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>

1 files changed, 11 insertions, 2 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) ||