diff options
Diffstat (limited to 'kernel/rcutree.c')
| -rw-r--r-- | kernel/rcutree.c | 45 |
1 files changed, 40 insertions, 5 deletions
diff --git a/kernel/rcutree.c b/kernel/rcutree.c index 49464aded7f7..b618d72bd8ec 100644 --- a/kernel/rcutree.c +++ b/kernel/rcutree.c @@ -804,8 +804,11 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp, static void record_gp_stall_check_time(struct rcu_state *rsp) { - rsp->gp_start = jiffies; - rsp->jiffies_stall = jiffies + rcu_jiffies_till_stall_check(); + unsigned long j = ACCESS_ONCE(jiffies); + + rsp->gp_start = j; + smp_wmb(); /* Record start time before stall time. */ + rsp->jiffies_stall = j + rcu_jiffies_till_stall_check(); } /* @@ -934,17 +937,48 @@ static void print_cpu_stall(struct rcu_state *rsp) static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp) { + unsigned long completed; + unsigned long gpnum; + unsigned long gps; unsigned long j; unsigned long js; struct rcu_node *rnp; - if (rcu_cpu_stall_suppress) + if (rcu_cpu_stall_suppress || !rcu_gp_in_progress(rsp)) return; j = ACCESS_ONCE(jiffies); + + /* + * Lots of memory barriers to reject false positives. + * + * The idea is to pick up rsp->gpnum, then rsp->jiffies_stall, + * then rsp->gp_start, and finally rsp->completed. These values + * are updated in the opposite order with memory barriers (or + * equivalent) during grace-period initialization and cleanup. + * Now, a false positive can occur if we get an new value of + * rsp->gp_start and a old value of rsp->jiffies_stall. But given + * the memory barriers, the only way that this can happen is if one + * grace period ends and another starts between these two fetches. + * Detect this by comparing rsp->completed with the previous fetch + * from rsp->gpnum. + * + * Given this check, comparisons of jiffies, rsp->jiffies_stall, + * and rsp->gp_start suffice to forestall false positives. + */ + gpnum = ACCESS_ONCE(rsp->gpnum); + smp_rmb(); /* Pick up ->gpnum first... */ js = ACCESS_ONCE(rsp->jiffies_stall); + smp_rmb(); /* ...then ->jiffies_stall before the rest... */ + gps = ACCESS_ONCE(rsp->gp_start); + smp_rmb(); /* ...and finally ->gp_start before ->completed. */ + completed = ACCESS_ONCE(rsp->completed); + if (ULONG_CMP_GE(completed, gpnum) || + ULONG_CMP_LT(j, js) || + ULONG_CMP_GE(gps, js)) + return; /* No stall or GP completed since entering function. */ rnp = rdp->mynode; if (rcu_gp_in_progress(rsp) && - (ACCESS_ONCE(rnp->qsmask) & rdp->grpmask) && ULONG_CMP_GE(j, js)) { + (ACCESS_ONCE(rnp->qsmask) & rdp->grpmask)) { /* We haven't checked in, so go dump stack. */ print_cpu_stall(rsp); @@ -1317,9 +1351,10 @@ static int rcu_gp_init(struct rcu_state *rsp) } /* Advance to a new grace period and initialize state. */ + record_gp_stall_check_time(rsp); + smp_wmb(); /* Record GP times before starting GP. */ rsp->gpnum++; trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start")); - record_gp_stall_check_time(rsp); raw_spin_unlock_irq(&rnp->lock); /* Exclude any concurrent CPU-hotplug operations. */ |
