Re: [v3 PATCH] arm64: replace read_lock to rcu lock in call_break_hook

[Paul E. McKenney]

On Mon, Oct 05, 2015 at 02:32:51PM -0700, Yang Shi wrote:
> BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:917
> in_atomic(): 0, irqs_disabled(): 128, pid: 342, name: perf
> 1 lock held by perf/342:
>  #0:  (break_hook_lock){+.+...}, at: [<ffffffc0000851ac>] call_break_hook+0x34/0xd0
> irq event stamp: 62224
> hardirqs last  enabled at (62223): [<ffffffc00010b7bc>] __call_rcu.constprop.59+0x104/0x270
> hardirqs last disabled at (62224): [<ffffffc0000fbe20>] vprintk_emit+0x68/0x640
> softirqs last  enabled at (0): [<ffffffc000097928>] copy_process.part.8+0x428/0x17f8
> softirqs last disabled at (0): [<          (null)>]           (null)
> CPU: 0 PID: 342 Comm: perf Not tainted 4.1.6-rt5 #4
> Hardware name: linux,dummy-virt (DT)
> Call trace:
> [<ffffffc000089968>] dump_backtrace+0x0/0x128
> [<ffffffc000089ab0>] show_stack+0x20/0x30
> [<ffffffc0007030d0>] dump_stack+0x7c/0xa0
> [<ffffffc0000c878c>] ___might_sleep+0x174/0x260
> [<ffffffc000708ac8>] __rt_spin_lock+0x28/0x40
> [<ffffffc000708db0>] rt_read_lock+0x60/0x80
> [<ffffffc0000851a8>] call_break_hook+0x30/0xd0
> [<ffffffc000085a70>] brk_handler+0x30/0x98
> [<ffffffc000082248>] do_debug_exception+0x50/0xb8
> Exception stack(0xffffffc00514fe30 to 0xffffffc00514ff50)
> fe20:                                     00000000 00000000 c1594680 0000007f
> fe40: ffffffff ffffffff 92063940 0000007f 0550dcd8 ffffffc0 00000000 00000000
> fe60: 0514fe70 ffffffc0 000be1f8 ffffffc0 0514feb0 ffffffc0 0008948c ffffffc0
> fe80: 00000004 00000000 0514fed0 ffffffc0 ffffffff ffffffff 9282a948 0000007f
> fea0: 00000000 00000000 9282b708 0000007f c1592820 0000007f 00083914 ffffffc0
> fec0: 00000000 00000000 00000010 00000000 00000064 00000000 00000001 00000000
> fee0: 005101e0 00000000 c1594680 0000007f c1594740 0000007f ffffffd8 ffffff80
> ff00: 00000000 00000000 00000000 00000000 c1594770 0000007f c1594770 0000007f
> ff20: 00665e10 00000000 7f7f7f7f 7f7f7f7f 01010101 01010101 00000000 00000000
> ff40: 928e4cc0 0000007f 91ff11e8 0000007f
> 
> call_break_hook is called in atomic context (hard irq disabled), so replace
> the sleepable lock to rcu lock, replace relevant list operations to rcu
> version and call synchronize_rcu() in unregister_break_hook().
> 
> And, replace write lock to spinlock in {un}register_break_hook.
> 
> Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxx>

Looks more plausible to me.  Does it look OK to you, Steven?

							Thanx, Paul

> ---
> v2 -> v3
> Add synchronize_rcu() in ungister_break_hook()
> Replace write lock to spinlock
> 
> v1 -> v2
> Replace list operations to rcu version.
> 
>  arch/arm64/kernel/debug-monitors.c | 21 +++++++++++----------
>  1 file changed, 11 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> index cebf786..13ca9cd 100644
> --- a/arch/arm64/kernel/debug-monitors.c
> +++ b/arch/arm64/kernel/debug-monitors.c
> @@ -271,20 +271,21 @@ static int single_step_handler(unsigned long addr, unsigned int esr,
>   * Use reader/writer locks instead of plain spinlock.
>   */
>  static LIST_HEAD(break_hook);
> -static DEFINE_RWLOCK(break_hook_lock);
> +static DEFINE_SPINLOCK(break_hook_lock);
> 
>  void register_break_hook(struct break_hook *hook)
>  {
> -	write_lock(&break_hook_lock);
> -	list_add(&hook->node, &break_hook);
> -	write_unlock(&break_hook_lock);
> +	spin_lock(&break_hook_lock);
> +	list_add_rcu(&hook->node, &break_hook);
> +	spin_unlock(&break_hook_lock);
>  }
> 
>  void unregister_break_hook(struct break_hook *hook)
>  {
> -	write_lock(&break_hook_lock);
> -	list_del(&hook->node);
> -	write_unlock(&break_hook_lock);
> +	spin_lock(&break_hook_lock);
> +	list_del_rcu(&hook->node);
> +	spin_unlock(&break_hook_lock);
> +	synchronize_rcu();
>  }
> 
>  static int call_break_hook(struct pt_regs *regs, unsigned int esr)
> @@ -292,11 +293,11 @@ static int call_break_hook(struct pt_regs *regs, unsigned int esr)
>  	struct break_hook *hook;
>  	int (*fn)(struct pt_regs *regs, unsigned int esr) = NULL;
> 
> -	read_lock(&break_hook_lock);
> -	list_for_each_entry(hook, &break_hook, node)
> +	rcu_read_lock();
> +	list_for_each_entry_rcu(hook, &break_hook, node)
>  		if ((esr & hook->esr_mask) == hook->esr_val)
>  			fn = hook->fn;
> -	read_unlock(&break_hook_lock);
> +	rcu_read_unlock();
> 
>  	return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
>  }
> -- 
> 2.0.2
> 

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