smp: convert cpu_hotplug_begin into a blocking lock acquisition

Don't allow cpu_hotplug_begin to fail by converting the trylock into a
blocking lock acquisition. Write users of the cpu_add_remove_lock are
limited to CPU plug/unplug operations, and cannot deadlock between
themselves or other users taking the lock in read mode as
cpu_add_remove_lock is always locked with interrupts enabled. There
are also no other locks taken during the plug/unplug operations.

The exclusive lock usage in register_cpu_notifier is also converted
into a blocking lock acquisition, as it was previously not allowed to
fail anyway.

This is meaningful when running Xen in shim mode, since VCPU_{up/down}
hypercalls use cpu hotplug/unplug operations in the background, and
hence failing to take the lock results in VPCU_{up/down} failing with
-EBUSY, which most users are not prepared to handle.

Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/smpboot.c

@@ -1255,8 +1255,7 @@ int cpu_add(uint32_t apic_id, uint32_t acpi_id, uint32_t pxm)
          (pxm >= 256) )
         return -EINVAL;
 
-    if ( !cpu_hotplug_begin() )
-        return -EBUSY;
+    cpu_hotplug_begin();
 
     /* Detect if the cpu has been added before */
     if ( x86_acpiid_to_apicid[acpi_id] != BAD_APICID )

xen/common/cpu.c

@@ -51,9 +51,9 @@ void put_cpu_maps(void)
     read_unlock(&cpu_add_remove_lock);
 }
 
-bool cpu_hotplug_begin(void)
+void cpu_hotplug_begin(void)
 {
-    return write_trylock(&cpu_add_remove_lock);
+    write_lock(&cpu_add_remove_lock);
 }
 
 void cpu_hotplug_done(void)
@@ -65,8 +65,7 @@ static NOTIFIER_HEAD(cpu_chain);
 
 void __init register_cpu_notifier(struct notifier_block *nb)
 {
-    if ( !write_trylock(&cpu_add_remove_lock) )
-        BUG(); /* Should never fail as we are called only during boot. */
+    write_lock(&cpu_add_remove_lock);
     notifier_chain_register(&cpu_chain, nb);
     write_unlock(&cpu_add_remove_lock);
 }
@@ -100,8 +99,7 @@ int cpu_down(unsigned int cpu)
     int err;
     struct notifier_block *nb = NULL;
 
-    if ( !cpu_hotplug_begin() )
-        return -EBUSY;
+    cpu_hotplug_begin();
 
     err = -EINVAL;
     if ( (cpu >= nr_cpu_ids) || (cpu == 0) )
@@ -142,8 +140,7 @@ int cpu_up(unsigned int cpu)
     int err;
     struct notifier_block *nb = NULL;
 
-    if ( !cpu_hotplug_begin() )
-        return -EBUSY;
+    cpu_hotplug_begin();
 
     err = -EINVAL;
     if ( (cpu >= nr_cpu_ids) || !cpu_present(cpu) )

xen/include/xen/cpu.h

@@ -10,7 +10,7 @@ bool get_cpu_maps(void);
 void put_cpu_maps(void);
 
 /* Safely perform CPU hotplug and update cpu_online_map, etc. */
-bool cpu_hotplug_begin(void);
+void cpu_hotplug_begin(void);
 void cpu_hotplug_done(void);
 
 /* Receive notification of CPU hotplug events. */