[Qemu-devel] [RFC 2/2] linux-user: Fix cpu_index generation

[David Gibson <david@gibson.dropbear.id.au>]

With CONFIG_USER_ONLY, generation of cpu_index values is done differently
than for full system targets.  This method turns out to be broken, since
it can fairly easily result in duplicate cpu_index values for
simultaneously active cpus (i.e. threads in the emulated process).

Consider this sequence:
    Create thread 1
    Create thread 2
    Exit thread 1
    Create thread 3

With the current logic thread 1 will get cpu_index 1, thread 2 will get
cpu_index 2 and thread 3 will also get cpu_index 2 (because there are 2
threads in the cpus list at the point of its creation).

We mostly get away with this because cpu_index values aren't that important
for userspace emulation.  Still, it can't be good, so this patch fixes it
by making CONFIG_USER_ONLY use the same bitmap based allocation that full
system targets already use.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
---
 exec.c | 19 -------------------
 1 file changed, 19 deletions(-)

diff --git a/exec.c b/exec.c
index 011babd..e410dab 100644
--- a/exec.c
+++ b/exec.c
@@ -596,7 +596,6 @@ AddressSpace *cpu_get_address_space(CPUState *cpu, int asidx)
 }
 #endif
 
-#ifndef CONFIG_USER_ONLY
 static DECLARE_BITMAP(cpu_index_map, MAX_CPUMASK_BITS);
 
 static int cpu_get_free_index(Error **errp)
@@ -617,24 +616,6 @@ static void cpu_release_index(CPUState *cpu)
 {
     bitmap_clear(cpu_index_map, cpu->cpu_index, 1);
 }
-#else
-
-static int cpu_get_free_index(Error **errp)
-{
-    CPUState *some_cpu;
-    int cpu_index = 0;
-
-    CPU_FOREACH(some_cpu) {
-        cpu_index++;
-    }
-    return cpu_index;
-}
-
-static void cpu_release_index(CPUState *cpu)
-{
-    return;
-}
-#endif
 
 void cpu_exec_exit(CPUState *cpu)
 {
-- 
2.7.4