The patch titled
Subject: CPU hotplug, debug: detect imbalance between get_online_cpus() and put_online_cpus()
has been added to the -mm tree. Its filename is
cpu-hotplug-debug-detect-imbalance-between-get_online_cpus-and-put_online_cpus.patch
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From: "Srivatsa S. Bhat" <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
Subject: CPU hotplug, debug: detect imbalance between get_online_cpus() and put_online_cpus()
The synchronization between CPU hotplug readers and writers is achieved by
means of refcounting, safeguarded by the cpu_hotplug.lock.
get_online_cpus() increments the refcount, whereas put_online_cpus()
decrements it. If we ever hit an imbalance between the two, we end up
compromising the guarantees of the hotplug synchronization i.e, for
example, an extra call to put_online_cpus() can end up allowing a hotplug
reader to execute concurrently with a hotplug writer. So, add a WARN_ON()
in put_online_cpus() to detect such cases where the refcount can go
negative, and also attempt to fix it up, so that we can continue to run.
Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
Reviewed-by: Yasuaki Ishimatsu <isimatu.yasuaki@xxxxxxxxxxxxxx>
Cc: Jiri Kosina <jkosina@xxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
kernel/cpu.c | 4 ++++
1 file changed, 4 insertions(+)
diff -puN kernel/cpu.c~cpu-hotplug-debug-detect-imbalance-between-get_online_cpus-and-put_online_cpus kernel/cpu.c
--- a/kernel/cpu.c~cpu-hotplug-debug-detect-imbalance-between-get_online_cpus-and-put_online_cpus
+++ a/kernel/cpu.c
@@ -80,6 +80,10 @@ void put_online_cpus(void)
if (cpu_hotplug.active_writer == current)
return;
mutex_lock(&cpu_hotplug.lock);
+
+ if (WARN_ON(!cpu_hotplug.refcount))
+ cpu_hotplug.refcount++; /* try to fix things up */
+
if (!--cpu_hotplug.refcount && unlikely(cpu_hotplug.active_writer))
wake_up_process(cpu_hotplug.active_writer);
mutex_unlock(&cpu_hotplug.lock);
_
Patches currently in -mm which might be from srivatsa.bhat@xxxxxxxxxxxxxxxxxx are
origin.patch
linux-next.patch
cpu-hotplug-debug-detect-imbalance-between-get_online_cpus-and-put_online_cpus.patch
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