Commit 765a3f4f authored by Paul E. McKenney's avatar Paul E. McKenney

rcu: Provide grace-period piggybacking API

The following pattern is currently not well supported by RCU:

1.	Make data element inaccessible to RCU readers.

2.	Do work that probably lasts for more than one grace period.

3.	Do something to make sure RCU readers in flight before #1 above
	have completed.

Here are some things that could currently be done:

a.	Do a synchronize_rcu() unconditionally at either #1 or #3 above.
	This works, but imposes needless work and latency.

b.	Post an RCU callback at #1 above that does a wakeup, then
	wait for the wakeup at #3.  This works well, but likely results
	in an extra unneeded grace period.  Open-coding this is also
	a bit more semi-tricky code than would be good.

This commit therefore adds get_state_synchronize_rcu() and
cond_synchronize_rcu() APIs.  Call get_state_synchronize_rcu() at #1
above and pass its return value to cond_synchronize_rcu() at #3 above.
This results in a call to synchronize_rcu() if no grace period has
elapsed between #1 and #3, but requires only a load, comparison, and
memory barrier if a full grace period did elapse.
Requested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
parent f5604f67
......@@ -27,6 +27,16 @@
#include <linux/cache.h>
static inline unsigned long get_state_synchronize_rcu(void)
{
return 0;
}
static inline void cond_synchronize_rcu(unsigned long oldstate)
{
might_sleep();
}
static inline void rcu_barrier_bh(void)
{
wait_rcu_gp(call_rcu_bh);
......
......@@ -76,6 +76,8 @@ static inline void synchronize_rcu_bh_expedited(void)
void rcu_barrier(void);
void rcu_barrier_bh(void);
void rcu_barrier_sched(void);
unsigned long get_state_synchronize_rcu(void);
void cond_synchronize_rcu(unsigned long oldstate);
extern unsigned long rcutorture_testseq;
extern unsigned long rcutorture_vernum;
......
......@@ -1421,13 +1421,14 @@ 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++;
/* Record GP times before starting GP, hence smp_store_release(). */
smp_store_release(&rsp->gpnum, rsp->gpnum + 1);
trace_rcu_grace_period(rsp->name, rsp->gpnum, TPS("start"));
raw_spin_unlock_irq(&rnp->lock);
/* Exclude any concurrent CPU-hotplug operations. */
mutex_lock(&rsp->onoff_mutex);
smp_mb__after_unlock_lock(); /* ->gpnum increment before GP! */
/*
* Set the quiescent-state-needed bits in all the rcu_node
......@@ -1555,10 +1556,11 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
}
rnp = rcu_get_root(rsp);
raw_spin_lock_irq(&rnp->lock);
smp_mb__after_unlock_lock();
smp_mb__after_unlock_lock(); /* Order GP before ->completed update. */
rcu_nocb_gp_set(rnp, nocb);
rsp->completed = rsp->gpnum; /* Declare grace period done. */
/* Declare grace period done. */
ACCESS_ONCE(rsp->completed) = rsp->gpnum;
trace_rcu_grace_period(rsp->name, rsp->completed, TPS("end"));
rsp->fqs_state = RCU_GP_IDLE;
rdp = this_cpu_ptr(rsp->rda);
......@@ -2637,6 +2639,58 @@ void synchronize_rcu_bh(void)
}
EXPORT_SYMBOL_GPL(synchronize_rcu_bh);
/**
* get_state_synchronize_rcu - Snapshot current RCU state
*
* Returns a cookie that is used by a later call to cond_synchronize_rcu()
* to determine whether or not a full grace period has elapsed in the
* meantime.
*/
unsigned long get_state_synchronize_rcu(void)
{
/*
* Any prior manipulation of RCU-protected data must happen
* before the load from ->gpnum.
*/
smp_mb(); /* ^^^ */
/*
* Make sure this load happens before the purportedly
* time-consuming work between get_state_synchronize_rcu()
* and cond_synchronize_rcu().
*/
return smp_load_acquire(&rcu_state->gpnum);
}
EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
/**
* cond_synchronize_rcu - Conditionally wait for an RCU grace period
*
* @oldstate: return value from earlier call to get_state_synchronize_rcu()
*
* If a full RCU grace period has elapsed since the earlier call to
* get_state_synchronize_rcu(), just return. Otherwise, invoke
* synchronize_rcu() to wait for a full grace period.
*
* Yes, this function does not take counter wrap into account. But
* counter wrap is harmless. If the counter wraps, we have waited for
* more than 2 billion grace periods (and way more on a 64-bit system!),
* so waiting for one additional grace period should be just fine.
*/
void cond_synchronize_rcu(unsigned long oldstate)
{
unsigned long newstate;
/*
* Ensure that this load happens before any RCU-destructive
* actions the caller might carry out after we return.
*/
newstate = smp_load_acquire(&rcu_state->completed);
if (ULONG_CMP_GE(oldstate, newstate))
synchronize_rcu();
}
EXPORT_SYMBOL_GPL(cond_synchronize_rcu);
static int synchronize_sched_expedited_cpu_stop(void *data)
{
/*
......
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