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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] [PATCH v3 2/2] xen/evtchn: rework per event channel lock
Currently the lock for a single event channel needs to be taken with
interrupts off, which causes deadlocks in some cases.
Rework the per event channel lock to be non-blocking for the case of
sending an event and removing the need for disabling interrupts for
taking the lock.
The lock is needed for avoiding races between sending an event or
querying the channel's state against removal of the event channel.
Use a locking scheme similar to a rwlock, but with some modifications:
- sending an event or querying the event channel's state uses an
operation similar to read_trylock(), in case of not obtaining the
lock the sending is omitted or a default state is returned
- closing an event channel is similar to write_lock(), but without
real fairness regarding multiple writers (this saves some space in
the event channel structure and multiple writers are impossible as
closing an event channel requires the domain's event_lock to be
held).
Fixes: e045199c7c9c54 ("evtchn: address races with evtchn_reset()")
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
V3:
- corrected a copy-and-paste error (Jan Beulich)
- corrected unlocking in two cases (Jan Beulich)
- renamed evtchn_read_trylock() (Jan Beulich)
- added some comments and an ASSERT() for evtchn_write_lock()
- set EVENT_WRITE_LOCK_INC to INT_MIN
V2:
- added needed barriers
Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
---
xen/arch/x86/irq.c | 6 +-
xen/arch/x86/pv/shim.c | 9 +--
xen/common/event_channel.c | 109 +++++++++++++++++--------------------
xen/include/xen/event.h | 76 ++++++++++++++++++++++----
xen/include/xen/sched.h | 2 +-
5 files changed, 125 insertions(+), 77 deletions(-)
diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
index 93c4fb9a79..8d1f9a9fc6 100644
--- a/xen/arch/x86/irq.c
+++ b/xen/arch/x86/irq.c
@@ -2495,14 +2495,12 @@ static void dump_irqs(unsigned char key)
pirq = domain_irq_to_pirq(d, irq);
info = pirq_info(d, pirq);
evtchn = evtchn_from_port(d, info->evtchn);
- local_irq_disable();
- if ( spin_trylock(&evtchn->lock) )
+ if ( evtchn_read_trylock(evtchn) )
{
pending = evtchn_is_pending(d, evtchn);
masked = evtchn_is_masked(d, evtchn);
- spin_unlock(&evtchn->lock);
+ evtchn_read_unlock(evtchn);
}
- local_irq_enable();
printk("d%d:%3d(%c%c%c)%c",
d->domain_id, pirq, "-P?"[pending],
"-M?"[masked], info->masked ? 'M' : '-',
diff --git a/xen/arch/x86/pv/shim.c b/xen/arch/x86/pv/shim.c
index 9aef7a860a..b4e83e0778 100644
--- a/xen/arch/x86/pv/shim.c
+++ b/xen/arch/x86/pv/shim.c
@@ -660,11 +660,12 @@ void pv_shim_inject_evtchn(unsigned int port)
if ( port_is_valid(guest, port) )
{
struct evtchn *chn = evtchn_from_port(guest, port);
- unsigned long flags;
- spin_lock_irqsave(&chn->lock, flags);
- evtchn_port_set_pending(guest, chn->notify_vcpu_id, chn);
- spin_unlock_irqrestore(&chn->lock, flags);
+ if ( evtchn_read_trylock(chn) )
+ {
+ evtchn_port_set_pending(guest, chn->notify_vcpu_id, chn);
+ evtchn_read_unlock(chn);
+ }
}
}
diff --git a/xen/common/event_channel.c b/xen/common/event_channel.c
index e365b5498f..3df73dbc71 100644
--- a/xen/common/event_channel.c
+++ b/xen/common/event_channel.c
@@ -131,7 +131,7 @@ static struct evtchn *alloc_evtchn_bucket(struct domain *d,
unsigned int port)
return NULL;
}
chn[i].port = port + i;
- spin_lock_init(&chn[i].lock);
+ atomic_set(&chn[i].lock, 0);
}
return chn;
}
@@ -253,7 +253,6 @@ static long evtchn_alloc_unbound(evtchn_alloc_unbound_t
*alloc)
int port;
domid_t dom = alloc->dom;
long rc;
- unsigned long flags;
d = rcu_lock_domain_by_any_id(dom);
if ( d == NULL )
@@ -269,14 +268,14 @@ static long evtchn_alloc_unbound(evtchn_alloc_unbound_t
*alloc)
if ( rc )
goto out;
- spin_lock_irqsave(&chn->lock, flags);
+ evtchn_write_lock(chn);
chn->state = ECS_UNBOUND;
if ( (chn->u.unbound.remote_domid = alloc->remote_dom) == DOMID_SELF )
chn->u.unbound.remote_domid = current->domain->domain_id;
evtchn_port_init(d, chn);
- spin_unlock_irqrestore(&chn->lock, flags);
+ evtchn_write_unlock(chn);
alloc->port = port;
@@ -289,32 +288,26 @@ static long evtchn_alloc_unbound(evtchn_alloc_unbound_t
*alloc)
}
-static unsigned long double_evtchn_lock(struct evtchn *lchn,
- struct evtchn *rchn)
+static void double_evtchn_lock(struct evtchn *lchn, struct evtchn *rchn)
{
- unsigned long flags;
-
if ( lchn <= rchn )
{
- spin_lock_irqsave(&lchn->lock, flags);
+ evtchn_write_lock(lchn);
if ( lchn != rchn )
- spin_lock(&rchn->lock);
+ evtchn_write_lock(rchn);
}
else
{
- spin_lock_irqsave(&rchn->lock, flags);
- spin_lock(&lchn->lock);
+ evtchn_write_lock(rchn);
+ evtchn_write_lock(lchn);
}
-
- return flags;
}
-static void double_evtchn_unlock(struct evtchn *lchn, struct evtchn *rchn,
- unsigned long flags)
+static void double_evtchn_unlock(struct evtchn *lchn, struct evtchn *rchn)
{
if ( lchn != rchn )
- spin_unlock(&lchn->lock);
- spin_unlock_irqrestore(&rchn->lock, flags);
+ evtchn_write_unlock(lchn);
+ evtchn_write_unlock(rchn);
}
static long evtchn_bind_interdomain(evtchn_bind_interdomain_t *bind)
@@ -324,7 +317,6 @@ static long
evtchn_bind_interdomain(evtchn_bind_interdomain_t *bind)
int lport, rport = bind->remote_port;
domid_t rdom = bind->remote_dom;
long rc;
- unsigned long flags;
if ( rdom == DOMID_SELF )
rdom = current->domain->domain_id;
@@ -360,7 +352,7 @@ static long
evtchn_bind_interdomain(evtchn_bind_interdomain_t *bind)
if ( rc )
goto out;
- flags = double_evtchn_lock(lchn, rchn);
+ double_evtchn_lock(lchn, rchn);
lchn->u.interdomain.remote_dom = rd;
lchn->u.interdomain.remote_port = rport;
@@ -377,7 +369,7 @@ static long
evtchn_bind_interdomain(evtchn_bind_interdomain_t *bind)
*/
evtchn_port_set_pending(ld, lchn->notify_vcpu_id, lchn);
- double_evtchn_unlock(lchn, rchn, flags);
+ double_evtchn_unlock(lchn, rchn);
bind->local_port = lport;
@@ -400,7 +392,6 @@ int evtchn_bind_virq(evtchn_bind_virq_t *bind,
evtchn_port_t port)
struct domain *d = current->domain;
int virq = bind->virq, vcpu = bind->vcpu;
int rc = 0;
- unsigned long flags;
if ( (virq < 0) || (virq >= ARRAY_SIZE(v->virq_to_evtchn)) )
return -EINVAL;
@@ -438,14 +429,14 @@ int evtchn_bind_virq(evtchn_bind_virq_t *bind,
evtchn_port_t port)
chn = evtchn_from_port(d, port);
- spin_lock_irqsave(&chn->lock, flags);
+ evtchn_write_lock(chn);
chn->state = ECS_VIRQ;
chn->notify_vcpu_id = vcpu;
chn->u.virq = virq;
evtchn_port_init(d, chn);
- spin_unlock_irqrestore(&chn->lock, flags);
+ evtchn_write_unlock(chn);
v->virq_to_evtchn[virq] = bind->port = port;
@@ -462,7 +453,6 @@ static long evtchn_bind_ipi(evtchn_bind_ipi_t *bind)
struct domain *d = current->domain;
int port, vcpu = bind->vcpu;
long rc = 0;
- unsigned long flags;
if ( domain_vcpu(d, vcpu) == NULL )
return -ENOENT;
@@ -474,13 +464,13 @@ static long evtchn_bind_ipi(evtchn_bind_ipi_t *bind)
chn = evtchn_from_port(d, port);
- spin_lock_irqsave(&chn->lock, flags);
+ evtchn_write_lock(chn);
chn->state = ECS_IPI;
chn->notify_vcpu_id = vcpu;
evtchn_port_init(d, chn);
- spin_unlock_irqrestore(&chn->lock, flags);
+ evtchn_write_unlock(chn);
bind->port = port;
@@ -524,7 +514,6 @@ static long evtchn_bind_pirq(evtchn_bind_pirq_t *bind)
struct pirq *info;
int port = 0, pirq = bind->pirq;
long rc;
- unsigned long flags;
if ( (pirq < 0) || (pirq >= d->nr_pirqs) )
return -EINVAL;
@@ -557,14 +546,14 @@ static long evtchn_bind_pirq(evtchn_bind_pirq_t *bind)
goto out;
}
- spin_lock_irqsave(&chn->lock, flags);
+ evtchn_write_lock(chn);
chn->state = ECS_PIRQ;
chn->u.pirq.irq = pirq;
link_pirq_port(port, chn, v);
evtchn_port_init(d, chn);
- spin_unlock_irqrestore(&chn->lock, flags);
+ evtchn_write_unlock(chn);
bind->port = port;
@@ -585,7 +574,6 @@ int evtchn_close(struct domain *d1, int port1, bool guest)
struct evtchn *chn1, *chn2;
int port2;
long rc = 0;
- unsigned long flags;
again:
spin_lock(&d1->event_lock);
@@ -686,14 +674,14 @@ int evtchn_close(struct domain *d1, int port1, bool guest)
BUG_ON(chn2->state != ECS_INTERDOMAIN);
BUG_ON(chn2->u.interdomain.remote_dom != d1);
- flags = double_evtchn_lock(chn1, chn2);
+ double_evtchn_lock(chn1, chn2);
evtchn_free(d1, chn1);
chn2->state = ECS_UNBOUND;
chn2->u.unbound.remote_domid = d1->domain_id;
- double_evtchn_unlock(chn1, chn2, flags);
+ double_evtchn_unlock(chn1, chn2);
goto out;
@@ -701,9 +689,9 @@ int evtchn_close(struct domain *d1, int port1, bool guest)
BUG();
}
- spin_lock_irqsave(&chn1->lock, flags);
+ evtchn_write_lock(chn1);
evtchn_free(d1, chn1);
- spin_unlock_irqrestore(&chn1->lock, flags);
+ evtchn_write_unlock(chn1);
out:
if ( d2 != NULL )
@@ -723,7 +711,6 @@ int evtchn_send(struct domain *ld, unsigned int lport)
struct evtchn *lchn, *rchn;
struct domain *rd;
int rport, ret = 0;
- unsigned long flags;
if ( !port_is_valid(ld, lport) )
return -EINVAL;
@@ -736,7 +723,8 @@ int evtchn_send(struct domain *ld, unsigned int lport)
lchn = evtchn_from_port(ld, lport);
- spin_lock_irqsave(&lchn->lock, flags);
+ if ( !evtchn_read_trylock(lchn) )
+ return 0;
/* Guest cannot send via a Xen-attached event channel. */
if ( unlikely(consumer_is_xen(lchn)) )
@@ -771,7 +759,7 @@ int evtchn_send(struct domain *ld, unsigned int lport)
}
out:
- spin_unlock_irqrestore(&lchn->lock, flags);
+ evtchn_read_unlock(lchn);
return ret;
}
@@ -798,9 +786,11 @@ void send_guest_vcpu_virq(struct vcpu *v, uint32_t virq)
d = v->domain;
chn = evtchn_from_port(d, port);
- spin_lock(&chn->lock);
- evtchn_port_set_pending(d, v->vcpu_id, chn);
- spin_unlock(&chn->lock);
+ if ( evtchn_read_trylock(chn) )
+ {
+ evtchn_port_set_pending(d, v->vcpu_id, chn);
+ evtchn_read_unlock(chn);
+ }
out:
spin_unlock_irqrestore(&v->virq_lock, flags);
@@ -829,9 +819,11 @@ void send_guest_global_virq(struct domain *d, uint32_t
virq)
goto out;
chn = evtchn_from_port(d, port);
- spin_lock(&chn->lock);
- evtchn_port_set_pending(d, chn->notify_vcpu_id, chn);
- spin_unlock(&chn->lock);
+ if ( evtchn_read_trylock(chn) )
+ {
+ evtchn_port_set_pending(d, chn->notify_vcpu_id, chn);
+ evtchn_read_unlock(chn);
+ }
out:
spin_unlock_irqrestore(&v->virq_lock, flags);
@@ -841,7 +833,6 @@ void send_guest_pirq(struct domain *d, const struct pirq
*pirq)
{
int port;
struct evtchn *chn;
- unsigned long flags;
/*
* PV guests: It should not be possible to race with __evtchn_close(). The
@@ -856,9 +847,11 @@ void send_guest_pirq(struct domain *d, const struct pirq
*pirq)
}
chn = evtchn_from_port(d, port);
- spin_lock_irqsave(&chn->lock, flags);
- evtchn_port_set_pending(d, chn->notify_vcpu_id, chn);
- spin_unlock_irqrestore(&chn->lock, flags);
+ if ( evtchn_read_trylock(chn) )
+ {
+ evtchn_port_set_pending(d, chn->notify_vcpu_id, chn);
+ evtchn_read_unlock(chn);
+ }
}
static struct domain *global_virq_handlers[NR_VIRQS] __read_mostly;
@@ -1060,15 +1053,16 @@ int evtchn_unmask(unsigned int port)
{
struct domain *d = current->domain;
struct evtchn *evtchn;
- unsigned long flags;
if ( unlikely(!port_is_valid(d, port)) )
return -EINVAL;
evtchn = evtchn_from_port(d, port);
- spin_lock_irqsave(&evtchn->lock, flags);
- evtchn_port_unmask(d, evtchn);
- spin_unlock_irqrestore(&evtchn->lock, flags);
+ if ( evtchn_read_trylock(evtchn) )
+ {
+ evtchn_port_unmask(d, evtchn);
+ evtchn_read_unlock(evtchn);
+ }
return 0;
}
@@ -1327,7 +1321,6 @@ int alloc_unbound_xen_event_channel(
{
struct evtchn *chn;
int port, rc;
- unsigned long flags;
spin_lock(&ld->event_lock);
@@ -1340,14 +1333,14 @@ int alloc_unbound_xen_event_channel(
if ( rc )
goto out;
- spin_lock_irqsave(&chn->lock, flags);
+ evtchn_write_lock(chn);
chn->state = ECS_UNBOUND;
chn->xen_consumer = get_xen_consumer(notification_fn);
chn->notify_vcpu_id = lvcpu;
chn->u.unbound.remote_domid = remote_domid;
- spin_unlock_irqrestore(&chn->lock, flags);
+ evtchn_write_unlock(chn);
/*
* Increment ->xen_evtchns /after/ ->active_evtchns. No explicit
@@ -1383,7 +1376,6 @@ void notify_via_xen_event_channel(struct domain *ld, int
lport)
{
struct evtchn *lchn, *rchn;
struct domain *rd;
- unsigned long flags;
if ( !port_is_valid(ld, lport) )
{
@@ -1398,7 +1390,8 @@ void notify_via_xen_event_channel(struct domain *ld, int
lport)
lchn = evtchn_from_port(ld, lport);
- spin_lock_irqsave(&lchn->lock, flags);
+ if ( !evtchn_read_trylock(lchn) )
+ return;
if ( likely(lchn->state == ECS_INTERDOMAIN) )
{
@@ -1408,7 +1401,7 @@ void notify_via_xen_event_channel(struct domain *ld, int
lport)
evtchn_port_set_pending(rd, rchn->notify_vcpu_id, rchn);
}
- spin_unlock_irqrestore(&lchn->lock, flags);
+ evtchn_read_unlock(lchn);
}
void evtchn_check_pollers(struct domain *d, unsigned int port)
diff --git a/xen/include/xen/event.h b/xen/include/xen/event.h
index 509d3ae861..592e0dc22d 100644
--- a/xen/include/xen/event.h
+++ b/xen/include/xen/event.h
@@ -105,6 +105,60 @@ void notify_via_xen_event_channel(struct domain *ld, int
lport);
#define bucket_from_port(d, p) \
((group_from_port(d, p))[((p) % EVTCHNS_PER_GROUP) / EVTCHNS_PER_BUCKET])
+#define EVENT_WRITE_LOCK_INC INT_MIN
+
+/*
+ * Lock an event channel exclusively. This is allowed only with holding
+ * d->event_lock AND when the channel is free or unbound either when taking
+ * or when releasing the lock, as any concurrent operation on the event
+ * channel using evtchn_read_trylock() will just assume the event channel is
+ * free or unbound at the moment.
+ */
+static inline void evtchn_write_lock(struct evtchn *evtchn)
+{
+ int val;
+
+ /*
+ * The lock can't be held by a writer already, as all writers need to
+ * hold d->event_lock.
+ */
+ ASSERT(atomic_read(&evtchn->lock) >= 0);
+
+ /* No barrier needed, atomic_add_return() is full barrier. */
+ for ( val = atomic_add_return(EVENT_WRITE_LOCK_INC, &evtchn->lock);
+ val != EVENT_WRITE_LOCK_INC;
+ val = atomic_read(&evtchn->lock) )
+ cpu_relax();
+}
+
+static inline void evtchn_write_unlock(struct evtchn *evtchn)
+{
+ arch_lock_release_barrier();
+
+ atomic_sub(EVENT_WRITE_LOCK_INC, &evtchn->lock);
+}
+
+static inline bool evtchn_read_trylock(struct evtchn *evtchn)
+{
+ if ( atomic_read(&evtchn->lock) < 0 )
+ return false;
+
+ /* No barrier needed, atomic_inc_return() is full barrier. */
+ if ( atomic_inc_return(&evtchn->lock) >= 0 )
+ return true;
+
+ atomic_dec(&evtchn->lock);
+
+ return false;
+}
+
+static inline void evtchn_read_unlock(struct evtchn *evtchn)
+{
+ arch_lock_release_barrier();
+
+ atomic_dec(&evtchn->lock);
+}
+
static inline unsigned int max_evtchns(const struct domain *d)
{
return d->evtchn_fifo ? EVTCHN_FIFO_NR_CHANNELS
@@ -249,12 +303,13 @@ static inline bool evtchn_is_masked(const struct domain
*d,
static inline bool evtchn_port_is_masked(struct domain *d, evtchn_port_t port)
{
struct evtchn *evtchn = evtchn_from_port(d, port);
- bool rc;
- unsigned long flags;
+ bool rc = true;
- spin_lock_irqsave(&evtchn->lock, flags);
- rc = evtchn_is_masked(d, evtchn);
- spin_unlock_irqrestore(&evtchn->lock, flags);
+ if ( evtchn_read_trylock(evtchn) )
+ {
+ rc = evtchn_is_masked(d, evtchn);
+ evtchn_read_unlock(evtchn);
+ }
return rc;
}
@@ -274,12 +329,13 @@ static inline int evtchn_port_poll(struct domain *d,
evtchn_port_t port)
if ( port_is_valid(d, port) )
{
struct evtchn *evtchn = evtchn_from_port(d, port);
- unsigned long flags;
- spin_lock_irqsave(&evtchn->lock, flags);
- if ( evtchn_usable(evtchn) )
- rc = evtchn_is_pending(d, evtchn);
- spin_unlock_irqrestore(&evtchn->lock, flags);
+ if ( evtchn_read_trylock(evtchn) )
+ {
+ if ( evtchn_usable(evtchn) )
+ rc = evtchn_is_pending(d, evtchn);
+ evtchn_read_unlock(evtchn);
+ }
}
return rc;
diff --git a/xen/include/xen/sched.h b/xen/include/xen/sched.h
index a298ff4df8..096e0ec6af 100644
--- a/xen/include/xen/sched.h
+++ b/xen/include/xen/sched.h
@@ -85,7 +85,7 @@ extern domid_t hardware_domid;
struct evtchn
{
- spinlock_t lock;
+ atomic_t lock; /* kind of rwlock, use evtchn_*_[un]lock() */
#define ECS_FREE 0 /* Channel is available for use. */
#define ECS_RESERVED 1 /* Channel is reserved. */
#define ECS_UNBOUND 2 /* Channel is waiting to bind to a remote domain. */
--
2.26.2
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