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rcu: Add full-sized polling for get_state() 

The get_state_synchronize_rcu() API compresses the combined expedited and
normal grace-period states into a single unsigned long, which conserves
storage, but can miss grace periods in certain cases involving overlapping
normal and expedited grace periods.  Missing the occasional grace period
is usually not a problem, but there are use cases that care about each
and every grace period.

This commit therefore adds the next member of the full-state RCU
grace-period polling API, namely the get_state_synchronize_rcu_full()
function.  This uses up to three times the storage (rcu_gp_oldstate
structure instead of unsigned long), but is guaranteed not to miss
grace periods.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
This commit is contained in:
Paul E. McKenney 2022-07-28 19:58:13 -07:00
parent 91a967fd69
commit 3fdefca9b4
4 changed files with 46 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
include/linux/rcutiny.h
View File

@ -19,6 +19,12 @@ struct rcu_gp_oldstate {
};
unsigned long get_state_synchronize_rcu(void);
static inline void get_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
{
rgosp->rgos_norm = get_state_synchronize_rcu();
}
unsigned long start_poll_synchronize_rcu(void);
bool poll_state_synchronize_rcu(unsigned long oldstate);

1
include/linux/rcutree.h
View File

@ -50,6 +50,7 @@ struct rcu_gp_oldstate {
unsigned long start_poll_synchronize_rcu_expedited(void);
void cond_synchronize_rcu_expedited(unsigned long oldstate);
unsigned long get_state_synchronize_rcu(void);
void get_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp);
unsigned long start_poll_synchronize_rcu(void);
bool poll_state_synchronize_rcu(unsigned long oldstate);
bool poll_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp);

10
kernel/rcu/rcutorture.c
View File

@ -335,6 +335,7 @@ struct rcu_torture_ops {
bool (*poll_gp_state_exp)(unsigned long oldstate);
void (*cond_sync_exp)(unsigned long oldstate);
unsigned long (*get_gp_state)(void);
void (*get_gp_state_full)(struct rcu_gp_oldstate *rgosp);
unsigned long (*get_gp_completed)(void);
void (*get_gp_completed_full)(struct rcu_gp_oldstate *rgosp);
unsigned long (*start_gp_poll)(void);
@ -504,6 +505,7 @@ static struct rcu_torture_ops rcu_ops = {
.sync = synchronize_rcu,
.exp_sync = synchronize_rcu_expedited,
.get_gp_state = get_state_synchronize_rcu,
.get_gp_state_full = get_state_synchronize_rcu_full,
.get_gp_completed = get_completed_synchronize_rcu,
.get_gp_completed_full = get_completed_synchronize_rcu_full,
.start_gp_poll = start_poll_synchronize_rcu,
@ -1293,12 +1295,12 @@ rcu_torture_writer(void *arg)
break;
case RTWS_EXP_SYNC:
rcu_torture_writer_state = RTWS_EXP_SYNC;
if (cur_ops->get_gp_state && cur_ops->poll_gp_state)
cookie = cur_ops->get_gp_state();
if (cur_ops->get_gp_state_full && cur_ops->poll_gp_state_full)
cur_ops->get_gp_state_full(&cookie_full);
cur_ops->exp_sync();
cur_ops->exp_sync();
if (cur_ops->get_gp_state && cur_ops->poll_gp_state)
WARN_ON_ONCE(!cur_ops->poll_gp_state(cookie));
if (cur_ops->get_gp_state_full && cur_ops->poll_gp_state_full)
WARN_ON_ONCE(!cur_ops->poll_gp_state_full(&cookie_full));
rcu_torture_pipe_update(old_rp);
break;
case RTWS_COND_GET:

33
kernel/rcu/tree.c
View File

@ -1755,6 +1755,8 @@ static noinline void rcu_gp_cleanup(void)
dump_blkd_tasks(rnp, 10);
WARN_ON_ONCE(rnp->qsmask);
WRITE_ONCE(rnp->gp_seq, new_gp_seq);
if (!rnp->parent)
smp_mb(); // Order against failing poll_state_synchronize_rcu_full().
rdp = this_cpu_ptr(&rcu_data);
if (rnp == rdp->mynode)
needgp = __note_gp_changes(rnp, rdp) || needgp;
@ -3556,6 +3558,37 @@ unsigned long get_state_synchronize_rcu(void)
}
EXPORT_SYMBOL_GPL(get_state_synchronize_rcu);
/**
* get_state_synchronize_rcu_full - Snapshot RCU state, both normal and expedited
* @rgosp: location to place combined normal/expedited grace-period state
*
* Places the normal and expedited grace-period states in @rgosp. This
* state value can be passed to a later call to cond_synchronize_rcu_full()
* or poll_state_synchronize_rcu_full() to determine whether or not a
* grace period (whether normal or expedited) has elapsed in the meantime.
* The rcu_gp_oldstate structure takes up twice the memory of an unsigned
* long, but is guaranteed to see all grace periods. In contrast, the
* combined state occupies less memory, but can sometimes fail to take
* grace periods into account.
*
* This does not guarantee that the needed grace period will actually
* start.
*/
void get_state_synchronize_rcu_full(struct rcu_gp_oldstate *rgosp)
{
struct rcu_node *rnp = rcu_get_root();
/*
* Any prior manipulation of RCU-protected data must happen
* before the loads from ->gp_seq and ->expedited_sequence.
*/
smp_mb(); /* ^^^ */
rgosp->rgos_norm = rcu_seq_snap(&rnp->gp_seq);
rgosp->rgos_exp = rcu_seq_snap(&rcu_state.expedited_sequence);
rgosp->rgos_polled = rcu_seq_snap(&rcu_state.gp_seq_polled);
}
EXPORT_SYMBOL_GPL(get_state_synchronize_rcu_full);
/**
* start_poll_synchronize_rcu - Snapshot and start RCU grace period
*