[PATCH] poll: allow f_op->poll to sleep, take #4

[Tejun Heo <htejun@gmail.com>]

f_op->poll is the only vfs operation which is not allowed to sleep.
It's because poll and select implementation used task state to
synchronize against wake ups, which doesn't have to be the case
anymore as wait/wake interface can now use custom wake up functions.
The non-sleep restriction can be a bit tricky because ->poll is not
called from an atomic context and the result of accidentally sleeping
in ->poll only shows up as temporary busy looping when the timing is
right or rather wrong.

This patch converts poll/select to use custom wake up function and use
separate triggered variable to synchronize against wake up events.
The only added overhead is an extra function call during wake up and
negligible.

This patch removes the one non-sleep exception from vfs locking rules
and is beneficial to userland filesystem implementations like FUSE, 9p
or peculiar fs like spufs as it's very difficult for those to
implement non-sleeping poll method.

While at it, make the following cosmetic changes to make poll.h and
select.c checkpatch friendly.

* s/type * symbol/type *symbol/		   : three places in poll.h
* remove blank line before EXPORT_SYMBOL() : two places in select.c

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Eric Van Hensbergen <ericvh@gmail.com>
Cc: Ron Minnich <rminnich@sandia.gov>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Cc: Davide Libenzi <davidel@xmailserver.org>
Cc: Brad Boyer <flar@allandria.com>
---
Added TODO comment to switch to wake_up_process() once @sync is gone.

Thanks.

 Documentation/filesystems/Locking |    2 -
 drivers/media/video/v4l1-compat.c |    4 --
 fs/select.c                       |   58
++++++++++++++++++++++++++++----------
 include/linux/poll.h              |   15 +++++++--
 4 files changed, 58 insertions(+), 21 deletions(-)

diff --git a/Documentation/filesystems/Locking
b/Documentation/filesystems/Locking
index 23d2f44..250f3c4 100644
--- a/Documentation/filesystems/Locking
+++ b/Documentation/filesystems/Locking
@@ -398,7 +398,7 @@ prototypes:
 };

 locking rules:
-	All except ->poll() may block.
+	All may block.
 			BKL
 llseek:			no	(see below)
 read:			no
diff --git a/drivers/media/video/v4l1-compat.c
b/drivers/media/video/v4l1-compat.c
index f13c0a9..d0253da 100644
--- a/drivers/media/video/v4l1-compat.c
+++ b/drivers/media/video/v4l1-compat.c
@@ -203,7 +203,6 @@ static int poll_one(struct file *file, struct
poll_wqueues *pwq)
 	table = &pwq->pt;
 	for (;;) {
 		int mask;
-		set_current_state(TASK_INTERRUPTIBLE);
 		mask = file->f_op->poll(file, table);
 		if (mask & POLLIN)
 			break;
@@ -212,9 +211,8 @@ static int poll_one(struct file *file, struct
poll_wqueues *pwq)
 			retval = -ERESTARTSYS;
 			break;
 		}
-		schedule();
+		poll_schedule(pwq, TASK_INTERRUPTIBLE);
 	}
-	set_current_state(TASK_RUNNING);
 	poll_freewait(pwq);
 	return retval;
 }
diff --git a/fs/select.c b/fs/select.c
index 87df51e..f2ef68b 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -109,11 +109,11 @@ static void __pollwait(struct file *filp,
wait_queue_head_t *wait_address,
 void poll_initwait(struct poll_wqueues *pwq)
 {
 	init_poll_funcptr(&pwq->pt, __pollwait);
+	pwq->polling_task = current;
 	pwq->error = 0;
 	pwq->table = NULL;
 	pwq->inline_index = 0;
 }
-
 EXPORT_SYMBOL(poll_initwait);

 static void free_poll_entry(struct poll_table_entry *entry)
@@ -142,12 +142,10 @@ void poll_freewait(struct poll_wqueues *pwq)
 		free_page((unsigned long) old);
 	}
 }
-
 EXPORT_SYMBOL(poll_freewait);

-static struct poll_table_entry *poll_get_entry(poll_table *_p)
+static struct poll_table_entry *poll_get_entry(struct poll_wqueues *p)
 {
-	struct poll_wqueues *p = container_of(_p, struct poll_wqueues, pt);
 	struct poll_table_page *table = p->table;

 	if (p->inline_index < N_INLINE_POLL_ENTRIES)
@@ -159,7 +157,6 @@ static struct poll_table_entry
*poll_get_entry(poll_table *_p)
 		new_table = (struct poll_table_page *) __get_free_page(GFP_KERNEL);
 		if (!new_table) {
 			p->error = -ENOMEM;
-			__set_current_state(TASK_RUNNING);
 			return NULL;
 		}
 		new_table->entry = new_table->entries;
@@ -171,20 +168,57 @@ static struct poll_table_entry
*poll_get_entry(poll_table *_p)
 	return table->entry++;
 }

+static int pollwake(wait_queue_t *wait, unsigned mode, int sync, void *key)
+{
+	struct poll_wqueues *pwq = wait->private;
+	DECLARE_WAITQUEUE(dummy_wait, pwq->polling_task);
+
+	set_mb(pwq->triggered, 1);
+
+	/*
+	 * Perform the default wake up operation using a dummy
+	 * waitqueue.
+	 *
+	 * TODO: This is hacky but there currently is no interface to
+	 * pass in @sync.  @sync is scheduled to be removed and once
+	 * that happens, wake_up_process() can be used directly.
+	 */
+	return default_wake_function(&dummy_wait, mode, sync, key);
+}
+
 /* Add a new entry */
 static void __pollwait(struct file *filp, wait_queue_head_t *wait_address,
 				poll_table *p)
 {
-	struct poll_table_entry *entry = poll_get_entry(p);
+	struct poll_wqueues *pwq = container_of(p, struct poll_wqueues, pt);
+	struct poll_table_entry *entry = poll_get_entry(pwq);
 	if (!entry)
 		return;
 	get_file(filp);
 	entry->filp = filp;
 	entry->wait_address = wait_address;
-	init_waitqueue_entry(&entry->wait, current);
+	init_waitqueue_func_entry(&entry->wait, pollwake);
+	entry->wait.private = pwq;
 	add_wait_queue(wait_address, &entry->wait);
 }

+int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
+			  ktime_t *expires, unsigned long slack)
+{
+	int rc = -EINTR;
+
+	set_current_state(state);
+	if (!pwq->triggered)
+		rc = schedule_hrtimeout_range(expires, slack, HRTIMER_MODE_ABS);
+	__set_current_state(TASK_RUNNING);
+
+	/* clear triggered for the next iteration */
+	pwq->triggered = 0;
+
+	return rc;
+}
+EXPORT_SYMBOL(poll_schedule_timeout);
+
 /**
  * poll_select_set_timeout - helper function to setup the timeout value
  * @to:		pointer to timespec variable for the final timeout
@@ -340,8 +374,6 @@ int do_select(int n, fd_set_bits *fds, struct
timespec *end_time)
 	for (;;) {
 		unsigned long *rinp, *routp, *rexp, *inp, *outp, *exp;

-		set_current_state(TASK_INTERRUPTIBLE);
-
 		inp = fds->in; outp = fds->out; exp = fds->ex;
 		rinp = fds->res_in; routp = fds->res_out; rexp = fds->res_ex;

@@ -411,10 +443,10 @@ int do_select(int n, fd_set_bits *fds, struct
timespec *end_time)
 			to = &expire;
 		}

-		if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
+		if (!poll_schedule_timeout(&table, TASK_INTERRUPTIBLE,
+					   to, slack))
 			timed_out = 1;
 	}
-	__set_current_state(TASK_RUNNING);

 	poll_freewait(&table);

@@ -666,7 +698,6 @@ static int do_poll(unsigned int nfds,  struct
poll_list *list,
 	for (;;) {
 		struct poll_list *walk;

-		set_current_state(TASK_INTERRUPTIBLE);
 		for (walk = list; walk != NULL; walk = walk->next) {
 			struct pollfd * pfd, * pfd_end;

@@ -709,10 +740,9 @@ static int do_poll(unsigned int nfds,  struct
poll_list *list,
 			to = &expire;
 		}

-		if (!schedule_hrtimeout_range(to, slack, HRTIMER_MODE_ABS))
+		if (!poll_schedule_timeout(wait, TASK_INTERRUPTIBLE, to, slack))
 			timed_out = 1;
 	}
-	__set_current_state(TASK_RUNNING);
 	return count;
 }

diff --git a/include/linux/poll.h b/include/linux/poll.h
index badd98a..8c24ef8 100644
--- a/include/linux/poll.h
+++ b/include/linux/poll.h
@@ -46,9 +46,9 @@ static inline void init_poll_funcptr(poll_table *pt,
poll_queue_proc qproc)
 }

 struct poll_table_entry {
-	struct file * filp;
+	struct file *filp;
 	wait_queue_t wait;
-	wait_queue_head_t * wait_address;
+	wait_queue_head_t *wait_address;
 };

 /*
@@ -56,7 +56,9 @@ struct poll_table_entry {
  */
 struct poll_wqueues {
 	poll_table pt;
-	struct poll_table_page * table;
+	struct poll_table_page *table;
+	struct task_struct *polling_task;
+	int triggered;
 	int error;
 	int inline_index;
 	struct poll_table_entry inline_entries[N_INLINE_POLL_ENTRIES];
@@ -64,6 +66,13 @@ struct poll_wqueues {

 extern void poll_initwait(struct poll_wqueues *pwq);
 extern void poll_freewait(struct poll_wqueues *pwq);
+extern int poll_schedule_timeout(struct poll_wqueues *pwq, int state,
+				 ktime_t *expires, unsigned long slack);
+
+static inline int poll_schedule(struct poll_wqueues *pwq, int state)
+{
+	return poll_schedule_timeout(pwq, state, NULL, 0);
+}

 /*
  * Scaleable version of the fd_set.