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@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210923165358.305969211@linutronix.de

diff --git a/include/linux/preempt.h b/include/linux/preempt.h
index 4d244e2..031898b 100644 (file)
--- 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 (file)
--- 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)