Re: [PATCH v2 2/5] PCI: endpoint: Replace spinlock with mutex

[Vidya Sagar <vidyas@xxxxxxxxxx>]

On 6/21/2021 7:07 PM, Kishon Vijay Abraham I wrote:
> External email: Use caution opening links or attachments
>
>
> Hi Vidya Sagar,
>
> On 21/06/21 3:08 pm, Vidya Sagar wrote:
> > On 6/21/2021 10:44 AM, Kishon Vijay Abraham I wrote:
> > > External email: Use caution opening links or attachments
> > >
> > >
> > > Hi Vidya Sagar,
> > >
> > > On 11/06/21 3:22 pm, Vidya Sagar wrote:
> > > > Hi Kishon,
> > > > Apologies for bringup it up this late.
> > > > I'm wondering if there was any issue which this patch tried to address?
> > >
> > > There was one function pci_epc_linkup() which was expected to be invoked
> > > in interrupt context (basically when the LINKUP interrupt is raised).
> > > But after it was moved to use atomic notifier, all the EPC core APIs
> > > were replaced to use mutex.
> > > > Actually, "The pci_epc_ops is not intended to be invoked from interrupt
> > > > context" isn't true in case of Tegra194. We do call
> > > > dw_pcie_ep_init_notify() API from threaded irq service routine and it
> > > > eventually calls mutext_lock() of pci_epc_get_features() which is
> > > > reusulting in the following warning log.
> > > > BUG: sleeping function called from invalid context at
> > > > kernel/locking/mutex.c:
> > > > Would like hear your comments on it.
> > After reviewing the logs and code again, I think it was my mistake to
> > come to early conclusion that it was because of calling mutex_lock() in
> > the atomic context. It is clear now.
> >
> > I would like to understand the reason behind putting locks in the epc
> > core driver before calling ops.
>
> There could be two different functions trying to configure endpoint
> controller (could be a multi-function endpoint) and the framework should
> guarantee the hardware is not accessed by both the functions simultaneously.
Not all ops functions need to be protected by the lock. for ex:- 
get/set_msi(x)(), raise_irq(), get_features() don't need to be protected 
by a global lock. So, transferring the synchronization responsibility to 
the controller driver gives an efficient control on locking.

> > I believe the ops callers should implement lock if they are concurrently
> > accessing the ops instead of adding a global lock in the epc core.
> This can only protect within a function and not across multiple functions.
> > This would help in scenarios like the one below.
> >
> >      We have a performance oriented endpoint function driver which calls
> > map, unmap & raise_irq ops from softirq context and because of
> > mutex_lock(), we can't do that now. epc core driver should not restrict
> > the function drivers to use only non-atomic functions.
>
> Not sure what exactly the function driver does but can't map/unmap be
> done for a big block once to optimize and operate on that buffer? I'd
> assume you are having a custom driver on the host side too?
We implemented a function driver that provides a virtual ethernet 
interface. To complement this, we have a PCIe device driver on the host 
that exposes a virtual ethernet interface in the host system. 
start_xmit() in the function driver of the virtual ethernet interface is 
a soft irq which maps/unmaps each skb buffer dynamically. So, a one time 
static mapping is not possible. Similarly, because of the global lock, 
start_xmit() can not raise_irq() to the host.

Thanks,
Vidya Sagar
> Thanks
> Kishon
>
> > Thanks,
> > Vidya Sagar
> > > I don't think it is ideal to initialize EPC in interrupt context (unless
> > > there is a specific reason for it). EPC initialization can be moved to
> > > bottom half similar to how commands are handled after LINKUP.
> >
> > > Thanks
> > > Kishon
> > >
> > > > Thanks,
> > > > Vidya Sagar
> > > >
> > > > On 2/12/2020 4:55 PM, Kishon Vijay Abraham I wrote:
> > > > > External email: Use caution opening links or attachments
> > > > >
> > > > >
> > > > > The pci_epc_ops is not intended to be invoked from interrupt context.
> > > > > Hence replace spin_lock_irqsave and spin_unlock_irqrestore with
> > > > > mutex_lock and mutex_unlock respectively.
> > > > >
> > > > > Signed-off-by: Kishon Vijay Abraham I <kishon@xxxxxx>
> > > > > ---
> > > > >     drivers/pci/endpoint/pci-epc-core.c | 82
> > > > > +++++++++++------------------
> > > > >     include/linux/pci-epc.h             |  6 +--
> > > > >     2 files changed, 34 insertions(+), 54 deletions(-)
> > > > >
> > > > > diff --git a/drivers/pci/endpoint/pci-epc-core.c
> > > > > b/drivers/pci/endpoint/pci-epc-core.c
> > > > > index 2f6436599fcb..e51a12ed85bb 100644
> > > > > --- a/drivers/pci/endpoint/pci-epc-core.c
> > > > > +++ b/drivers/pci/endpoint/pci-epc-core.c
> > > > > @@ -120,7 +120,6 @@ const struct pci_epc_features
> > > > > *pci_epc_get_features(struct pci_epc *epc,
> > > > >                                                        u8 func_no)
> > > > >     {
> > > > >            const struct pci_epc_features *epc_features;
> > > > > -       unsigned long flags;
> > > > >
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions)
> > > > >                    return NULL;
> > > > > @@ -128,9 +127,9 @@ const struct pci_epc_features
> > > > > *pci_epc_get_features(struct pci_epc *epc,
> > > > >            if (!epc->ops->get_features)
> > > > >                    return NULL;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            epc_features = epc->ops->get_features(epc, func_no);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            return epc_features;
> > > > >     }
> > > > > @@ -144,14 +143,12 @@ EXPORT_SYMBOL_GPL(pci_epc_get_features);
> > > > >      */
> > > > >     void pci_epc_stop(struct pci_epc *epc)
> > > > >     {
> > > > > -       unsigned long flags;
> > > > > -
> > > > >            if (IS_ERR(epc) || !epc->ops->stop)
> > > > >                    return;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            epc->ops->stop(epc);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >     }
> > > > >     EXPORT_SYMBOL_GPL(pci_epc_stop);
> > > > >
> > > > > @@ -164,7 +161,6 @@ EXPORT_SYMBOL_GPL(pci_epc_stop);
> > > > >     int pci_epc_start(struct pci_epc *epc)
> > > > >     {
> > > > >            int ret;
> > > > > -       unsigned long flags;
> > > > >
> > > > >            if (IS_ERR(epc))
> > > > >                    return -EINVAL;
> > > > > @@ -172,9 +168,9 @@ int pci_epc_start(struct pci_epc *epc)
> > > > >            if (!epc->ops->start)
> > > > >                    return 0;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            ret = epc->ops->start(epc);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            return ret;
> > > > >     }
> > > > > @@ -193,7 +189,6 @@ int pci_epc_raise_irq(struct pci_epc *epc, u8
> > > > > func_no,
> > > > >                          enum pci_epc_irq_type type, u16 interrupt_num)
> > > > >     {
> > > > >            int ret;
> > > > > -       unsigned long flags;
> > > > >
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions)
> > > > >                    return -EINVAL;
> > > > > @@ -201,9 +196,9 @@ int pci_epc_raise_irq(struct pci_epc *epc, u8
> > > > > func_no,
> > > > >            if (!epc->ops->raise_irq)
> > > > >                    return 0;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            ret = epc->ops->raise_irq(epc, func_no, type, interrupt_num);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            return ret;
> > > > >     }
> > > > > @@ -219,7 +214,6 @@ EXPORT_SYMBOL_GPL(pci_epc_raise_irq);
> > > > >     int pci_epc_get_msi(struct pci_epc *epc, u8 func_no)
> > > > >     {
> > > > >            int interrupt;
> > > > > -       unsigned long flags;
> > > > >
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions)
> > > > >                    return 0;
> > > > > @@ -227,9 +221,9 @@ int pci_epc_get_msi(struct pci_epc *epc, u8
> > > > > func_no)
> > > > >            if (!epc->ops->get_msi)
> > > > >                    return 0;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            interrupt = epc->ops->get_msi(epc, func_no);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            if (interrupt < 0)
> > > > >                    return 0;
> > > > > @@ -252,7 +246,6 @@ int pci_epc_set_msi(struct pci_epc *epc, u8
> > > > > func_no, u8 interrupts)
> > > > >     {
> > > > >            int ret;
> > > > >            u8 encode_int;
> > > > > -       unsigned long flags;
> > > > >
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions ||
> > > > >                interrupts > 32)
> > > > > @@ -263,9 +256,9 @@ int pci_epc_set_msi(struct pci_epc *epc, u8
> > > > > func_no, u8 interrupts)
> > > > >
> > > > >            encode_int = order_base_2(interrupts);
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            ret = epc->ops->set_msi(epc, func_no, encode_int);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            return ret;
> > > > >     }
> > > > > @@ -281,7 +274,6 @@ EXPORT_SYMBOL_GPL(pci_epc_set_msi);
> > > > >     int pci_epc_get_msix(struct pci_epc *epc, u8 func_no)
> > > > >     {
> > > > >            int interrupt;
> > > > > -       unsigned long flags;
> > > > >
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions)
> > > > >                    return 0;
> > > > > @@ -289,9 +281,9 @@ int pci_epc_get_msix(struct pci_epc *epc, u8
> > > > > func_no)
> > > > >            if (!epc->ops->get_msix)
> > > > >                    return 0;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            interrupt = epc->ops->get_msix(epc, func_no);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            if (interrupt < 0)
> > > > >                    return 0;
> > > > > @@ -311,7 +303,6 @@ EXPORT_SYMBOL_GPL(pci_epc_get_msix);
> > > > >     int pci_epc_set_msix(struct pci_epc *epc, u8 func_no, u16
> > > > > interrupts)
> > > > >     {
> > > > >            int ret;
> > > > > -       unsigned long flags;
> > > > >
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions ||
> > > > >                interrupts < 1 || interrupts > 2048)
> > > > > @@ -320,9 +311,9 @@ int pci_epc_set_msix(struct pci_epc *epc, u8
> > > > > func_no, u16 interrupts)
> > > > >            if (!epc->ops->set_msix)
> > > > >                    return 0;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            ret = epc->ops->set_msix(epc, func_no, interrupts - 1);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            return ret;
> > > > >     }
> > > > > @@ -339,17 +330,15 @@ EXPORT_SYMBOL_GPL(pci_epc_set_msix);
> > > > >     void pci_epc_unmap_addr(struct pci_epc *epc, u8 func_no,
> > > > >                            phys_addr_t phys_addr)
> > > > >     {
> > > > > -       unsigned long flags;
> > > > > -
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions)
> > > > >                    return;
> > > > >
> > > > >            if (!epc->ops->unmap_addr)
> > > > >                    return;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            epc->ops->unmap_addr(epc, func_no, phys_addr);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >     }
> > > > >     EXPORT_SYMBOL_GPL(pci_epc_unmap_addr);
> > > > >
> > > > > @@ -367,7 +356,6 @@ int pci_epc_map_addr(struct pci_epc *epc, u8
> > > > > func_no,
> > > > >                         phys_addr_t phys_addr, u64 pci_addr, size_t
> > > > > size)
> > > > >     {
> > > > >            int ret;
> > > > > -       unsigned long flags;
> > > > >
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions)
> > > > >                    return -EINVAL;
> > > > > @@ -375,9 +363,9 @@ int pci_epc_map_addr(struct pci_epc *epc, u8
> > > > > func_no,
> > > > >            if (!epc->ops->map_addr)
> > > > >                    return 0;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            ret = epc->ops->map_addr(epc, func_no, phys_addr, pci_addr,
> > > > > size);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            return ret;
> > > > >     }
> > > > > @@ -394,8 +382,6 @@ EXPORT_SYMBOL_GPL(pci_epc_map_addr);
> > > > >     void pci_epc_clear_bar(struct pci_epc *epc, u8 func_no,
> > > > >                           struct pci_epf_bar *epf_bar)
> > > > >     {
> > > > > -       unsigned long flags;
> > > > > -
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions ||
> > > > >                (epf_bar->barno == BAR_5 &&
> > > > >                 epf_bar->flags & PCI_BASE_ADDRESS_MEM_TYPE_64))
> > > > > @@ -404,9 +390,9 @@ void pci_epc_clear_bar(struct pci_epc *epc, u8
> > > > > func_no,
> > > > >            if (!epc->ops->clear_bar)
> > > > >                    return;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            epc->ops->clear_bar(epc, func_no, epf_bar);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >     }
> > > > >     EXPORT_SYMBOL_GPL(pci_epc_clear_bar);
> > > > >
> > > > > @@ -422,7 +408,6 @@ int pci_epc_set_bar(struct pci_epc *epc, u8
> > > > > func_no,
> > > > >                        struct pci_epf_bar *epf_bar)
> > > > >     {
> > > > >            int ret;
> > > > > -       unsigned long irq_flags;
> > > > >            int flags = epf_bar->flags;
> > > > >
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions ||
> > > > > @@ -437,9 +422,9 @@ int pci_epc_set_bar(struct pci_epc *epc, u8
> > > > > func_no,
> > > > >            if (!epc->ops->set_bar)
> > > > >                    return 0;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, irq_flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            ret = epc->ops->set_bar(epc, func_no, epf_bar);
> > > > > -       spin_unlock_irqrestore(&epc->lock, irq_flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            return ret;
> > > > >     }
> > > > > @@ -460,7 +445,6 @@ int pci_epc_write_header(struct pci_epc *epc, u8
> > > > > func_no,
> > > > >                             struct pci_epf_header *header)
> > > > >     {
> > > > >            int ret;
> > > > > -       unsigned long flags;
> > > > >
> > > > >            if (IS_ERR_OR_NULL(epc) || func_no >= epc->max_functions)
> > > > >                    return -EINVAL;
> > > > > @@ -468,9 +452,9 @@ int pci_epc_write_header(struct pci_epc *epc, u8
> > > > > func_no,
> > > > >            if (!epc->ops->write_header)
> > > > >                    return 0;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            ret = epc->ops->write_header(epc, func_no, header);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            return ret;
> > > > >     }
> > > > > @@ -487,8 +471,6 @@ EXPORT_SYMBOL_GPL(pci_epc_write_header);
> > > > >      */
> > > > >     int pci_epc_add_epf(struct pci_epc *epc, struct pci_epf *epf)
> > > > >     {
> > > > > -       unsigned long flags;
> > > > > -
> > > > >            if (epf->epc)
> > > > >                    return -EBUSY;
> > > > >
> > > > > @@ -500,9 +482,9 @@ int pci_epc_add_epf(struct pci_epc *epc, struct
> > > > > pci_epf *epf)
> > > > >
> > > > >            epf->epc = epc;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            list_add_tail(&epf->list, &epc->pci_epf);
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >
> > > > >            return 0;
> > > > >     }
> > > > > @@ -517,15 +499,13 @@ EXPORT_SYMBOL_GPL(pci_epc_add_epf);
> > > > >      */
> > > > >     void pci_epc_remove_epf(struct pci_epc *epc, struct pci_epf *epf)
> > > > >     {
> > > > > -       unsigned long flags;
> > > > > -
> > > > >            if (!epc || IS_ERR(epc) || !epf)
> > > > >                    return;
> > > > >
> > > > > -       spin_lock_irqsave(&epc->lock, flags);
> > > > > +       mutex_lock(&epc->lock);
> > > > >            list_del(&epf->list);
> > > > >            epf->epc = NULL;
> > > > > -       spin_unlock_irqrestore(&epc->lock, flags);
> > > > > +       mutex_unlock(&epc->lock);
> > > > >     }
> > > > >     EXPORT_SYMBOL_GPL(pci_epc_remove_epf);
> > > > >
> > > > > @@ -604,7 +584,7 @@ __pci_epc_create(struct device *dev, const struct
> > > > > pci_epc_ops *ops,
> > > > >                    goto err_ret;
> > > > >            }
> > > > >
> > > > > -       spin_lock_init(&epc->lock);
> > > > > +       mutex_init(&epc->lock);
> > > > >            INIT_LIST_HEAD(&epc->pci_epf);
> > > > >            ATOMIC_INIT_NOTIFIER_HEAD(&epc->notifier);
> > > > >
> > > > > diff --git a/include/linux/pci-epc.h b/include/linux/pci-epc.h
> > > > > index 36644ccd32ac..9dd60f2e9705 100644
> > > > > --- a/include/linux/pci-epc.h
> > > > > +++ b/include/linux/pci-epc.h
> > > > > @@ -88,7 +88,7 @@ struct pci_epc_mem {
> > > > >      * @mem: address space of the endpoint controller
> > > > >      * @max_functions: max number of functions that can be configured in
> > > > > this EPC
> > > > >      * @group: configfs group representing the PCI EPC device
> > > > > - * @lock: spinlock to protect pci_epc ops
> > > > > + * @lock: mutex to protect pci_epc ops
> > > > >      * @notifier: used to notify EPF of any EPC events (like linkup)
> > > > >      */
> > > > >     struct pci_epc {
> > > > > @@ -98,8 +98,8 @@ struct pci_epc {
> > > > >            struct pci_epc_mem              *mem;
> > > > >            u8                              max_functions;
> > > > >            struct config_group             *group;
> > > > > -       /* spinlock to protect against concurrent access of EP
> > > > > controller */
> > > > > -       spinlock_t                      lock;
> > > > > +       /* mutex to protect against concurrent access of EP
> > > > > controller */
> > > > > +       struct mutex                    lock;
> > > > >            struct atomic_notifier_head     notifier;
> > > > >     };
> > > > >
> > > > > --
> > > > > 2.17.1