Hi Kelvin,
I have noticed lockdep_rcu complaining about suspicious rcu_dereference inside: switchtec_dma_chans_enumerate and switchtec_dma_chan_free functions
In the switchtec_dma_chans_enumerate:
```
static int switchtec_dma_chans_enumerate(struct switchtec_dma_dev *swdma_dev,
int chan_cnt)
{
struct dma_device *dma = &swdma_dev->dma_dev;
struct pci_dev *pdev = rcu_dereference(swdma_dev->pdev); <<< suspicious RCU usage
int base;
int cnt;
int rc;
int i;
```
And inside switchtec_dma_chan_free
```
static int switchtec_dma_chan_free(struct switchtec_dma_chan
*swdma_chan) {
struct pci_dev *pdev = rcu_dereference(swdma_chan->swdma_dev->pdev); <<< suspicious RCU usage
spin_lock_bh(&swdma_chan->submit_lock);
swdma_chan->ring_active = false;
spin_unlock_bh(&swdma_chan->submit_lock);
```
Is going to trigger lockdep_rcu below log from my system:
```
[ 296.678725] =============================
[ 296.682890] WARNING: suspicious RCU usage
[ 296.687017] 6.8.0-rc4+ #2 Tainted: G E
[ 296.687028] -----------------------------
[ 296.691166] drivers/dma/switchtec_dma.c:1256 suspicious rcu_dereference_check() usage!
[ 296.699263]
other info that might help us debug this:
[ 296.707446]
rcu_scheduler_active = 2, debug_locks = 1
[ 296.714142] 1 lock held by insmod/1536:
[ 296.714153] #0: ffff8881191d91b0 (&dev->mutex){....}-{4:4}, at: __driver_attach+0x172/0x3b0
[ 296.714207]
stack backtrace:
[ 296.718689] CPU: 45 PID: 1536 Comm: insmod Tainted: G E 6.8.0-rc4+ #2
[ 296.718710] Call Trace:
[ 296.718720] <TASK>
[ 296.718731] dump_stack_lvl+0xac/0xc0
[ 296.718750] lockdep_rcu_suspicious+0x1dd/0x380
[ 296.718782] switchtec_dma_probe+0xf0a/0x1400 [switchtec_dma]
[ 296.718819] ? _raw_spin_unlock_irqrestore+0x66/0x80
```
I think it should be also guarded by rcu_read_lock/unlock in order to avoid such situations.
