Re: [PATCH] PCI: vmd: Use raw spinlock for cfg_lock
From: Paul M Stillwell Jr
Date: Thu Jun 20 2024 - 11:11:20 EST
On 6/20/2024 1:57 AM, Jiwei Sun wrote:
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@xxxxxxxxxx>
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.
It looks like this only happens when CONFIG_PREEMPT_RT is set so I would
mention that in the commit message (as Bjorn mentioned). I also think
thsi level of detail is helpful and should be in the commit message as
well since it's not obvious to the casual observer :)
Paul
[1] https://lore.kernel.org/lkml/YBBA81osV7cHN2fb@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
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@xxxxxxxxxx>
Suggested-by: Adrian Huang <ahuang12@xxxxxxxxxx>
---
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