Do not spin on locks that may be held by stop_machine_run() callers.

Currently stop_machine_run() will try to bring all CPUs to softirq
context, with some locks held, like xenpf_lock or cpu_add_remove_lock
etc. However, if another CPU is trying to get these locks, it may
cause deadlock.

This patch replace all such spin_lock with spin_trylock. For
xenpf_lock and sysctl_lock, we try to use hypercall_continuation, so
that we will not cause trouble to user space tools. For
cpu_hot_remove_lock, we simply return EBUSY if failure, since it will
only impact small number of user space tools.

In the end, we should try to make the stop_machine_run as spinlock
free.

Signed-off-by: Jiang, Yunhong <yunhong.jiang@intel.com>

3 files changed, 33 insertions, 6 deletions

diff --git a/xen/arch/x86/platform_hypercall.c b/xen/arch/x86/platform_hypercall.c
index 6461d4f0bc..dc2214930a 100644
--- a/xen/arch/x86/platform_hypercall.c
+++ b/xen/arch/x86/platform_hypercall.c
@@ -73,7 +73,11 @@ ret_t do_platform_op(XEN_GUEST_HANDLE(xen_platform_op_t) u_xenpf_op)
     if ( op->interface_version != XENPF_INTERFACE_VERSION )
         return -EACCES;
 
-    spin_lock(&xenpf_lock);
+    /* spin_trylock() avoids deadlock with stop_machine_run(). */
+    while ( !spin_trylock(&xenpf_lock) )
+        if ( hypercall_preempt_check() )
+            return hypercall_create_continuation(
+                __HYPERVISOR_platform_op, "h", u_xenpf_op);
 
     switch ( op->cmd )
     {
@@ -398,7 +402,12 @@ ret_t do_platform_op(XEN_GUEST_HANDLE(xen_platform_op_t) u_xenpf_op)
 
         g_info = &op->u.pcpu_info;
 
-        spin_lock(&cpu_add_remove_lock);
+        /* spin_trylock() avoids deadlock with stop_machine_run(). */
+        if ( !spin_trylock(&cpu_add_remove_lock) )
+        {
+            ret = -EBUSY;
+            break;
+        }
 
         if ( (g_info->xen_cpuid >= NR_CPUS) ||
              (g_info->xen_cpuid < 0) ||
diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index 8d5a103b80..98f2c1155c 100644
--- a/xen/arch/x86/smpboot.c
+++ b/xen/arch/x86/smpboot.c
@@ -1342,7 +1342,12 @@ int cpu_down(unsigned int cpu)
 {
 	int err = 0;
 
-	spin_lock(&cpu_add_remove_lock);
+	/* spin_trylock() avoids deadlock with stop_machine_run(). */
+	if (!spin_trylock(&cpu_add_remove_lock)) {
+		err = -EBUSY;
+		goto out;
+	}
+
 	if (num_online_cpus() == 1) {
 		err = -EBUSY;
 		goto out;
@@ -1384,7 +1389,10 @@ int cpu_up(unsigned int cpu)
 {
 	int err = 0;
 
-	spin_lock(&cpu_add_remove_lock);
+	/* spin_trylock() avoids deadlock with stop_machine_run(). */
+	if (!spin_trylock(&cpu_add_remove_lock))
+	    return -EBUSY;
+
 	if (cpu_online(cpu)) {
 		printk("Bring up a online cpu. Bogus!\n");
 		err = -EBUSY;
@@ -1419,6 +1427,8 @@ void disable_nonboot_cpus(void)
 		if (cpu == 0)
 			continue;
 		error = cpu_down(cpu);
+		/* No need to check EBUSY here */
+		ASSERT(error != -EBUSY);
 		if (!error) {
 			cpu_set(cpu, frozen_cpus);
 			printk("CPU%d is down\n", cpu);
@@ -1439,6 +1449,8 @@ void enable_nonboot_cpus(void)
 	mtrr_aps_sync_begin();
 	for_each_cpu_mask(cpu, frozen_cpus) {
 		error = cpu_up(cpu);
+		/* No conflict will happen here */
+		ASSERT(error != -EBUSY);
 		if (!error) {
 			printk("CPU%d is up\n", cpu);
 			continue;
@@ -1481,7 +1493,9 @@ int cpu_add(uint32_t apic_id, uint32_t acpi_id, uint32_t pxm)
 	if ( physid_isset(apic_id, phys_cpu_present_map) )
 		return -EEXIST;
 
-	spin_lock(&cpu_add_remove_lock);
+	/* spin_trylock() avoids deadlock with stop_machine_run(). */
+	if (!spin_trylock(&cpu_add_remove_lock))
+		return -EBUSY;
 
 	cpu = mp_register_lapic(apic_id, 1);
 
diff --git a/xen/common/sysctl.c b/xen/common/sysctl.c
index dbe6fc1083..1799eb3a94 100644
--- a/xen/common/sysctl.c
+++ b/xen/common/sysctl.c
@@ -48,7 +48,11 @@ long do_sysctl(XEN_GUEST_HANDLE(xen_sysctl_t) u_sysctl)
     if ( op->interface_version != XEN_SYSCTL_INTERFACE_VERSION )
         return -EACCES;
 
-    spin_lock(&sysctl_lock);
+    /* spin_trylock() avoids deadlock with stop_machine_run(). */
+    while ( !spin_trylock(&sysctl_lock) )
+        if ( hypercall_preempt_check() )
+            return hypercall_create_continuation(
+                __HYPERVISOR_sysctl, "h", u_sysctl);
 
     switch ( op->cmd )
     {