The following commit has been merged into the core/rcu branch of tip: Commit-ID: ac3caf827488d3bc4d4101ff34931abbfa33839d Gitweb: https://git.kernel.org/tip/ac3caf827488d3bc4d4101ff34931abbfa33839d Author: Paul E. McKenney <paulmck@xxxxxxxxxx> AuthorDate: Thu, 12 Mar 2020 17:01:57 -07:00 Committer: Paul E. McKenney <paulmck@xxxxxxxxxx> CommitterDate: Mon, 27 Apr 2020 11:03:50 -07:00 rcu: Add comments marking transitions between RCU watching and not It is not as clear as it might be just where in RCU's idle entry/exit code RCU stops and starts watching the current CPU. This commit therefore adds comments calling out the transitions. Reported-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx> Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx> --- kernel/rcu/tree.c | 29 +++++++++++++++++++++++++---- 1 file changed, 25 insertions(+), 4 deletions(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index 156ac8d..0bbcbf3 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -238,7 +238,9 @@ void rcu_softirq_qs(void) /* * Record entry into an extended quiescent state. This is only to be - * called when not already in an extended quiescent state. + * called when not already in an extended quiescent state, that is, + * RCU is watching prior to the call to this function and is no longer + * watching upon return. */ static void rcu_dynticks_eqs_enter(void) { @@ -251,7 +253,7 @@ static void rcu_dynticks_eqs_enter(void) * next idle sojourn. */ seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks); - /* Better be in an extended quiescent state! */ + // RCU is no longer watching. Better be in extended quiescent state! WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && (seq & RCU_DYNTICK_CTRL_CTR)); /* Better not have special action (TLB flush) pending! */ @@ -261,7 +263,8 @@ static void rcu_dynticks_eqs_enter(void) /* * Record exit from an extended quiescent state. This is only to be - * called from an extended quiescent state. + * called from an extended quiescent state, that is, RCU is not watching + * prior to the call to this function and is watching upon return. */ static void rcu_dynticks_eqs_exit(void) { @@ -274,6 +277,7 @@ static void rcu_dynticks_eqs_exit(void) * critical section. */ seq = atomic_add_return(RCU_DYNTICK_CTRL_CTR, &rdp->dynticks); + // RCU is now watching. Better not be in an extended quiescent state! WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !(seq & RCU_DYNTICK_CTRL_CTR)); if (seq & RCU_DYNTICK_CTRL_MASK) { @@ -584,6 +588,7 @@ static void rcu_eqs_enter(bool user) WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && rdp->dynticks_nesting == 0); if (rdp->dynticks_nesting != 1) { + // RCU will still be watching, so just do accounting and leave. rdp->dynticks_nesting--; return; } @@ -596,7 +601,9 @@ static void rcu_eqs_enter(bool user) rcu_prepare_for_idle(); rcu_preempt_deferred_qs(current); WRITE_ONCE(rdp->dynticks_nesting, 0); /* Avoid irq-access tearing. */ + // RCU is watching here ... rcu_dynticks_eqs_enter(); + // ... but is no longer watching here. rcu_dynticks_task_enter(); } @@ -676,7 +683,9 @@ static __always_inline void rcu_nmi_exit_common(bool irq) if (irq) rcu_prepare_for_idle(); + // RCU is watching here ... rcu_dynticks_eqs_enter(); + // ... but is no longer watching here. if (irq) rcu_dynticks_task_enter(); @@ -751,11 +760,14 @@ static void rcu_eqs_exit(bool user) oldval = rdp->dynticks_nesting; WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && oldval < 0); if (oldval) { + // RCU was already watching, so just do accounting and leave. rdp->dynticks_nesting++; return; } rcu_dynticks_task_exit(); + // RCU is not watching here ... rcu_dynticks_eqs_exit(); + // ... but is watching here. rcu_cleanup_after_idle(); trace_rcu_dyntick(TPS("End"), rdp->dynticks_nesting, 1, atomic_read(&rdp->dynticks)); WARN_ON_ONCE(IS_ENABLED(CONFIG_RCU_EQS_DEBUG) && !user && !is_idle_task(current)); @@ -832,7 +844,9 @@ static __always_inline void rcu_nmi_enter_common(bool irq) if (irq) rcu_dynticks_task_exit(); + // RCU is not watching here ... rcu_dynticks_eqs_exit(); + // ... but is watching here. if (irq) rcu_cleanup_after_idle(); @@ -842,9 +856,16 @@ static __always_inline void rcu_nmi_enter_common(bool irq) rdp->dynticks_nmi_nesting == DYNTICK_IRQ_NONIDLE && READ_ONCE(rdp->rcu_urgent_qs) && !READ_ONCE(rdp->rcu_forced_tick)) { + // We get here only if we had already exited the extended + // quiescent state and this was an interrupt (not an NMI). + // Therefore, (1) RCU is already watching and (2) The fact + // that we are in an interrupt handler and that the rcu_node + // lock is an irq-disabled lock prevents self-deadlock. + // So we can safely recheck under the lock. raw_spin_lock_rcu_node(rdp->mynode); - // Recheck under lock. if (rdp->rcu_urgent_qs && !rdp->rcu_forced_tick) { + // A nohz_full CPU is in the kernel and RCU + // needs a quiescent state. Turn on the tick! WRITE_ONCE(rdp->rcu_forced_tick, true); tick_dep_set_cpu(rdp->cpu, TICK_DEP_BIT_RCU); }
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