[for-416 PATCH 1/2] bcache: Fix, improve efficiency of closure_sync()

Michael Lyle
2017-12-30T23:09:46Z

From: Kent Overstreet <kent.overstreet@gmail.com>

Eliminates cases where sync can race and fail to complete / get stuck.
Removes many status flags and simplifies entering-and-exiting closure
sleeping behaviors.

[mlyle: fixed conflicts due to changed return behavior in mainline.
extended commit comment, and squashed down two commits that were mostly
contradictory to get to this state]

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Michael Lyle <mlyle@lyle.org>
Reviewed-by: Michael Lyle <mlyle@lyle.org>
---
drivers/md/bcache/closure.c | 47 ++++++++++++++++++-----------------
drivers/md/bcache/closure.h | 60 +++++++++++++++++----------------------------
2 files changed, 47 insertions(+), 60 deletions(-)

diff --git a/drivers/md/bcache/closure.c b/drivers/md/bcache/closure.c
index 1841d0359bac..8a4f4e4f5537 100644
--- a/drivers/md/bcache/closure.c
+++ b/drivers/md/bcache/closure.c
@@ -18,10 +18,6 @@ static inline void closure_put_after_sub(struct closure *cl, int flags)
BUG_ON(flags & CLOSURE_GUARD_MASK);
BUG_ON(!r && (flags & ~CLOSURE_DESTRUCTOR));

- /* Must deliver precisely one wakeup */
- if (r == 1 && (flags & CLOSURE_SLEEPING))
- wake_up_process(cl->task);
-
if (!r) {
if (cl->fn && !(flags & CLOSURE_DESTRUCTOR)) {
atomic_set(&cl->remaining,
@@ -100,28 +96,35 @@ bool closure_wait(struct closure_waitlist *waitlist, struct closure *cl)
}
EXPORT_SYMBOL(closure_wait);

-/**
- * closure_sync - sleep until a closure has nothing left to wait on
- *
- * Sleeps until the refcount hits 1 - the thread that's running the closure owns
- * the last refcount.
- */
-void closure_sync(struct closure *cl)
+struct closure_syncer {
+ struct task_struct *task;
+ int done;
+};
+
+static void closure_sync_fn(struct closure *cl)
{
- while (1) {
- __closure_start_sleep(cl);
- closure_set_ret_ip(cl);
+ cl->s->done = 1;
+ wake_up_process(cl->s->task);
+}

- if ((atomic_read(&cl->remaining) &
- CLOSURE_REMAINING_MASK) == 1)
- break;
+void __closure_sync(struct closure *cl)
+{
+ struct closure_syncer s = { .task = current };

+ cl->s = &s;
+ continue_at(cl, closure_sync_fn, NULL);
+
+ while (1) {
+ __set_current_state(TASK_UNINTERRUPTIBLE);
+ smp_mb(); /* Ensure task state set before load of done flag */
+ if (s.done)
+ break;
schedule();
}

- __closure_end_sleep(cl);
+ __set_current_state(TASK_RUNNING);
}
-EXPORT_SYMBOL(closure_sync);
+EXPORT_SYMBOL(__closure_sync);

#ifdef CONFIG_BCACHE_CLOSURES_DEBUG

@@ -168,12 +171,10 @@ static int debug_seq_show(struct seq_file *f, void *data)
cl, (void *) cl->ip, cl->fn, cl->parent,
r & CLOSURE_REMAINING_MASK);

- seq_printf(f, "%s%s%s%s\n",
+ seq_printf(f, "%s%s\n",
test_bit(WORK_STRUCT_PENDING_BIT,
work_data_bits(&cl->work)) ? "Q" : "",
- r & CLOSURE_RUNNING ? "R" : "",
- r & CLOSURE_STACK ? "S" : "",
- r & CLOSURE_SLEEPING ? "Sl" : "");
+ r & CLOSURE_RUNNING ? "R" : "");

if (r & CLOSURE_WAITING)
seq_printf(f, " W %pF\n",
diff --git a/drivers/md/bcache/closure.h b/drivers/md/bcache/closure.h
index ccfbea6f9f6b..392a87cf1b92 100644
--- a/drivers/md/bcache/closure.h
+++ b/drivers/md/bcache/closure.h
@@ -103,6 +103,7 @@
*/

struct closure;
+struct closure_syncer;
typedef void (closure_fn) (struct closure *);

struct closure_waitlist {
@@ -115,10 +116,6 @@ enum closure_state {
* the thread that owns the closure, and cleared by the thread that's
* waking up the closure.
*
- * CLOSURE_SLEEPING: Must be set before a thread uses a closure to sleep
- * - indicates that cl->task is valid and closure_put() may wake it up.
- * Only set or cleared by the thread that owns the closure.
- *
* The rest are for debugging and don't affect behaviour:
*
* CLOSURE_RUNNING: Set when a closure is running (i.e. by
@@ -128,22 +125,16 @@ enum closure_state {
* continue_at() and closure_return() clear it for you, if you're doing
* something unusual you can use closure_set_dead() which also helps
* annotate where references are being transferred.
- *
- * CLOSURE_STACK: Sanity check - remaining should never hit 0 on a
- * closure with this flag set
*/

- CLOSURE_BITS_START = (1 << 23),
- CLOSURE_DESTRUCTOR = (1 << 23),
- CLOSURE_WAITING = (1 << 25),
- CLOSURE_SLEEPING = (1 << 27),
- CLOSURE_RUNNING = (1 << 29),
- CLOSURE_STACK = (1 << 31),
+ CLOSURE_BITS_START = (1U << 27),
+ CLOSURE_DESTRUCTOR = (1U << 27),
+ CLOSURE_WAITING = (1U << 29),
+ CLOSURE_RUNNING = (1U << 31),
};

#define CLOSURE_GUARD_MASK \
- ((CLOSURE_DESTRUCTOR|CLOSURE_WAITING|CLOSURE_SLEEPING| \
- CLOSURE_RUNNING|CLOSURE_STACK) << 1)
+ ((CLOSURE_DESTRUCTOR|CLOSURE_WAITING|CLOSURE_RUNNING) << 1)

#define CLOSURE_REMAINING_MASK (CLOSURE_BITS_START - 1)
#define CLOSURE_REMAINING_INITIALIZER (1|CLOSURE_RUNNING)
@@ -152,7 +143,7 @@ struct closure {
union {
struct {
struct workqueue_struct *wq;
- struct task_struct *task;
+ struct closure_syncer *s;
struct llist_node list;
closure_fn *fn;
};
@@ -178,7 +169,19 @@ void closure_sub(struct closure *cl, int v);
void closure_put(struct closure *cl);
void __closure_wake_up(struct closure_waitlist *list);
bool closure_wait(struct closure_waitlist *list, struct closure *cl);
-void closure_sync(struct closure *cl);
+void __closure_sync(struct closure *cl);
+
+/**
+ * closure_sync - sleep until a closure a closure has nothing left to wait on
+ *
+ * Sleeps until the refcount hits 1 - the thread that's running the closure owns
+ * the last refcount.
+ */
+static inline void closure_sync(struct closure *cl)
+{
+ if ((atomic_read(&cl->remaining) & CLOSURE_REMAINING_MASK) != 1)
+ __closure_sync(cl);
+}

#ifdef CONFIG_BCACHE_CLOSURES_DEBUG

@@ -215,24 +218,6 @@ static inline void closure_set_waiting(struct closure *cl, unsigned long f)
#endif
}

-static inline void __closure_end_sleep(struct closure *cl)
-{
- __set_current_state(TASK_RUNNING);
-
- if (atomic_read(&cl->remaining) & CLOSURE_SLEEPING)
- atomic_sub(CLOSURE_SLEEPING, &cl->remaining);
-}
-
-static inline void __closure_start_sleep(struct closure *cl)
-{
- closure_set_ip(cl);
- cl->task = current;
- set_current_state(TASK_UNINTERRUPTIBLE);
-
- if (!(atomic_read(&cl->remaining) & CLOSURE_SLEEPING))
- atomic_add(CLOSURE_SLEEPING, &cl->remaining);
-}
-
static inline void closure_set_stopped(struct closure *cl)
{
atomic_sub(CLOSURE_RUNNING, &cl->remaining);
@@ -241,7 +226,6 @@ static inline void closure_set_stopped(struct closure *cl)
static inline void set_closure_fn(struct closure *cl, closure_fn *fn,
struct workqueue_struct *wq)
{
- BUG_ON(object_is_on_stack(cl));
closure_set_ip(cl);
cl->fn = fn;
cl->wq = wq;
@@ -300,7 +284,7 @@ static inline void closure_init(struct closure *cl, struct closure *parent)
static inline void closure_init_stack(struct closure *cl)
{
memset(cl, 0, sizeof(struct closure));
- atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER|CLOSURE_STACK);
+ atomic_set(&cl->remaining, CLOSURE_REMAINING_INITIALIZER);
}

/**
@@ -322,6 +306,8 @@ static inline void closure_wake_up(struct closure_waitlist *list)
* This is because after calling continue_at() you no longer have a ref on @cl,
* and whatever @cl owns may be freed out from under you - a running closure fn
* has a ref on its own closure which continue_at() drops.
+ *
+ * Note you are expected to immediately return after using this macro.
*/
#define continue_at(_cl, _fn, _wq) \
do { \
--
2.14.1