[xen stable-4.12] evtchn: arrange for preemption in evtchn_destroy()

[patchbot@xxxxxxx]

commit 2aa4864b8a25f2a4b3f2647dbf75abd88e3514ad
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Tue Sep 22 17:11:33 2020 +0200
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Tue Sep 22 17:11:33 2020 +0200

    evtchn: arrange for preemption in evtchn_destroy()
    
    Especially closing of fully established interdomain channels can take
    quite some time, due to the locking involved. Therefore we shouldn't
    assume we can clean up still active ports all in one go. Besides adding
    the necessary preemption check, also avoid pointlessly starting from
    (or now really ending at) 0; 1 is the lowest numbered port which may
    need closing.
    
    Since we're now reducing ->valid_evtchns, free_xen_event_channel(),
    and (at least to be on the safe side) notify_via_xen_event_channel()
    need to cope with attempts to close / unbind from / send through already
    closed (and no longer valid, as per port_is_valid()) ports.
    
    This is part of XSA-344.
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Julien Grall <jgrall@xxxxxxxxxx>
    Reviewed-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
---
 xen/common/domain.c        |  4 +++-
 xen/common/event_channel.c | 43 ++++++++++++++++++++++++++++++++++++++-----
 xen/include/xen/sched.h    |  2 +-
 3 files changed, 42 insertions(+), 7 deletions(-)

diff --git a/xen/common/domain.c b/xen/common/domain.c
index 5f94651f11..8e44e88bc4 100644
--- a/xen/common/domain.c
+++ b/xen/common/domain.c
@@ -724,7 +724,6 @@ int domain_kill(struct domain *d)
             return domain_kill(d);
         d->is_dying = DOMDYING_dying;
         argo_destroy(d);
-        evtchn_destroy(d);
         gnttab_release_mappings(d);
         tmem_destroy(d->tmem_client);
         vnuma_destroy(d->vnuma);
@@ -732,6 +731,9 @@ int domain_kill(struct domain *d)
         d->tmem_client = NULL;
         /* fallthrough */
     case DOMDYING_dying:
+        rc = evtchn_destroy(d);
+        if ( rc )
+            break;
         rc = domain_relinquish_resources(d);
         if ( rc != 0 )
             break;
diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index 878e4250ed..3b1fe423df 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -1297,7 +1297,16 @@ int alloc_unbound_xen_event_channel(
 
 void free_xen_event_channel(struct domain *d, int port)
 {
-    BUG_ON(!port_is_valid(d, port));
+    if ( !port_is_valid(d, port) )
+    {
+        /*
+         * Make sure ->is_dying is read /after/ ->valid_evtchns, pairing
+         * with the spin_barrier() and BUG_ON() in evtchn_destroy().
+         */
+        smp_rmb();
+        BUG_ON(!d->is_dying);
+        return;
+    }
 
     evtchn_close(d, port, 0);
 }
@@ -1309,7 +1318,17 @@ void notify_via_xen_event_channel(struct domain *ld, int 
lport)
     struct domain *rd;
     unsigned long flags;
 
-    ASSERT(port_is_valid(ld, lport));
+    if ( !port_is_valid(ld, lport) )
+    {
+        /*
+         * Make sure ->is_dying is read /after/ ->valid_evtchns, pairing
+         * with the spin_barrier() and BUG_ON() in evtchn_destroy().
+         */
+        smp_rmb();
+        ASSERT(ld->is_dying);
+        return;
+    }
+
     lchn = evtchn_from_port(ld, lport);
 
     spin_lock_irqsave(&lchn->lock, flags);
@@ -1380,8 +1399,7 @@ int evtchn_init(struct domain *d, unsigned int max_port)
     return 0;
 }
 
-
-void evtchn_destroy(struct domain *d)
+int evtchn_destroy(struct domain *d)
 {
     unsigned int i;
 
@@ -1390,14 +1408,29 @@ void evtchn_destroy(struct domain *d)
     spin_barrier(&d->event_lock);
 
     /* Close all existing event channels. */
-    for ( i = 0; port_is_valid(d, i); i++ )
+    for ( i = d->valid_evtchns; --i; )
+    {
         evtchn_close(d, i, 0);
 
+        /*
+         * Avoid preempting when called from domain_create()'s error path,
+         * and don't check too often (choice of frequency is arbitrary).
+         */
+        if ( i && !(i & 0x3f) && d->is_dying != DOMDYING_dead &&
+             hypercall_preempt_check() )
+        {
+            write_atomic(&d->valid_evtchns, i);
+            return -ERESTART;
+        }
+    }
+
     ASSERT(!d->active_evtchns);
 
     clear_global_virq_handlers(d);
 
     evtchn_fifo_destroy(d);
+
+    return 0;
 }
 
 
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index 8d0c817d7d..f70d990dbb 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -136,7 +136,7 @@ struct evtchn
 } __attribute__((aligned(64)));
 
 int  evtchn_init(struct domain *d, unsigned int max_port);
-void evtchn_destroy(struct domain *d); /* from domain_kill */
+int  evtchn_destroy(struct domain *d); /* from domain_kill */
 void evtchn_destroy_final(struct domain *d); /* from complete_domain_destroy */
 
 struct waitqueue_vcpu;
--
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