sched/wait: Introduce wait_var_event()

As a replacement for the wait_on_atomic_t() API provide the
wait_var_event() API.

The wait_var_event() API is based on the very same hashed-waitqueue
idea, but doesn't care about the type (atomic_t) or the specific
condition (atomic_read() == 0). IOW. it's much more widely
applicable/flexible.

It shares all the benefits/disadvantages of a hashed-waitqueue
approach with the existing wait_on_atomic_t/wait_on_bit() APIs.

The API is modeled after the existing wait_event() API, but instead of
taking a wait_queue_head, it takes an address. This addresses is
hashed to obtain a wait_queue_head from the bit_wait_table.

Similar to the wait_event() API, it takes a condition expression as
second argument and will wait until this expression becomes true.

The following are (mostly) identical replacements:

	wait_on_atomic_t(&my_atomic, atomic_t_wait, TASK_UNINTERRUPTIBLE);
	wake_up_atomic_t(&my_atomic);

	wait_var_event(&my_atomic, !atomic_read(&my_atomic));
	wake_up_var(&my_atomic);

The only difference is that wake_up_var() is an unconditional wakeup
and doesn't check the previously hard-coded (atomic_read() == 0)
condition here. This is of little concequence, since most callers are
already conditional on atomic_dec_and_test() and the ones that are
not, are trivial to make so.

Tested-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: David Howells <dhowells@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-kernel@vger.kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

2 files changed, 118 insertions, 0 deletions

diff --git a/include/linux/wait_bit.h b/include/linux/wait_bit.h
index 61b39eaf7cad..3fcdb75d69cf 100644
--- a/include/linux/wait_bit.h
+++ b/include/linux/wait_bit.h
@@ -262,4 +262,74 @@ int wait_on_atomic_t(atomic_t *val, wait_atomic_t_action_f action, unsigned mode
         return out_of_line_wait_on_atomic_t(val, action, mode);
 }
 
+extern void init_wait_var_entry(struct wait_bit_queue_entry *wbq_entry, void *var, int flags);
+extern void wake_up_var(void *var);
+extern wait_queue_head_t *__var_waitqueue(void *p);
+
+#define ___wait_var_event(var, condition, state, exclusive, ret, cmd)   \
+({                                                                      \
+        __label__ __out;                                                \
+        struct wait_queue_head *__wq_head = __var_waitqueue(var);       \
+        struct wait_bit_queue_entry __wbq_entry;                        \
+        long __ret = ret; /* explicit shadow */                         \
+                                                                        \
+        init_wait_var_entry(&__wbq_entry, var,                          \
+                            exclusive ? WQ_FLAG_EXCLUSIVE : 0);         \
+        for (;;) {                                                      \
+                long __int = prepare_to_wait_event(__wq_head,           \
+                                                   &__wbq_entry.wq_entry, \
+                                                   state);              \
+                if (condition)                                          \
+                        break;                                          \
+                                                                        \
+                if (___wait_is_interruptible(state) && __int) {         \
+                        __ret = __int;                                  \
+                        goto __out;                                     \
+                }                                                       \
+                                                                        \
+                cmd;                                                    \
+        }                                                               \
+        finish_wait(__wq_head, &__wbq_entry.wq_entry);                  \
+__out:  __ret;                                                          \
+})
+
+#define __wait_var_event(var, condition)                                \
+        ___wait_var_event(var, condition, TASK_UNINTERRUPTIBLE, 0, 0,   \
+                          schedule())
+
+#define wait_var_event(var, condition)                                  \
+do {                                                                    \
+        might_sleep();                                                  \
+        if (condition)                                                  \
+                break;                                                  \
+        __wait_var_event(var, condition);                               \
+} while (0)
+
+#define __wait_var_event_killable(var, condition)                       \
+        ___wait_var_event(var, condition, TASK_KILLABLE, 0, 0,          \
+                          schedule())
+
+#define wait_var_event_killable(var, condition)                         \
+({                                                                      \
+        int __ret = 0;                                                  \
+        might_sleep();                                                  \
+        if (!(condition))                                               \
+                __ret = __wait_var_event_killable(var, condition);      \
+        __ret;                                                          \
+})
+
+#define __wait_var_event_timeout(var, condition, timeout)               \
+        ___wait_var_event(var, ___wait_cond_timeout(condition),         \
+                          TASK_UNINTERRUPTIBLE, 0, timeout,             \
+                          __ret = schedule_timeout(__ret))
+
+#define wait_var_event_timeout(var, condition, timeout)                 \
+({                                                                      \
+        long __ret = timeout;                                           \
+        might_sleep();                                                  \
+        if (!___wait_cond_timeout(condition))                           \
+                __ret = __wait_var_event_timeout(var, condition, timeout); \
+        __ret;                                                          \
+})
+
 #endif /* _LINUX_WAIT_BIT_H */
diff --git a/kernel/sched/wait_bit.c b/kernel/sched/wait_bit.c
index 4239c78f5cd3..ed84ab245a05 100644
--- a/kernel/sched/wait_bit.c
+++ b/kernel/sched/wait_bit.c
@@ -149,6 +149,54 @@ void wake_up_bit(void *word, int bit)
 }
 EXPORT_SYMBOL(wake_up_bit);
 
+wait_queue_head_t *__var_waitqueue(void *p)
+{
+        if (BITS_PER_LONG == 64) {
+                unsigned long q = (unsigned long)p;
+
+                return bit_waitqueue((void *)(q & ~1), q & 1);
+        }
+        return bit_waitqueue(p, 0);
+}
+EXPORT_SYMBOL(__var_waitqueue);
+
+static int
+var_wake_function(struct wait_queue_entry *wq_entry, unsigned int mode,
+                  int sync, void *arg)
+{
+        struct wait_bit_key *key = arg;
+        struct wait_bit_queue_entry *wbq_entry =
+                container_of(wq_entry, struct wait_bit_queue_entry, wq_entry);
+
+        if (wbq_entry->key.flags != key->flags ||
+            wbq_entry->key.bit_nr != key->bit_nr)
+                return 0;
+
+        return autoremove_wake_function(wq_entry, mode, sync, key);
+}
+
+void init_wait_var_entry(struct wait_bit_queue_entry *wbq_entry, void *var, int flags)
+{
+        *wbq_entry = (struct wait_bit_queue_entry){
+                .key = {
+                        .flags  = (var),
+                        .bit_nr = -1,
+                },
+                .wq_entry = {
+                        .private = current,
+                        .func    = var_wake_function,
+                        .entry   = LIST_HEAD_INIT(wbq_entry->wq_entry.entry),
+                },
+        };
+}
+EXPORT_SYMBOL(init_wait_var_entry);
+
+void wake_up_var(void *var)
+{
+        __wake_up_bit(__var_waitqueue(var), var, -1);
+}
+EXPORT_SYMBOL(wake_up_var);
+
 /*
  * Manipulate the atomic_t address to produce a better bit waitqueue table hash
  * index (we're keying off bit -1, but that would produce a horrible hash