[PATCH] hrtimer: Don't reinitialize a cpu_base's lock on CPU_UP

From: Michael Bohan
Date: Tue Mar 19 2013 - 22:19:53 EST

The current code makes the assumption that a cpu_base lock won't
be held if the CPU corresponding to that cpu_base is offline,
which isn't always true.

If a hrtimer is not queued, then it will not be migrated by
migrate_hrtimers() when a CPU is offlined. Therefore, the
hrtimer's cpu_base may still point to a CPU which has
subsequently gone offline if the timer wasn't enqueued at the
time the CPU went down.

Normally this wouldn't be a problem, but a cpu_base's lock is
blindly reinitialized each time a CPU is brought up. If a CPU is
brought online during the period that another thread is
performing a hrtimer operation on a stale hrtimer, then the lock
will be reinitialized under its feet, and a SPIN_BUG() like the
following will be observed:

<0>[ 28.082085] BUG: spinlock already unlocked on CPU#0, swapper/0/0
<0>[ 28.087078] lock: 0xc4780b40, value 0x0 .magic: dead4ead, .owner: <none>/-1, .owner_cpu: -1
<4>[ 42.451150] [<c0014398>] (unwind_backtrace+0x0/0x120) from [<c0269220>] (do_raw_spin_unlock+0x44/0xdc)
<4>[ 42.460430] [<c0269220>] (do_raw_spin_unlock+0x44/0xdc) from [<c071b5bc>] (_raw_spin_unlock+0x8/0x30)
<4>[ 42.469632] [<c071b5bc>] (_raw_spin_unlock+0x8/0x30) from [<c00a9ce0>] (__hrtimer_start_range_ns+0x1e4/0x4f8)
<4>[ 42.479521] [<c00a9ce0>] (__hrtimer_start_range_ns+0x1e4/0x4f8) from [<c00aa014>] (hrtimer_start+0x20/0x28)
<4>[ 42.489247] [<c00aa014>] (hrtimer_start+0x20/0x28) from [<c00e6190>] (rcu_idle_enter_common+0x1ac/0x320)
<4>[ 42.498709] [<c00e6190>] (rcu_idle_enter_common+0x1ac/0x320) from [<c00e6440>] (rcu_idle_enter+0xa0/0xb8)
<4>[ 42.508259] [<c00e6440>] (rcu_idle_enter+0xa0/0xb8) from [<c000f268>] (cpu_idle+0x24/0xf0)
<4>[ 42.516503] [<c000f268>] (cpu_idle+0x24/0xf0) from [<c06ed3c0>] (rest_init+0x88/0xa0)
<4>[ 42.524319] [<c06ed3c0>] (rest_init+0x88/0xa0) from [<c0c00978>] (start_kernel+0x3d0/0x434)

As an example, this particular crash occurred when
hrtimer_start() was executed on CPU #0. The code locked the
hrtimer's current cpu_base corresponding to CPU #1. CPU #0
then tried to switch the hrtimer's cpu_base to an optimal CPU
which was online. In this case, it selected the cpu_base
corresponding to CPU #3.

Before it could proceed, CPU #1 came online and reinitialized the
spinlock corresponding to its cpu_base. Thus now CPU #0 held a
lock which was reinitialized. When CPU #0 finally ended up
unlocking the old cpu_base corresponding to CPU #1 so that it
could switch to CPU #3, we hit this SPIN_BUG() above while in

CPU #0 CPU #1
---- ----
... <offline>
lock_hrtimer_base(base #1)
... init_hrtimers_cpu()
switch_hrtimer_base() ...
... raw_spin_lock_init(&cpu_base->lock)
raw_spin_unlock(&cpu_base->lock) ...

Signed-off-by: Michael Bohan <mbohan@xxxxxxxxxxxxxx>
kernel/hrtimer.c | 3 +--
1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index cc47812..14be27f 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -63,6 +63,7 @@
DEFINE_PER_CPU(struct hrtimer_cpu_base, hrtimer_bases) =

+ .lock = __RAW_SPIN_LOCK_UNLOCKED(hrtimer_bases.lock),
.clock_base =
@@ -1642,8 +1643,6 @@ static void __cpuinit init_hrtimers_cpu(int cpu)
struct hrtimer_cpu_base *cpu_base = &per_cpu(hrtimer_bases, cpu);
int i;

- raw_spin_lock_init(&cpu_base->lock);
for (i = 0; i < HRTIMER_MAX_CLOCK_BASES; i++) {
cpu_base->clock_base[i].cpu_base = cpu_base;
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