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McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, "Paul E. McKenney" Subject: [PATCH tip/core/rcu 10/19] rcu: Update comments and help text for no more RCU-bh updaters Date: Wed, 29 Aug 2018 15:20:38 -0700 X-Mailer: git-send-email 2.17.1 In-Reply-To: <20180829222021.GA29944@linux.vnet.ibm.com> References: <20180829222021.GA29944@linux.vnet.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 18082922-0068-0000-0000-00000331B7B5 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009636; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01080738; UDB=6.00557494; IPR=6.00860723; MB=3.00023001; MTD=3.00000008; XFM=3.00000015; UTC=2018-08-29 22:20:54 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18082922-0069-0000-0000-0000458F0E11 Message-Id: <20180829222047.319-10-paulmck@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-08-29_06:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=1 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1807170000 definitions=main-1808290217 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This commit updates comments and help text to account for the fact that RCU-bh update-side functions are now simple wrappers for their RCU or RCU-sched counterparts. Signed-off-by: Paul E. McKenney --- include/linux/rcupdate.h | 12 ++++-------- include/linux/rcupdate_wait.h | 6 +++--- include/linux/rcutree.h | 14 ++------------ kernel/rcu/Kconfig | 10 +++++----- kernel/rcu/tree.c | 17 +++++++++-------- kernel/rcu/update.c | 2 +- 6 files changed, 24 insertions(+), 37 deletions(-) diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h index 9ebfd436cec7..8d5740edd63c 100644 --- a/include/linux/rcupdate.h +++ b/include/linux/rcupdate.h @@ -688,14 +688,10 @@ static inline void rcu_read_unlock(void) /** * rcu_read_lock_bh() - mark the beginning of an RCU-bh critical section * - * This is equivalent of rcu_read_lock(), but to be used when updates - * are being done using call_rcu_bh() or synchronize_rcu_bh(). Since - * both call_rcu_bh() and synchronize_rcu_bh() consider completion of a - * softirq handler to be a quiescent state, a process in RCU read-side - * critical section must be protected by disabling softirqs. Read-side - * critical sections in interrupt context can use just rcu_read_lock(), - * though this should at least be commented to avoid confusing people - * reading the code. + * This is equivalent of rcu_read_lock(), but also disables softirqs. + * Note that synchronize_rcu() and friends may be used for the update + * side, although synchronize_rcu_bh() is available as a wrapper in the + * short term. Longer term, the _bh update-side API will be eliminated. * * Note that rcu_read_lock_bh() and the matching rcu_read_unlock_bh() * must occur in the same context, for example, it is illegal to invoke diff --git a/include/linux/rcupdate_wait.h b/include/linux/rcupdate_wait.h index 57f371344152..bc104699560e 100644 --- a/include/linux/rcupdate_wait.h +++ b/include/linux/rcupdate_wait.h @@ -36,13 +36,13 @@ do { \ * @...: List of call_rcu() functions for the flavors to wait on. * * This macro waits concurrently for multiple flavors of RCU grace periods. - * For example, synchronize_rcu_mult(call_rcu, call_rcu_bh) would wait - * on concurrent RCU and RCU-bh grace periods. Waiting on a give SRCU + * For example, synchronize_rcu_mult(call_rcu, call_rcu_sched) would wait + * on concurrent RCU and RCU-sched grace periods. Waiting on a give SRCU * domain requires you to write a wrapper function for that SRCU domain's * call_srcu() function, supplying the corresponding srcu_struct. * * If Tiny RCU, tell _wait_rcu_gp() not to bother waiting for RCU - * or RCU-bh, given that anywhere synchronize_rcu_mult() can be called + * or RCU-sched, given that anywhere synchronize_rcu_mult() can be called * is automatically a grace period. */ #define synchronize_rcu_mult(...) \ diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h index c789c302a2c9..f7a41323aa54 100644 --- a/include/linux/rcutree.h +++ b/include/linux/rcutree.h @@ -58,18 +58,8 @@ void kfree_call_rcu(struct rcu_head *head, rcu_callback_t func); /** * synchronize_rcu_bh_expedited - Brute-force RCU-bh grace period * - * Wait for an RCU-bh grace period to elapse, but use a "big hammer" - * approach to force the grace period to end quickly. This consumes - * significant time on all CPUs and is unfriendly to real-time workloads, - * so is thus not recommended for any sort of common-case code. In fact, - * if you are using synchronize_rcu_bh_expedited() in a loop, please - * restructure your code to batch your updates, and then use a single - * synchronize_rcu_bh() instead. - * - * Note that it is illegal to call this function while holding any lock - * that is acquired by a CPU-hotplug notifier. And yes, it is also illegal - * to call this function from a CPU-hotplug notifier. Failing to observe - * these restriction will result in deadlock. + * This is a transitional API and will soon be removed, with all + * callers converted to synchronize_rcu_expedited(). */ static inline void synchronize_rcu_bh_expedited(void) { diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig index 9210379c0353..a0b7f0103ca9 100644 --- a/kernel/rcu/Kconfig +++ b/kernel/rcu/Kconfig @@ -229,11 +229,11 @@ config RCU_NOCB_CPU CPUs specified at boot time by the rcu_nocbs parameter. For each such CPU, a kthread ("rcuox/N") will be created to invoke callbacks, where the "N" is the CPU being offloaded, - and where the "x" is "b" for RCU-bh, "p" for RCU-preempt, and - "s" for RCU-sched. Nothing prevents this kthread from running - on the specified CPUs, but (1) the kthreads may be preempted - between each callback, and (2) affinity or cgroups can be used - to force the kthreads to run on whatever set of CPUs is desired. + and where the "p" for RCU-preempt and "s" for RCU-sched. + Nothing prevents this kthread from running on the specified + CPUs, but (1) the kthreads may be preempted between each + callback, and (2) affinity or cgroups can be used to force + the kthreads to run on whatever set of CPUs is desired. Say Y here if you want to help to debug reduced OS jitter. Say N here if you are unsure. diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index b602d60462ba..8d04cf2c6f76 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -565,7 +565,8 @@ unsigned long rcu_sched_get_gp_seq(void) EXPORT_SYMBOL_GPL(rcu_sched_get_gp_seq); /* - * Return the number of RCU-bh GPs completed thus far for debug & stats. + * Return the number of RCU GPs completed thus far for debug & stats. + * This is a transitional API and will soon be removed. */ unsigned long rcu_bh_get_gp_seq(void) { @@ -3068,13 +3069,13 @@ void kfree_call_rcu(struct rcu_head *head, EXPORT_SYMBOL_GPL(kfree_call_rcu); /* - * Because a context switch is a grace period for RCU-sched and RCU-bh, - * any blocking grace-period wait automatically implies a grace period - * if there is only one CPU online at any point time during execution - * of either synchronize_sched() or synchronize_rcu_bh(). It is OK to - * occasionally incorrectly indicate that there are multiple CPUs online - * when there was in fact only one the whole time, as this just adds - * some overhead: RCU still operates correctly. + * Because a context switch is a grace period for RCU-sched, any blocking + * grace-period wait automatically implies a grace period if there + * is only one CPU online at any point time during execution of either + * synchronize_sched() or synchronize_rcu_bh(). It is OK to occasionally + * incorrectly indicate that there are multiple CPUs online when there + * was in fact only one the whole time, as this just adds some overhead: + * RCU still operates correctly. */ static int rcu_blocking_is_gp(void) { diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c index 39cb23d22109..9ea87d0aa386 100644 --- a/kernel/rcu/update.c +++ b/kernel/rcu/update.c @@ -298,7 +298,7 @@ EXPORT_SYMBOL_GPL(rcu_read_lock_held); * * Check debug_lockdep_rcu_enabled() to prevent false positives during boot. * - * Note that rcu_read_lock() is disallowed if the CPU is either idle or + * Note that rcu_read_lock_bh() is disallowed if the CPU is either idle or * offline from an RCU perspective, so check for those as well. */ int rcu_read_lock_bh_held(void) -- 2.17.1