Subject: Re: [PATCH 03/31] fs: introduce new ->get_poll_head and ->poll_mask methods

From: Christoph Hellwig <[email-masked]>
Date: Wed, 10 Jan 2018 18:42:40 +0100

On Wed, Jan 10, 2018 at 04:31:35PM +0000, Al Viro wrote:
> *snort*
> 
> Seeing that random drivers are, by far, the majority of instances...
> What I wonder is how many of them conform to that pattern and how
> many can be massaged to that form.
> 
> How painful would it be, to pick an instance with more than one wait
> queue involved, to convert drivers/char/random.c to that form?

Attached.  Not very painful at all.  Would be even nicer if we had a
wait_event version that can deal with keys, but I can look into that.

> static unsigned int vtpm_proxy_fops_poll(struct file *filp, poll_table *wait)
> {
>         struct proxy_dev *proxy_dev = filp->private_data;
>         unsigned ret;
> 
>         poll_wait(filp, &proxy_dev->wq, wait);
> 
>         ret = POLLOUT;
> 
>         mutex_lock(&proxy_dev->buf_lock);
> 
>         if (proxy_dev->req_len)
>                 ret |= POLLIN | POLLRDNORM;
> 
>         if (!(proxy_dev->state & STATE_OPENED_FLAG))
>                 ret |= POLLHUP;
> 
>         mutex_unlock(&proxy_dev->buf_lock);
> 
>         return ret;
> } 
> (mainline drivers/char/tpm/tpm_vtpm_proxy.c)

Yeah.  And what exactly is the lock protecting given that each
of them checks a single smaller than register sized variable?

> Is that mutex_lock() in there a bug?  Another fun case is dma_buf_poll()...

Right now it is not a bug, but it is not very helpful behavior.  We
actually have quite a few of those, and not allowing ->poll_mask to
is a good thing to catch these.
>>From 5f5549dc78b39799db01d48dc346d14561ec2003 Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <[email-masked]>
Date: Wed, 10 Jan 2018 18:37:51 +0100
Subject: random: convert to ->poll_mask

The big change is that random_read_wait and random_write_wait are merged
into a single waitqueue that uses keyed wakeups.  Because wait_event_*
doesn't know about that this will lead to occassional spurious wakeups
in _random_read and add_hwgenerator_randomness, but wait_event_* is
designed to handle these and were are not in a a hot path there.

Signed-off-by: Christoph Hellwig <[email-masked]>
---
 drivers/char/random.c | 27 +++++++++++++++------------
 1 file changed, 15 insertions(+), 12 deletions(-)

diff --git a/drivers/char/random.c b/drivers/char/random.c
index 64b59562c872..9a1b85928de3 100644
--- a/drivers/char/random.c
+++ b/drivers/char/random.c
@@ -401,8 +401,7 @@ static struct poolinfo {
 /*
  * Static global variables
  */
-static DECLARE_WAIT_QUEUE_HEAD(random_read_wait);
-static DECLARE_WAIT_QUEUE_HEAD(random_write_wait);
+static DECLARE_WAIT_QUEUE_HEAD(random_wait);
 static struct fasync_struct *fasync;
 
 static DEFINE_SPINLOCK(random_ready_list_lock);
@@ -710,7 +709,7 @@ static void credit_entropy_bits(struct entropy_store *r, int nbits)
 
 		/* should we wake readers? */
 		if (entropy_bits >= random_read_wakeup_bits) {
-			wake_up_interruptible(&random_read_wait);
+			wake_up_interruptible_poll(&random_wait, POLLIN);
 			kill_fasync(&fasync, SIGIO, POLL_IN);
 		}
 		/* If the input pool is getting full, send some
@@ -1293,7 +1292,7 @@ static size_t account(struct entropy_store *r, size_t nbytes, int min,
 	trace_debit_entropy(r->name, 8 * ibytes);
 	if (ibytes &&
 	    (r->entropy_count >> ENTROPY_SHIFT) < random_write_wakeup_bits) {
-		wake_up_interruptible(&random_write_wait);
+		wake_up_interruptible_poll(&random_wait, POLLOUT);
 		kill_fasync(&fasync, SIGIO, POLL_OUT);
 	}
 
@@ -1748,7 +1747,7 @@ _random_read(int nonblock, char __user *buf, size_t nbytes)
 		if (nonblock)
 			return -EAGAIN;
 
-		wait_event_interruptible(random_read_wait,
+		wait_event_interruptible(random_wait,
 			ENTROPY_BITS(&input_pool) >=
 			random_read_wakeup_bits);
 		if (signal_pending(current))
@@ -1784,14 +1783,17 @@ urandom_read(struct file *file, char __user *buf, size_t nbytes, loff_t *ppos)
 	return ret;
 }
 
+static struct wait_queue_head *
+random_get_poll_head(struct file *file, __poll_t events)
+{
+	return &random_wait;
+}
+
 static __poll_t
-random_poll(struct file *file, poll_table * wait)
+random_poll_mask(struct file *file, __poll_t events)
 {
-	__poll_t mask;
+	__poll_t mask = 0;
 
-	poll_wait(file, &random_read_wait, wait);
-	poll_wait(file, &random_write_wait, wait);
-	mask = 0;
 	if (ENTROPY_BITS(&input_pool) >= random_read_wakeup_bits)
 		mask |= POLLIN | POLLRDNORM;
 	if (ENTROPY_BITS(&input_pool) < random_write_wakeup_bits)
@@ -1890,7 +1892,8 @@ static int random_fasync(int fd, struct file *filp, int on)
 const struct file_operations random_fops = {
 	.read  = random_read,
 	.write = random_write,
-	.poll  = random_poll,
+	.get_poll_head  = random_get_poll_head,
+	.poll_mask  = random_poll_mask,
 	.unlocked_ioctl = random_ioctl,
 	.fasync = random_fasync,
 	.llseek = noop_llseek,
@@ -2223,7 +2226,7 @@ void add_hwgenerator_randomness(const char *buffer, size_t count,
 	 * We'll be woken up again once below random_write_wakeup_thresh,
 	 * or when the calling thread is about to terminate.
 	 */
-	wait_event_interruptible(random_write_wait, kthread_should_stop() ||
+	wait_event_interruptible(random_wait, kthread_should_stop() ||
 			ENTROPY_BITS(&input_pool) <= random_write_wakeup_bits);
 	mix_pool_bytes(poolp, buffer, count);
 	credit_entropy_bits(poolp, entropy);
-- 
2.14.2