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@xxxxxxxxxxxxxxx
Cc: Naresh Kamboju <naresh.kamboju@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Will Deacon <will@xxxxxxxxxx>
Signed-off-by: Paul Gortmaker <paul.gortmaker@xxxxxxxxxxxxx>
---
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
