rcu: Prohibit starting new grace periods while forcing quiescent states

Reduce the number and variety of race conditions by prohibiting
the start of a new grace period while force_quiescent_state() is
active. A new fqs_active flag in the rcu_state structure is used
to trace whether or not force_quiescent_state() is active, and
this new flag is tested by rcu_start_gp().  If the CPU that
closed out the last grace period needs another grace period,
this new grace period may be delayed up to one scheduling-clock
tick, but it will eventually get started.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <126264655052-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index d42ad30c4d703ab64a027bc2e4871b9544d8264f..41688ff60e0729b8486961b176b18240284e547f 100644 (file)
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -659,7 +659,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
        struct rcu_data *rdp = rsp->rda[smp_processor_id()];
        struct rcu_node *rnp = rcu_get_root(rsp);
 
-       if (!cpu_needs_another_gp(rsp, rdp)) {
+       if (!cpu_needs_another_gp(rsp, rdp) || rsp->fqs_active) {
                if (rnp->completed == rsp->completed) {
                        spin_unlock_irqrestore(&rnp->lock, flags);
                        return;
@@ -1195,6 +1195,7 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
        struct rcu_node *rnp = rcu_get_root(rsp);
        u8 signaled;
        u8 forcenow;
+       u8 gpdone;
 
        if (!rcu_gp_in_progress(rsp))
                return;  /* No grace period in progress, nothing to force. */
@@ -1206,15 +1207,16 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
            (long)(rsp->jiffies_force_qs - jiffies) >= 0)
                goto unlock_fqs_ret; /* no emergency and done recently. */
        rsp->n_force_qs++;
-       spin_lock(&rnp->lock);
+       spin_lock(&rnp->lock);  /* irqs already disabled */
        lastcomp = rsp->gpnum - 1;
        signaled = rsp->signaled;
        rsp->jiffies_force_qs = jiffies + RCU_JIFFIES_TILL_FORCE_QS;
        if(!rcu_gp_in_progress(rsp)) {
                rsp->n_force_qs_ngp++;
-               spin_unlock(&rnp->lock);
+               spin_unlock(&rnp->lock);  /* irqs remain disabled */
                goto unlock_fqs_ret;  /* no GP in progress, time updated. */
        }
+       rsp->fqs_active = 1;
        switch (signaled) {
        case RCU_GP_IDLE:
        case RCU_GP_INIT:
@@ -1223,15 +1225,16 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
 
        case RCU_SAVE_DYNTICK:
 
-               spin_unlock(&rnp->lock);
+               spin_unlock(&rnp->lock);  /* irqs remain disabled */
                if (RCU_SIGNAL_INIT != RCU_SAVE_DYNTICK)
                        break; /* So gcc recognizes the dead code. */
 
                /* Record dyntick-idle state. */
-               if (rcu_process_dyntick(rsp, lastcomp,
-                                       dyntick_save_progress_counter))
-                       goto unlock_fqs_ret;
-               spin_lock(&rnp->lock);
+               gpdone = rcu_process_dyntick(rsp, lastcomp,
+                                            dyntick_save_progress_counter);
+               spin_lock(&rnp->lock);  /* irqs already disabled */
+               if (gpdone)
+                       break;
                /* fall into next case. */
 
        case RCU_SAVE_COMPLETED:
@@ -1252,17 +1255,17 @@ static void force_quiescent_state(struct rcu_state *rsp, int relaxed)
        case RCU_FORCE_QS:
 
                /* Check dyntick-idle state, send IPI to laggarts. */
-               spin_unlock(&rnp->lock);
-               if (rcu_process_dyntick(rsp, rsp->completed_fqs,
-                                       rcu_implicit_dynticks_qs))
-                       goto unlock_fqs_ret;
+               spin_unlock(&rnp->lock);  /* irqs remain disabled */
+               gpdone = rcu_process_dyntick(rsp, rsp->completed_fqs,
+                                            rcu_implicit_dynticks_qs);
 
                /* Leave state in case more forcing is required. */
 
-               spin_lock(&rnp->lock);
+               spin_lock(&rnp->lock);  /* irqs already disabled */
                break;
        }
-       spin_unlock(&rnp->lock);
+       rsp->fqs_active = 0;
+       spin_unlock(&rnp->lock);  /* irqs remain disabled */
 unlock_fqs_ret:
        spin_unlock_irqrestore(&rsp->fqslock, flags);
 }

diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index d2a0046f63b297b41e4605b25ae178ee57883367..dc386a7c634f8038ab678cc961d1213ff4b8affd 100644 (file)
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -277,6 +277,8 @@ struct rcu_state {
 
        u8      signaled ____cacheline_internodealigned_in_smp;
                                                /* Force QS state. */
+       u8      fqs_active;                     /* force_quiescent_state() */
+                                               /*  is running. */
        long    gpnum;                          /* Current gp number. */
        long    completed;                      /* # of last completed gp. */