Balance atomic/!atomic migrate_enable calls in futex_[un]lock_pi.
The clever use of migrate_disable/enable in rt patch
"futex: workaround migrate_disable/enable in different"
has balanced atomic/!atomic context only for the rt kernel.
This workaround makes it balanced for both rt and !rt.
The 'solution' presented here is for reference only.
A better solution might be for !rt to go back to using
migrate_enable/disable == preempt_enable/disable.
This patch passes the futex selftests for rt and !rt.
Sample kernel splat, edited for brevity. This happens
near the end of boot on a CentOS 7 installation.
WARNING: CPU: 1 PID: 5966 at kernel/sched/core.c:6994 migrate_enable+0x24e/0x2f0
CPU: 1 PID: 5966 Comm: threaded-ml Not tainted 4.14.40-rt31 #1
Hardware name: Supermicro X9DRL-3F/iF/X9DRL-3F/iF, BIOS 3.2 09/22/2015
task: ffff88046b67a6c0 task.stack: ffffc900053a0000
RIP: 0010:migrate_enable+0x24e/0x2f0
RSP: 0018:ffffc900053a3df8 EFLAGS: 00010246
Call Trace:
futex_unlock_pi+0x134/0x210
do_futex+0x13f/0x190
SyS_futex+0x6e/0x150
do_syscall_64+0x6f/0x190
entry_SYSCALL_64_after_hwframe+0x42/0xb7
WARNING: CPU: 1 PID: 5966 at kernel/sched/core.c:6998 migrate_enable+0x75/0x2f0
CPU: 1 PID: 5966 Comm: threaded-ml Tainted: G W 4.14.40-rt31 #1
Hardware name: Supermicro X9DRL-3F/iF/X9DRL-3F/iF, BIOS 3.2 09/22/2015
task: ffff88046b67a6c0 task.stack: ffffc900053a0000
RIP: 0010:migrate_enable+0x75/0x2f0
RSP: 0018:ffffc900053a3df8 EFLAGS: 00010246
Call Trace:
futex_unlock_pi+0x134/0x210
do_futex+0x13f/0x190
SyS_futex+0x6e/0x150
do_syscall_64+0x6f/0x190
entry_SYSCALL_64_after_hwframe+0x42/0xb7
This patch was developed against 4.14.40-rt31. Should be
applicatible to all rt releases in which migrate_enable !=
preempt_enable for !rt kernels.
Signed-off-by: Joe Korty <joe.korty@xxxxxxxxxxxxxxxxx>
Index: b/kernel/futex.c
===================================================================
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2838,7 +2838,14 @@ retry_private:
spin_unlock(q.lock_ptr);
ret = __rt_mutex_start_proxy_lock(&q.pi_state->pi_mutex, &rt_waiter, current);
raw_spin_unlock_irq(&q.pi_state->pi_mutex.wait_lock);
+#ifdef CONFIG_PREEMPT_RT_FULL
migrate_enable();
+#else
+ /* !rt has to force balanced atomic/!atomic migrate_enable/disable uses */
+ preempt_disable();
+ migrate_enable();
+ preempt_enable();
+#endif
if (ret) {
if (ret == 1)
@@ -2998,7 +3005,14 @@ retry:
/* drops pi_state->pi_mutex.wait_lock */
ret = wake_futex_pi(uaddr, uval, pi_state);
+#ifdef CONFIG_PREEMPT_RT_FULL
+ migrate_enable();
+#else
+ /* !rt has to force balanced atomic/!atomic uses */
+ preempt_disable();
migrate_enable();
+ preempt_enable();
+#endif
put_pi_state(pi_state);
--
To unsubscribe from this list: send the line "unsubscribe linux-rt-users" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
