Re: [v2 PATCH] arm64: replace read_lock to rcu lock in call_break_hook

[Paul E. McKenney]

On Thu, Oct 01, 2015 at 01:53:51PM -0700, Shi, Yang wrote:
> On 10/1/2015 10:08 AM, Steven Rostedt wrote:
> >On Thu,  1 Oct 2015 09:37:37 -0700
> >Yang Shi <yang.shi@xxxxxxxxxx> 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 and replace relevant list operations to rcu
> >>version.
> >>
> >>Signed-off-by: Yang Shi <yang.shi@xxxxxxxxxx>
> >>---
> >>v1-> v2
> >>Replace list operations to rcu version.
> >>
> >>  arch/arm64/kernel/debug-monitors.c | 10 +++++-----
> >>  1 file changed, 5 insertions(+), 5 deletions(-)
> >>
> >>diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
> >>index cebf786..cf0e4fc 100644
> >>--- a/arch/arm64/kernel/debug-monitors.c
> >>+++ b/arch/arm64/kernel/debug-monitors.c
> >>@@ -276,14 +276,14 @@ static DEFINE_RWLOCK(break_hook_lock);
> >>  void register_break_hook(struct break_hook *hook)
> >>  {
> >>  	write_lock(&break_hook_lock);
> >>-	list_add(&hook->node, &break_hook);
> >>+	list_add_rcu(&hook->node, &break_hook);
> >>  	write_unlock(&break_hook_lock);
> >>  }
> >>
> >>  void unregister_break_hook(struct break_hook *hook)
> >>  {
> >>  	write_lock(&break_hook_lock);
> >>-	list_del(&hook->node);
> >>+	list_del_rcu(&hook->node);
> >>  	write_unlock(&break_hook_lock);
> >>  }
> >
> >Shouldn't there be a synchronize_rcu() somewhere?
> 
> So far kgdb is the only user of unregister_break_hook in mainline kernel.
> 
> Just read Documentation/RCU/checklist.txt, it says:
> 
> Note that synchronize_rcu() -only- guarantees to wait until
> all currently executing rcu_read_lock()-protected RCU read-side
> critical sections complete.
> 
> For kgdb, the unregister is just called in kgdb_arch_exit by
> kgdb_unregister_io_module, which is called when rmmod kgdb module.
> 
> The break point handler is done synchronously. So, it sounds should
> be not a problem without calling synchronize_rcu().

OK, I will bite...  What does "synchronously" mean here?  Unless you
have somehow guaranteed that all current readers in call_break_hook()
are done between the time you call unregister_break_hook() to remove a
given break_hook structure and the time you call register_break_hook()
to add that same structure back in, you have a problem.

What you have now only protects against invoking register_break_hook()
on newly allocated and initialized break_hook structure.  But the only
calls to register_break_hook() that I see in v4.2 use compile-time
initialized structures.  So the only failure from using non-RCU list
primitives would be due to the list_head's ->next pointer initialization.
This could momentarily make the list appear to have only the new element,
but not the old element.

Unless you do a series of register_break_hook() and unregister_break_hook()
calls, in which case a previously deleted structure could momentarily
appear to already (or still) be in the list.

Are those the sorts of failures you are seeing?

							Thanx, Paul

> Yang
> 
> >-- Steve
> >
> >>
> >>@@ -292,11 +292,11 @@ 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;
> >>  }
> >
> 

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