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authorMichael Ellerman <michael@ellerman.id.au>2013-12-23 23:46:04 +1100
committerBenjamin Herrenschmidt <benh@kernel.crashing.org>2014-02-11 11:24:50 +1100
commit730efb6193f8568354fd80849612291afa9fa81e (patch)
treeb66d851be73038d03b3322442f720d04138ff97b
parentb4d6c06c8de81517320a9964b0c26e096aeadd7b (diff)
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>
-rw-r--r--arch/powerpc/xmon/xmon.c10
1 files changed, 5 insertions, 5 deletions
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index a90731b3d44a..598cdc7c7adc 100644
--- 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)

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