[PATCH] x86_64: Fix race in TSC synchronization

Plug a race in TSC synchronization

We need to do tsc_sync_wait() before the CPU is set online to prevent
multiple CPUs from doing it in parallel - which won't work because TSC
sync has global unprotected state.

Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

diff --git a/arch/x86_64/kernel/smpboot.c b/arch/x86_64/kernel/smpboot.c
index b15761f..fa25e39 100644 (file)
--- a/arch/x86_64/kernel/smpboot.c
+++ b/arch/x86_64/kernel/smpboot.c
@@ -492,6 +492,14 @@ void __cpuinit start_secondary(void)
         */
        set_cpu_sibling_map(smp_processor_id());
 
+       /* 
+        * Wait for TSC sync to not schedule things before.
+        * We still process interrupts, which could see an inconsistent
+        * time in that window unfortunately. 
+        * Do this here because TSC sync has global unprotected state.
+        */
+       tsc_sync_wait();
+
        /*
         * We need to hold call_lock, so there is no inconsistency
         * between the time smp_call_function() determines number of
@@ -509,13 +517,6 @@ void __cpuinit start_secondary(void)
        per_cpu(cpu_state, smp_processor_id()) = CPU_ONLINE;
        unlock_ipi_call_lock();
 
-       mb();
-
-       /* Wait for TSC sync to not schedule things before.
-          We still process interrupts, which could see an inconsistent
-          time in that window unfortunately. */
-       tsc_sync_wait();
-
        cpu_idle();
 }