The following commit has been merged into the locking/core branch of tip:
Commit-ID: 3e9cc688e56cc2abb9b6067f57c8397f6c96d42c
Gitweb: https://git.kernel.org/tip/3e9cc688e56cc2abb9b6067f57c8397f6c96d42c
Author: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate: Thu, 23 Sep 2021 18:54:44 +02:00
Committer: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Fri, 01 Oct 2021 13:57:51 +02:00
sched: Make cond_resched_lock() variants RT aware
The __might_resched() checks in the cond_resched_lock() variants use
PREEMPT_LOCK_OFFSET for preempt count offset checking which takes the
preemption disable by the spin_lock() which is still held at that point
into account.
On PREEMPT_RT enabled kernels spin/rw_lock held sections stay preemptible
which means PREEMPT_LOCK_OFFSET is 0, but that still triggers the
__might_resched() check because that takes RCU read side nesting into
account.
On RT enabled kernels spin/read/write_lock() issue rcu_read_lock() to
resemble the !RT semantics, which means in cond_resched_lock() the might
resched check will see preempt_count() == 0 and rcu_preempt_depth() == 1.
Introduce PREEMPT_LOCK_SCHED_OFFSET for those might resched checks and map
them depending on CONFIG_PREEMPT_RT.
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20210923165358.305969211@xxxxxxxxxxxxx
---
include/linux/preempt.h | 5 +++--
include/linux/sched.h | 34 +++++++++++++++++++++++++---------
2 files changed, 28 insertions(+), 11 deletions(-)
diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 4d244e2..031898b 100644
--- a/include/linux/preempt.h
+++ b/include/linux/preempt.h
@@ -122,9 +122,10 @@
* The preempt_count offset after spin_lock()
*/
#if !defined(CONFIG_PREEMPT_RT)
-#define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET
+#define PREEMPT_LOCK_OFFSET PREEMPT_DISABLE_OFFSET
#else
-#define PREEMPT_LOCK_OFFSET 0
+/* Locks on RT do not disable preemption */
+#define PREEMPT_LOCK_OFFSET 0
#endif
/*
diff --git a/include/linux/sched.h b/include/linux/sched.h
index b448c74..21b7cd0 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -2049,19 +2049,35 @@ extern int __cond_resched_rwlock_write(rwlock_t *lock);
#define MIGHT_RESCHED_RCU_SHIFT 8
#define MIGHT_RESCHED_PREEMPT_MASK ((1U << MIGHT_RESCHED_RCU_SHIFT) - 1)
-#define cond_resched_lock(lock) ({ \
- __might_resched(__FILE__, __LINE__, PREEMPT_LOCK_OFFSET); \
- __cond_resched_lock(lock); \
+#ifndef CONFIG_PREEMPT_RT
+/*
+ * Non RT kernels have an elevated preempt count due to the held lock,
+ * but are not allowed to be inside a RCU read side critical section
+ */
+# define PREEMPT_LOCK_RESCHED_OFFSETS PREEMPT_LOCK_OFFSET
+#else
+/*
+ * spin/rw_lock() on RT implies rcu_read_lock(). The might_sleep() check in
+ * cond_resched*lock() has to take that into account because it checks for
+ * preempt_count() and rcu_preempt_depth().
+ */
+# define PREEMPT_LOCK_RESCHED_OFFSETS \
+ (PREEMPT_LOCK_OFFSET + (1U << MIGHT_RESCHED_RCU_SHIFT))
+#endif
+
+#define cond_resched_lock(lock) ({ \
+ __might_resched(__FILE__, __LINE__, PREEMPT_LOCK_RESCHED_OFFSETS); \
+ __cond_resched_lock(lock); \
})
-#define cond_resched_rwlock_read(lock) ({ \
- __might_resched(__FILE__, __LINE__, PREEMPT_LOCK_OFFSET); \
- __cond_resched_rwlock_read(lock); \
+#define cond_resched_rwlock_read(lock) ({ \
+ __might_resched(__FILE__, __LINE__, PREEMPT_LOCK_RESCHED_OFFSETS); \
+ __cond_resched_rwlock_read(lock); \
})
-#define cond_resched_rwlock_write(lock) ({ \
- __might_resched(__FILE__, __LINE__, PREEMPT_LOCK_OFFSET); \
- __cond_resched_rwlock_write(lock); \
+#define cond_resched_rwlock_write(lock) ({ \
+ __might_resched(__FILE__, __LINE__, PREEMPT_LOCK_RESCHED_OFFSETS); \
+ __cond_resched_rwlock_write(lock); \
})
static inline void cond_resched_rcu(void)
|