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srcu: Provide polling interfaces for Tiny SRCU grace periods

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commit 8b5bd67cf6422b63ee100d76d8de8960ca2df7f0 upstream.

There is a need for a polling interface for SRCU grace
periods, so this commit supplies get_state_synchronize_srcu(),
start_poll_synchronize_srcu(), and poll_state_synchronize_srcu() for this
purpose.  The first can be used if future grace periods are inevitable
(perhaps due to a later call_srcu() invocation), the second if future
grace periods might not otherwise happen, and the third to check if a
grace period has elapsed since the corresponding call to either of the
first two.

As with get_state_synchronize_rcu() and cond_synchronize_rcu(),
the return value from either get_state_synchronize_srcu() or
start_poll_synchronize_srcu() must be passed in to a later call to
poll_state_synchronize_srcu().

Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
[ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
[ paulmck: Apply feedback from Neeraj Upadhyay. ]
Link: https://lore.kernel.org/lkml/20201117004017.GA7444@paulmck-ThinkPad-P72/
Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Paul E. McKenney
2020-11-13 12:54:48 -08:00
committed by Greg Kroah-Hartman
parent 450948b06c
commit b6ae385407
4 changed files with 59 additions and 2 deletions
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55
kernel/rcu/srcutiny.c
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@@ -34,6 +34,7 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp)
ssp->srcu_gp_running = false;
ssp->srcu_gp_waiting = false;
ssp->srcu_idx = 0;
ssp->srcu_idx_max = 0;
INIT_WORK(&ssp->srcu_work, srcu_drive_gp);
INIT_LIST_HEAD(&ssp->srcu_work.entry);
return 0;
@@ -84,6 +85,8 @@ void cleanup_srcu_struct(struct srcu_struct *ssp)
WARN_ON(ssp->srcu_gp_waiting);
WARN_ON(ssp->srcu_cb_head);
WARN_ON(&ssp->srcu_cb_head != ssp->srcu_cb_tail);
WARN_ON(ssp->srcu_idx != ssp->srcu_idx_max);
WARN_ON(ssp->srcu_idx & 0x1);
}
EXPORT_SYMBOL_GPL(cleanup_srcu_struct);
@@ -114,7 +117,7 @@ void srcu_drive_gp(struct work_struct *wp)
struct srcu_struct *ssp;
ssp = container_of(wp, struct srcu_struct, srcu_work);
if (ssp->srcu_gp_running || !READ_ONCE(ssp->srcu_cb_head))
if (ssp->srcu_gp_running || USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
return; /* Already running or nothing to do. */
/* Remove recently arrived callbacks and wait for readers. */
@@ -147,13 +150,19 @@ void srcu_drive_gp(struct work_struct *wp)
* straighten that out.
*/
WRITE_ONCE(ssp->srcu_gp_running, false);
if (READ_ONCE(ssp->srcu_cb_head))
if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
schedule_work(&ssp->srcu_work);
}
EXPORT_SYMBOL_GPL(srcu_drive_gp);
static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
{
unsigned short cookie;
cookie = get_state_synchronize_srcu(ssp);
if (USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx_max), cookie))
return;
WRITE_ONCE(ssp->srcu_idx_max, cookie);
if (!READ_ONCE(ssp->srcu_gp_running)) {
if (likely(srcu_init_done))
schedule_work(&ssp->srcu_work);
@@ -196,6 +205,48 @@ void synchronize_srcu(struct srcu_struct *ssp)
}
EXPORT_SYMBOL_GPL(synchronize_srcu);
/*
* get_state_synchronize_srcu - Provide an end-of-grace-period cookie
*/
unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp)
{
unsigned long ret;
barrier();
ret = (READ_ONCE(ssp->srcu_idx) + 3) & ~0x1;
barrier();
return ret & USHRT_MAX;
}
EXPORT_SYMBOL_GPL(get_state_synchronize_srcu);
/*
* start_poll_synchronize_srcu - Provide cookie and start grace period
*
* The difference between this and get_state_synchronize_srcu() is that
* this function ensures that the poll_state_synchronize_srcu() will
* eventually return the value true.
*/
unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp)
{
unsigned long ret = get_state_synchronize_srcu(ssp);
srcu_gp_start_if_needed(ssp);
return ret;
}
EXPORT_SYMBOL_GPL(start_poll_synchronize_srcu);
/*
* poll_state_synchronize_srcu - Has cookie's grace period ended?
*/
bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie)
{
bool ret = USHORT_CMP_GE(READ_ONCE(ssp->srcu_idx), cookie);
barrier();
return ret;
}
EXPORT_SYMBOL_GPL(poll_state_synchronize_srcu);
/* Lockdep diagnostics. */
void __init rcu_scheduler_starting(void)
{