[PATCH] timer: Don't reinitialize the cpu base lock during CPU_UP_PREPARE

[Tirupathi Reddy]

An inactive timer's base can refer to a offline cpu's base.

In the current code, 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 trying to modify an
inactive timer on that CPU with holding its timer base lock,
then the lock will be reinitialized under its feet. This leads
to following SPIN_BUG().

<0> BUG: spinlock already unlocked on CPU#3, kworker/u:3/1466
<0> lock: 0xe3ebe000, .magic: dead4ead, .owner: kworker/u:3/1466, .owner_cpu: 1
<4> [<c0013dc4>] (unwind_backtrace+0x0/0x11c) from [<c026e794>] (do_raw_spin_unlock+0x40/0xcc)
<4> [<c026e794>] (do_raw_spin_unlock+0x40/0xcc) from [<c076c160>] (_raw_spin_unlock+0x8/0x30)
<4> [<c076c160>] (_raw_spin_unlock+0x8/0x30) from [<c009b858>] (mod_timer+0x294/0x310)
<4> [<c009b858>] (mod_timer+0x294/0x310) from [<c00a5e04>] (queue_delayed_work_on+0x104/0x120)
<4> [<c00a5e04>] (queue_delayed_work_on+0x104/0x120) from [<c04eae00>] (sdhci_msm_bus_voting+0x88/0x9c)
<4> [<c04eae00>] (sdhci_msm_bus_voting+0x88/0x9c) from [<c04d8780>] (sdhci_disable+0x40/0x48)
<4> [<c04d8780>] (sdhci_disable+0x40/0x48) from [<c04bf300>] (mmc_release_host+0x4c/0xb0)
<4> [<c04bf300>] (mmc_release_host+0x4c/0xb0) from [<c04c7aac>] (mmc_sd_detect+0x90/0xfc)
<4> [<c04c7aac>] (mmc_sd_detect+0x90/0xfc) from [<c04c2504>] (mmc_rescan+0x7c/0x2c4)
<4> [<c04c2504>] (mmc_rescan+0x7c/0x2c4) from [<c00a6a7c>] (process_one_work+0x27c/0x484)
<4> [<c00a6a7c>] (process_one_work+0x27c/0x484) from [<c00a6e94>] (worker_thread+0x210/0x3b0)
<4> [<c00a6e94>] (worker_thread+0x210/0x3b0) from [<c00aad9c>] (kthread+0x80/0x8c)
<4> [<c00aad9c>] (kthread+0x80/0x8c) from [<c000ea80>] (kernel_thread_exit+0x0/0x8)

As an example, this particular crash occurred when CPU #3 is executing
mod_timer() on an inactive timer whose base is refered to offlined CPU #2.
The code locked the timer_base corresponding to CPU #2. Before it could
proceed, CPU #2 came online and reinitialized the spinlock corresponding
to its base. Thus now CPU #3 held a lock which was reinitialized. When
CPU #3 finally ended up unlocking the old cpu_base corresponding to CPU #2,
we hit the above SPIN_BUG().

CPU #0			CPU #3				       CPU #2
------			-------				       -------
.....			 ......				      <Offline>
			mod_timer()
			 lock_timer_base
			   spin_lock_irqsave(&base->lock)

cpu_up(2)		 .....				        ......
								init_timers_cpu()
....			 .....				    	  spin_lock_init(&base->lock)
.....		 	 spin_unlock_irqrestore(&base->lock)    ......
			   <spin_bug>

Allocation of per_cpu timer vector bases is done only once under
"tvec_base_done[]" check. In the current code, spinlock_initialization
of base->lock isn't under this check. When a CPU is up each time the base
lock is reinitialized. Move base spinlock initialization under the check.

Signed-off-by: Tirupathi Reddy <tirupath@xxxxxxxxxxxxxx>
---
 kernel/timer.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/timer.c b/kernel/timer.c
index dbf7a78..1b399c8 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -1678,12 +1678,12 @@ static int __cpuinit init_timers_cpu(int cpu)
 			boot_done = 1;
 			base = &boot_tvec_bases;
 		}
+		spin_lock_init(&base->lock);
 		tvec_base_done[cpu] = 1;
 	} else {
 		base = per_cpu(tvec_bases, cpu);
 	}

-	spin_lock_init(&base->lock);

 	for (j = 0; j < TVN_SIZE; j++) {
 		INIT_LIST_HEAD(base->tv5.vec + j);
--
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