Re: [PATCH RFC tip/core/rcu 4/5] srcu: Provide polling interfaces for Tiny SRCU grace periods
From: Neeraj Upadhyay
Date: Fri Nov 20 2020 - 06:58:53 EST
Hi Paul,
On 11/17/2020 6:10 AM, paulmck@xxxxxxxxxx wrote:
From: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
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@xxxxxxxxxxxxxx/
Reported-by: Kent Overstreet <kent.overstreet@xxxxxxxxx>
[ paulmck: Add EXPORT_SYMBOL_GPL() per kernel test robot feedback. ]
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
include/linux/rcupdate.h | 2 ++
include/linux/srcu.h | 3 +++
include/linux/srcutiny.h | 1 +
kernel/rcu/srcutiny.c | 52 ++++++++++++++++++++++++++++++++++++++++++++++--
4 files changed, 56 insertions(+), 2 deletions(-)
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index de08264..e09c0d8 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -33,6 +33,8 @@
#define ULONG_CMP_GE(a, b) (ULONG_MAX / 2 >= (a) - (b))
#define ULONG_CMP_LT(a, b) (ULONG_MAX / 2 < (a) - (b))
#define ulong2long(a) (*(long *)(&(a)))
+#define USHORT_CMP_GE(a, b) (USHRT_MAX / 2 >= (unsigned short)((a) - (b)))
+#define USHORT_CMP_LT(a, b) (USHRT_MAX / 2 < (unsigned short)((a) - (b)))
/* Exported common interfaces */
void call_rcu(struct rcu_head *head, rcu_callback_t func);
diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index e432cc9..a0895bb 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -60,6 +60,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp);
int __srcu_read_lock(struct srcu_struct *ssp) __acquires(ssp);
void __srcu_read_unlock(struct srcu_struct *ssp, int idx) __releases(ssp);
void synchronize_srcu(struct srcu_struct *ssp);
+unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp);
+unsigned long start_poll_synchronize_srcu(struct srcu_struct *ssp);
+bool poll_state_synchronize_srcu(struct srcu_struct *ssp, unsigned long cookie);
#ifdef CONFIG_DEBUG_LOCK_ALLOC
diff --git a/include/linux/srcutiny.h b/include/linux/srcutiny.h
index fed4a2d..e9bd6fb 100644
--- a/include/linux/srcutiny.h
+++ b/include/linux/srcutiny.h
@@ -16,6 +16,7 @@
struct srcu_struct {
short srcu_lock_nesting[2]; /* srcu_read_lock() nesting depth. */
unsigned short srcu_idx; /* Current reader array element in bit 0x2. */
+ unsigned short srcu_idx_max; /* Furthest future srcu_idx request. */
u8 srcu_gp_running; /* GP workqueue running? */
u8 srcu_gp_waiting; /* GP waiting for readers? */
struct swait_queue_head srcu_wq;
diff --git a/kernel/rcu/srcutiny.c b/kernel/rcu/srcutiny.c
index 3bac1db..b405811 100644
--- a/kernel/rcu/srcutiny.c
+++ b/kernel/rcu/srcutiny.c
@@ -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;
@@ -114,7 +115,7 @@ 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. */
@@ -147,14 +148,19 @@ 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_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
Should this be USHORT_CMP_LT ?
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;
+
if (!READ_ONCE(ssp->srcu_gp_running)) {
+ cookie = get_state_synchronize_srcu(ssp);
+ if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
+ WRITE_ONCE(ssp->srcu_idx_max, cookie);
I was thinking of a case which might break with this.
Consider a scenario, where GP is in progress and kworker is right
before below point, after executing callbacks:
void srcu_drive_gp(struct work_struct *wp) {
<snip>
while (lh) {
<cb execution loop>
}
>>> CURRENT EXECUTION POINT
WRITE_ONCE(ssp->srcu_gp_running, false);
if (USHORT_CMP_LT(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
schedule_work(&ssp->srcu_work);
}
Now, at this instance, srcu_gp_start_if_needed() runs and samples
srcu_gp_running and returns, without updating srcu_idx_max
static void srcu_gp_start_if_needed(struct srcu_struct *ssp)
{
if (!READ_ONCE(ssp->srcu_gp_running)) returns true
<snip>
}
kworker running srcu_drive_gp() resumes and returns without queueing a
new schedule_work(&ssp->srcu_work); for new GP?
Prior to this patch, call_srcu() enqueues a cb before entering
srcu_gp_start_if_needed(), and srcu_drive_gp() observes this
queuing, and schedule a work for the new GP, for this scenario.
WRITE_ONCE(ssp->srcu_gp_running, false);
- if (READ_ONCE(ssp->srcu_cb_head))
+ if (USHORT_CMP_GE(ssp->srcu_idx, READ_ONCE(ssp->srcu_idx_max)))
schedule_work(&ssp->srcu_work);
So, should the "cookie" calculation and "srcu_idx_max" setting be moved
outside of ssp->srcu_gp_running check and maybe return the same cookie
to caller and use that as the returned cookie from
start_poll_synchronize_srcu() ?
srcu_gp_start_if_needed()
cookie = get_state_synchronize_srcu(ssp);
if (USHORT_CMP_LT(READ_ONCE(ssp->srcu_idx_max), cookie))
WRITE_ONCE(ssp->srcu_idx_max, cookie);
if (!READ_ONCE(ssp->srcu_gp_running)) {
<snip>
}
Thanks
Neeraj
if (likely(srcu_init_done))
schedule_work(&ssp->srcu_work);
else if (list_empty(&ssp->srcu_work.entry))
@@ -196,6 +202,48 @@ 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)
{
--
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