[PATCH] arm64: don't preempt_disable in do_debug_exception

[Paul Gortmaker <paul.gortmaker@windriver.com>]

In commit d8bb6718c4db ("arm64: Make debug exception handlers visible
from RCU") debug_exception_enter and exit were added to deal with better
tracking of RCU state - however, in addition to that, but not mentioned
in the commit log, a preempt_disable/enable pair were also added.

Based on the comment (being removed here) it would seem that the pair
were not added to address a specific problem, but just as a proactive,
preventative measure - as in "seemed like a good idea at the time".

The problem is that do_debug_exception() eventually calls out to
generic kernel code like do_force_sig_info() which takes non-raw locks
and on -rt enabled kernels, results in things looking like the following,
since on -rt kernels, it is noticed that preemption is still disabled.

 BUG: sleeping function called from invalid context at kernel/locking/rtmutex.c:975
 in_atomic(): 1, irqs_disabled(): 0, pid: 35658, name: gdbtest
 Preemption disabled at:
 [<ffff000010081578>] do_debug_exception+0x38/0x1a4
 Call trace:
 dump_backtrace+0x0/0x138
 show_stack+0x24/0x30
 dump_stack+0x94/0xbc
 ___might_sleep+0x13c/0x168
 rt_spin_lock+0x40/0x80
 do_force_sig_info+0x30/0xe0
 force_sig_fault+0x64/0x90
 arm64_force_sig_fault+0x50/0x80
 send_user_sigtrap+0x50/0x80
 brk_handler+0x98/0xc8
 do_debug_exception+0x70/0x1a4
 el0_dbg+0x18/0x20

The reproducer was basically an automated gdb test that set a breakpoint
on a simple "hello world" program and then quit gdb once the breakpoint
was hit - i.e. "(gdb) A debugging session is active.  Quit anyway? "

Fixes: d8bb6718c4db ("arm64: Make debug exception handlers visible from RCU")
Cc: stable@vger.kernel.org
Cc: Naresh Kamboju <naresh.kamboju@linaro.org>
Cc: Paul E. McKenney <paulmck@linux.ibm.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Will Deacon <will@kernel.org>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
---
 arch/arm64/mm/fault.c | 11 -----------
 1 file changed, 11 deletions(-)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 8afb238ff335..4d83ec991b33 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -788,13 +788,6 @@ void __init hook_debug_fault_code(int nr,
 	debug_fault_info[nr].name	= name;
 }
 
-/*
- * In debug exception context, we explicitly disable preemption despite
- * having interrupts disabled.
- * This serves two purposes: it makes it much less likely that we would
- * accidentally schedule in exception context and it will force a warning
- * if we somehow manage to schedule by accident.
- */
 static void debug_exception_enter(struct pt_regs *regs)
 {
 	/*
@@ -816,8 +809,6 @@ static void debug_exception_enter(struct pt_regs *regs)
 		rcu_nmi_enter();
 	}
 
-	preempt_disable();
-
 	/* This code is a bit fragile.  Test it. */
 	RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
 }
@@ -825,8 +816,6 @@ NOKPROBE_SYMBOL(debug_exception_enter);
 
 static void debug_exception_exit(struct pt_regs *regs)
 {
-	preempt_enable_no_resched();
-
 	if (!user_mode(regs))
 		rcu_nmi_exit();
 
-- 
2.7.4