From 058e87782c91696020bdb0aa28ddf77d89aed266 Mon Sep 17 00:00:00 2001 From: "Uladzislau Rezki (Sony)" Date: Wed, 14 Feb 2024 14:45:53 -0800 Subject: rcu: Update lockdep while in RCU read-side critical section With Ankur's lazy-/auto-preemption patches applied and with a lazy-preemptible kernel in combination with a non-preemptible RCU, lockdep sometimes complains about context switches within RCU read-side critical sections. This is a false positive due to rcu_read_unlock() updating lockdep state too late: __release(RCU); __rcu_read_unlock(); // Context switch here results in lockdep false positive!!! rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */ Although this complaint could also happen with preemptible RCU in a preemptible kernel, the odds of that happening aer quite low. In constrast, with non-preemptible RCU, a long critical section has a high probability of performing a context switch from the preempt_enable() in __rcu_read_unlock(). The fix is straightforward, just move the rcu_lock_release() within rcu_read_unlock() to obtain the reverse order from that of rcu_read_lock(): rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */ __release(RCU); __rcu_read_unlock(); This commit makes this change. Co-developed-by: Frederic Weisbecker Signed-off-by: Frederic Weisbecker Co-developed-by: Joel Fernandes (Google) Signed-off-by: Joel Fernandes (Google) Co-developed-by: Boqun Feng Signed-off-by: Boqun Feng Signed-off-by: Uladzislau Rezki (Sony) Reviewed-by: Paul E. McKenney Cc: Ankur Arora Cc: Thomas Gleixner --- include/linux/rcupdate.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'include/linux/rcupdate.h') diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 17d7ed5f3ae6..2c54750e36a0 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -809,9 +809,9 @@ static inline void rcu_read_unlock(void) { RCU_LOCKDEP_WARN(!rcu_is_watching(), "rcu_read_unlock() used illegally while idle"); + rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */ __release(RCU); __rcu_read_unlock(); - rcu_lock_release(&rcu_lock_map); /* Keep acq info for rls diags. */ } /** -- cgit v1.2.3 From 10c9e40f297d3f1b0d4df6e73618d4466a0b2cfc Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Fri, 15 Mar 2024 23:29:59 -0700 Subject: rcu: Remove redundant CONFIG_PROVE_RCU #if condition The #if condition controlling the rcu_preempt_sleep_check() definition has a redundant check for CONFIG_PREEMPT_RCU, which is already checked for by an enclosing #ifndef. This commit therefore removes this redundant condition from the inner #if. Signed-off-by: Paul E. McKenney Signed-off-by: Uladzislau Rezki (Sony) --- include/linux/rcupdate.h | 6 +++--- 1 file changed, 3 insertions(+), 3 deletions(-) (limited to 'include/linux/rcupdate.h') diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 2c54750e36a0..382780bb60f4 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -401,15 +401,15 @@ static inline int debug_lockdep_rcu_enabled(void) } \ } while (0) -#if defined(CONFIG_PROVE_RCU) && !defined(CONFIG_PREEMPT_RCU) +#ifndef CONFIG_PREEMPT_RCU static inline void rcu_preempt_sleep_check(void) { RCU_LOCKDEP_WARN(lock_is_held(&rcu_lock_map), "Illegal context switch in RCU read-side critical section"); } -#else /* #ifdef CONFIG_PROVE_RCU */ +#else // #ifndef CONFIG_PREEMPT_RCU static inline void rcu_preempt_sleep_check(void) { } -#endif /* #else #ifdef CONFIG_PROVE_RCU */ +#endif // #else // #ifndef CONFIG_PREEMPT_RCU #define rcu_sleep_check() \ do { \ -- cgit v1.2.3 From 80cd613a9ae091dbf52e27a409d58da988ffc8f3 Mon Sep 17 00:00:00 2001 From: Johannes Berg Date: Mon, 25 Mar 2024 18:39:08 +0100 Subject: rcu: Mollify sparse with RCU guard When using "guard(rcu)();" sparse will complain, because even though it now understands the cleanup attribute, it doesn't evaluate the calls from it at function exit, and thus doesn't count the context correctly. Given that there's a conditional in the resulting code: static inline void class_rcu_destructor(class_rcu_t *_T) { if (_T->lock) { rcu_read_unlock(); } } it seems that even trying to teach sparse to evalulate the cleanup attribute function it'd still be difficult to really make it understand the full context here. Suppress the sparse warning by just releasing the context in the acquisition part of the function, after all we know it's safe with the guard, that's the whole point of it. Signed-off-by: Johannes Berg Reviewed-by: Boqun Feng Signed-off-by: Paul E. McKenney Signed-off-by: Uladzislau Rezki (Sony) --- include/linux/rcupdate.h | 14 +++++++++++++- 1 file changed, 13 insertions(+), 1 deletion(-) (limited to 'include/linux/rcupdate.h') diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 382780bb60f4..dfd2399f2cde 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -1090,6 +1090,18 @@ rcu_head_after_call_rcu(struct rcu_head *rhp, rcu_callback_t f) extern int rcu_expedited; extern int rcu_normal; -DEFINE_LOCK_GUARD_0(rcu, rcu_read_lock(), rcu_read_unlock()) +DEFINE_LOCK_GUARD_0(rcu, + do { + rcu_read_lock(); + /* + * sparse doesn't call the cleanup function, + * so just release immediately and don't track + * the context. We don't need to anyway, since + * the whole point of the guard is to not need + * the explicit unlock. + */ + __release(RCU); + } while (0), + rcu_read_unlock()) #endif /* __LINUX_RCUPDATE_H */ -- cgit v1.2.3