Re: [Xen-devel] Re: [PATCH] xen/pciback: Use mutexes when working with Xenbus state transitions.

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Sep 19, 2011 at 12:01:56PM +0100, Jan Beulich wrote:
> >>> On 19.09.11 at 12:43, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
> >>>> On 16.09.11 at 21:06, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> 
> >>>> wrote:
> > 
> >> The caller that orchestrates the state changes is xenwatch_thread
> >> and it takes a mutex. In our processing of Xenbus states we can take
> >> the luxery of going to sleep on a mutex, so lets do that and
> > 
> > This is only the direct conversion of existing spinlock accesses in
> > xenbus.c. However, in the course of converting from the legacy
> > implementation you stripped a couple more (in xen_pcibk_attach(),
> > xen_pcibk_reconfigure(), and xen_pcibk_setup_backend()), and
> 
> Actually, xen_pcibk_attach() has its lock taken in xen_pcibk_do_attach(),
> so no change needed there.
> 
> In xen_pcibk_reconfigure() and xen_pcibk_setup_backend() the locking
> may be redundant with the one in passthrough.c/vpci.c - is that the
> basis upon which you removed the locks taken there?

No. I believe the reason was much simpler.. it was b/c of this patch (see 
below).
But for the life of me I don't recall what deadlock we could hit.

I think the better choice will be to restore the call-sites of these
spinlocks but use mutex instead.

So let me post two patches - one for xenbus.c and one for vpci.c to
complement "xen/pciback: use mutex rather than spinlock in passthrough backend"
you posted and I queued up.


commit 3a8d1841ae2dd32452b79284da03eda596f30827
Author: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Date:   Fri Jul 23 14:35:47 2010 -0400

    xen/pciback: Redo spinlock usage.
    
    We were using coarse spinlocks that could end up with a deadlock.
    This patch fixes that and makes the spinlocks much more fine-grained.
    
    We also drop be->watchding state spinlocks as they are already
    guarded by the xenwatch_thread against multiple customers.
    
    Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>

diff --git a/drivers/xen/pciback/xenbus.c b/drivers/xen/pciback/xenbus.c
index d448bf5..993b659 100644
--- a/drivers/xen/pciback/xenbus.c
+++ b/drivers/xen/pciback/xenbus.c
@@ -54,23 +54,29 @@ static void pciback_disconnect(struct pciback_device *pdev)
                unbind_from_irqhandler(pdev->evtchn_irq, pdev);
                pdev->evtchn_irq = INVALID_EVTCHN_IRQ;
        }
+       spin_unlock(&pdev->dev_lock);
 
        /* If the driver domain started an op, make sure we complete it
         * before releasing the shared memory */
+
+       /* Note, the workqueue does not use spinlocks at all.*/
        flush_workqueue(pciback_wq);
 
+       spin_lock(&pdev->dev_lock);
        if (pdev->sh_info != NULL) {
                xenbus_unmap_ring_vfree(pdev->xdev, pdev->sh_info);
                pdev->sh_info = NULL;
        }
-
        spin_unlock(&pdev->dev_lock);
+
 }
 
 static void free_pdev(struct pciback_device *pdev)
 {
-       if (pdev->be_watching)
+       if (pdev->be_watching) {
                unregister_xenbus_watch(&pdev->be_watch);
+               pdev->be_watching = 0;
+       }
 
        pciback_disconnect(pdev);
 
@@ -98,7 +104,10 @@ static int pciback_do_attach(struct pciback_device *pdev, 
int gnt_ref,
                                "Error mapping other domain page in ours.");
                goto out;
        }
+
+       spin_lock(&pdev->dev_lock);
        pdev->sh_info = vaddr;
+       spin_unlock(&pdev->dev_lock);
 
        err = bind_interdomain_evtchn_to_irqhandler(
                pdev->xdev->otherend_id, remote_evtchn, pciback_handle_event,
@@ -108,7 +117,10 @@ static int pciback_do_attach(struct pciback_device *pdev, 
int gnt_ref,
                                 "Error binding event channel to IRQ");
                goto out;
        }
+
+       spin_lock(&pdev->dev_lock);
        pdev->evtchn_irq = err;
+       spin_unlock(&pdev->dev_lock);
        err = 0;
 
        dev_dbg(&pdev->xdev->dev, "Attached!\n");
@@ -122,7 +134,6 @@ static int pciback_attach(struct pciback_device *pdev)
        int gnt_ref, remote_evtchn;
        char *magic = NULL;
 
-       spin_lock(&pdev->dev_lock);
 
        /* Make sure we only do this setup once */
        if (xenbus_read_driver_state(pdev->xdev->nodename) !=
@@ -168,7 +179,6 @@ static int pciback_attach(struct pciback_device *pdev)
 
        dev_dbg(&pdev->xdev->dev, "Connected? %d\n", err);
 out:
-       spin_unlock(&pdev->dev_lock);
 
        kfree(magic);
 
@@ -340,7 +350,6 @@ static int pciback_reconfigure(struct pciback_device *pdev)
        char state_str[64];
        char dev_str[64];
 
-       spin_lock(&pdev->dev_lock);
 
        dev_dbg(&pdev->xdev->dev, "Reconfiguring device ...\n");
 
@@ -481,8 +490,6 @@ static int pciback_reconfigure(struct pciback_device *pdev)
        }
 
 out:
-       spin_unlock(&pdev->dev_lock);
-
        return 0;
 }
 
@@ -539,8 +546,6 @@ static int pciback_setup_backend(struct pciback_device 
*pdev)
        char dev_str[64];
        char state_str[64];
 
-       spin_lock(&pdev->dev_lock);
-
        /* It's possible we could get the call to setup twice, so make sure
         * we're not already connected.
         */
@@ -621,8 +626,6 @@ static int pciback_setup_backend(struct pciback_device 
*pdev)
                                 "Error switching to initialised state!");
 
 out:
-       spin_unlock(&pdev->dev_lock);
-
        if (!err)
                /* see if pcifront is already configured (if not, we'll wait) */
                pciback_attach(pdev);
@@ -669,6 +672,7 @@ static int pciback_xenbus_probe(struct xenbus_device *dev,
                                pciback_be_watch);
        if (err)
                goto out;
+
        pdev->be_watching = 1;
 
        /* We need to force a call to our callback here in case
@@ -708,8 +712,8 @@ int __init pciback_xenbus_register(void)
 {
        pciback_wq = create_workqueue("pciback_workqueue");
        if (!pciback_wq) {
-               printk(KERN_ERR "pciback_xenbus_register: create"
-                       "pciback_workqueue failed\n");
+               printk(KERN_ERR "%s: create"
+                       "pciback_workqueue failed\n",__FUNCTION__);
                return -EFAULT;
        }
        return xenbus_register_backend(&xenbus_pciback_driver);

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