn, int reg,
>>  	if (!addr)
>>  		return -EFAULT;
>>  
>> -	spin_lock_irqsave(&vmd->cfg_lock, flags);
>> +	raw_spin_lock_irqsave(&vmd->cfg_lock, flags);
>>  	switch (len) {
>>  	case 1:
>>  		*value = readb(addr);
>> @@ -417,7 +417,7 @@ static int vmd_pci_read(struct pci_bus *bus, unsigned int devfn, int reg,
>>  		ret = -EINVAL;
>>  		break;
>>  	}
>> -	spin_unlock_irqrestore(&vmd->cfg_lock, flags);
>> +	raw_spin_unlock_irqrestore(&vmd->cfg_lock, flags);
>>  	return ret;
>>  }
>>  
>> @@ -437,7 +437,7 @@ static int vmd_pci_write(struct pci_bus *bus, unsigned int devfn, int reg,
>>  	if (!addr)
>>  		return -EFAULT;
>>  
>> -	spin_lock_irqsave(&vmd->cfg_lock, flags);
>> +	raw_spin_lock_irqsave(&vmd->cfg_lock, flags);
>>  	switch (len) {
>>  	case 1:
>>  		writeb(value, addr);
>> @@ -455,7 +455,7 @@ static int vmd_pci_write(struct pci_bus *bus, unsigned int devfn, int reg,
>>  		ret = -EINVAL;
>>  		break;
>>  	}
>> -	spin_unlock_irqrestore(&vmd->cfg_lock, flags);
>> +	raw_spin_unlock_irqrestore(&vmd->cfg_lock, flags);
>>  	return ret;
>>  }
>>  
>> @@ -1015,7 +1015,7 @@ static int vmd_probe(struct pci_dev *dev, const struct pci_device_id *id)
>>  	if (features & VMD_FEAT_OFFSET_FIRST_VECTOR)
>>  		vmd->first_vec = 1;
>>  
>> -	spin_lock_init(&vmd->cfg_lock);
>> +	raw_spin_lock_init(&vmd->cfg_lock);
>>  	pci_set_drvdata(dev, vmd);
>>  	err = vmd_enable_domain(vmd, features);
>>  	if (err)
>> -- 
>> 2.27.0
>>

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