[Xen-changelog] [xen master] smp: convert cpu_hotplug_begin into a blocking lock acquisition

[patchbot@xxxxxxx]

commit 5b74f5e42884a01646e9ecf12d402ca20d3ddf7a
Author:     Roger Pau Monné <roger.pau@xxxxxxxxxx>
AuthorDate: Wed Feb 26 10:55:22 2020 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Wed Feb 26 10:55:22 2020 +0100

    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@xxxxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/smpboot.c |  3 +--
 xen/common/cpu.c       | 13 +++++--------
 xen/include/xen/cpu.h  |  2 +-
 3 files changed, 7 insertions(+), 11 deletions(-)

diff --git a/xen/arch/x86/smpboot.c b/xen/arch/x86/smpboot.c
index e83e4564a4..ad49f2dcd7 100644
--- a/xen/arch/x86/smpboot.c
+++ b/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 )
diff --git a/xen/common/cpu.c b/xen/common/cpu.c
index 0d7a10878c..31953f32e4 100644
--- a/xen/common/cpu.c
+++ b/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) )
diff --git a/xen/include/xen/cpu.h b/xen/include/xen/cpu.h
index e49172f64c..e8eeb217a0 100644
--- a/xen/include/xen/cpu.h
+++ b/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. */
--
generated by git-patchbot for /home/xen/git/xen.git#master

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