[Xen-changelog] [xen staging] xen/cpu: Distinguish "cpu already in that state" in cpu_{up, down}()

[patchbot@xxxxxxx]

commit ab0e50c03f01c51c091f232d1a096a90085c2a24
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Tue Apr 2 14:21:56 2019 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Thu Apr 4 14:32:41 2019 +0100

    xen/cpu: Distinguish "cpu already in that state" in cpu_{up,down}()
    
    All methods of querying the online state of a CPU are racy without the 
hotplug
    lock held, which can lead to a TOCTOU race trying to online or offline CPUs.
    
    Distinguish this case with -EEXIST rather than -EINVAL, so the caller can 
take
    other actions if necessary.
    
    While adjusting this, rework the code slightly to fold the exit paths, which
    results in a minor reduction in compiled code size.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/common/cpu.c | 26 ++++++++++++++++----------
 1 file changed, 16 insertions(+), 10 deletions(-)

diff --git a/xen/common/cpu.c b/xen/common/cpu.c
index a6efc5e604..f388d8977d 100644
--- a/xen/common/cpu.c
+++ b/xen/common/cpu.c
@@ -103,11 +103,13 @@ int cpu_down(unsigned int cpu)
     if ( !cpu_hotplug_begin() )
         return -EBUSY;
 
-    if ( (cpu >= nr_cpu_ids) || (cpu == 0) || !cpu_online(cpu) )
-    {
-        cpu_hotplug_done();
-        return -EINVAL;
-    }
+    err = -EINVAL;
+    if ( (cpu >= nr_cpu_ids) || (cpu == 0) )
+        goto out;
+
+    err = -EEXIST;
+    if ( !cpu_online(cpu) )
+        goto out;
 
     err = cpu_notifier_call_chain(cpu, CPU_DOWN_PREPARE, &nb, false);
     if ( err )
@@ -130,6 +132,7 @@ int cpu_down(unsigned int cpu)
 
  fail:
     cpu_notifier_call_chain(cpu, CPU_DOWN_FAILED, &nb, true);
+ out:
     cpu_hotplug_done();
     return err;
 }
@@ -142,11 +145,13 @@ int cpu_up(unsigned int cpu)
     if ( !cpu_hotplug_begin() )
         return -EBUSY;
 
-    if ( (cpu >= nr_cpu_ids) || cpu_online(cpu) || !cpu_present(cpu) )
-    {
-        cpu_hotplug_done();
-        return -EINVAL;
-    }
+    err = -EINVAL;
+    if ( (cpu >= nr_cpu_ids) || !cpu_present(cpu) )
+        goto out;
+
+    err = -EEXIST;
+    if ( cpu_online(cpu) )
+        goto out;
 
     err = cpu_notifier_call_chain(cpu, CPU_UP_PREPARE, &nb, false);
     if ( err )
@@ -165,6 +170,7 @@ int cpu_up(unsigned int cpu)
 
  fail:
     cpu_notifier_call_chain(cpu, CPU_UP_CANCELED, &nb, true);
+ out:
     cpu_hotplug_done();
     return err;
 }
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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