powerpc: Fix RCU idle and hcall tracing

Tracepoints should not be called inside an rcu_idle_enter/rcu_idle_exit
region. Since pSeries calls H_CEDE in the idle loop, we were violating
this rule.

commit a7b152d5342c (powerpc: Tell RCU about idle after hcall tracing)
tried to work around it by delaying the rcu_idle_enter until after we
called the hcall tracepoint, but there are a number of issues with it.

The hcall tracepoint trampoline code is called conditionally when the
tracepoint is enabled. If the tracepoint is not enabled we never call
rcu_idle_enter. The idle_uses_rcu check was also done at compile time
which breaks multiplatform builds.

The simple fix is to avoid tracing H_CEDE and rely on other tracepoints
and the hypervisor dispatch trace log to work out if we called H_CEDE.

This fixes a hang during boot on pSeries.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

1 files changed, 10 insertions, 4 deletions

diff --git a/arch/powerpc/platforms/pseries/lpar.c b/arch/powerpc/platforms/pseries/lpar.c
index 948e0e3b3547..7bc73af6c7b9 100644
--- a/arch/powerpc/platforms/pseries/lpar.c
+++ b/arch/powerpc/platforms/pseries/lpar.c
@@ -546,6 +546,13 @@ void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
         unsigned long flags;
         unsigned int *depth;
 
+        /*
+         * We cannot call tracepoints inside RCU idle regions which
+         * means we must not trace H_CEDE.
+         */
+        if (opcode == H_CEDE)
+                return;
+
         local_irq_save(flags);
 
         depth = &__get_cpu_var(hcall_trace_depth);
@@ -556,8 +563,6 @@ void __trace_hcall_entry(unsigned long opcode, unsigned long *args)
         (*depth)++;
         preempt_disable();
         trace_hcall_entry(opcode, args);
-        if (opcode == H_CEDE)
-                rcu_idle_enter();
         (*depth)--;
 
 out:
@@ -570,6 +575,9 @@ void __trace_hcall_exit(long opcode, unsigned long retval,
         unsigned long flags;
         unsigned int *depth;
 
+        if (opcode == H_CEDE)
+                return;
+
         local_irq_save(flags);
 
         depth = &__get_cpu_var(hcall_trace_depth);
@@ -578,8 +586,6 @@ void __trace_hcall_exit(long opcode, unsigned long retval,
                 goto out;
 
         (*depth)++;
-        if (opcode == H_CEDE)
-                rcu_idle_exit();
         trace_hcall_exit(opcode, retval, retbuf);
         preempt_enable();
         (*depth)--;