powerpc/xmon: Don't loop forever in get_output_lock()

If we enter with xmon_speaker != 0 we skip the first cmpxchg(), we also
skip the while loop because xmon_speaker != last_speaker (0) - meaning we
skip the second cmpxchg() also.

Following that code path the compiler sees no memory barriers and so is
within its rights to never reload xmon_speaker. The end result is we loop
forever.

This manifests as all cpus being in xmon ('c' command), but they refuse
to take control when you switch to them ('c x' for cpu # x).

I have seen this deadlock in practice and also checked the generated code to
confirm this is what's happening.

The simplest fix is just to always try the cmpxchg().

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index a90731b3d44a3ea41052690975aefcd8b3ce2db7..598cdc7c7adc099bf9a8dd6893fa8e820c23fa8c 100644 (file)
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -309,12 +309,12 @@ static void get_output_lock(void)
 
        if (xmon_speaker == me)
                return;
+
        for (;;) {
-               if (xmon_speaker == 0) {
-                       last_speaker = cmpxchg(&xmon_speaker, 0, me);
-                       if (last_speaker == 0)
-                               return;
-               }
+               last_speaker = cmpxchg(&xmon_speaker, 0, me);
+               if (last_speaker == 0)
+                       return;
+
                timeout = 10000000;
                while (xmon_speaker == last_speaker) {
                        if (--timeout > 0)