linux/drivers/tty/tty_buffer.c
Linus Torvalds 81de916f19 tty_buffer: get rid of 'seen_tail' logic in flush_to_ldisc
The flush_to_ldisc() work entry has special logic to notice when it has
seen the original tail of the data queue, and it avoids continuing the
flush if it sees that _original_ tail rather than the current tail.

This logic can trigger in case somebody is constantly adding new data to
the tty while the flushing is active - and the intent is to avoid
excessive CPU usage while flushing the tty, especially as we used to do
this from a softirq context which made it non-preemptible.

However, since we no longer re-arm the work-queue from within itself
(because that causes other trouble: see commit a5660b41af "tty: fix
endless work loop when the buffer fills up"), this just leads to
possible hung tty's (most easily seen in SMP and with a test-program
that floods a pty with data - nobody seems to have reported this for any
real-life situation yet).

And since the workqueue isn't done from timers and softirq's any more,
it's doubtful whether the CPU useage issue is really relevant any more.
So just remove the logic entirely, and see if anybody ever notices.

Alternatively, we might want to re-introduce the "re-arm the work" for
just this case, but then we'd have to re-introduce the delayed work
model or some explicit timer, which really doesn't seem worth it for
this.

Reported-and-tested-by: Guillaume Chazarain <guichaz@gmail.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2011-06-08 07:46:30 -07:00

523 lines
14 KiB
C

/*
* Tty buffer allocation management
*/
#include <linux/types.h>
#include <linux/errno.h>
#include <linux/tty.h>
#include <linux/tty_driver.h>
#include <linux/tty_flip.h>
#include <linux/timer.h>
#include <linux/string.h>
#include <linux/slab.h>
#include <linux/sched.h>
#include <linux/init.h>
#include <linux/wait.h>
#include <linux/bitops.h>
#include <linux/delay.h>
#include <linux/module.h>
/**
* tty_buffer_free_all - free buffers used by a tty
* @tty: tty to free from
*
* Remove all the buffers pending on a tty whether queued with data
* or in the free ring. Must be called when the tty is no longer in use
*
* Locking: none
*/
void tty_buffer_free_all(struct tty_struct *tty)
{
struct tty_buffer *thead;
while ((thead = tty->buf.head) != NULL) {
tty->buf.head = thead->next;
kfree(thead);
}
while ((thead = tty->buf.free) != NULL) {
tty->buf.free = thead->next;
kfree(thead);
}
tty->buf.tail = NULL;
tty->buf.memory_used = 0;
}
/**
* tty_buffer_alloc - allocate a tty buffer
* @tty: tty device
* @size: desired size (characters)
*
* Allocate a new tty buffer to hold the desired number of characters.
* Return NULL if out of memory or the allocation would exceed the
* per device queue
*
* Locking: Caller must hold tty->buf.lock
*/
static struct tty_buffer *tty_buffer_alloc(struct tty_struct *tty, size_t size)
{
struct tty_buffer *p;
if (tty->buf.memory_used + size > 65536)
return NULL;
p = kmalloc(sizeof(struct tty_buffer) + 2 * size, GFP_ATOMIC);
if (p == NULL)
return NULL;
p->used = 0;
p->size = size;
p->next = NULL;
p->commit = 0;
p->read = 0;
p->char_buf_ptr = (char *)(p->data);
p->flag_buf_ptr = (unsigned char *)p->char_buf_ptr + size;
tty->buf.memory_used += size;
return p;
}
/**
* tty_buffer_free - free a tty buffer
* @tty: tty owning the buffer
* @b: the buffer to free
*
* Free a tty buffer, or add it to the free list according to our
* internal strategy
*
* Locking: Caller must hold tty->buf.lock
*/
static void tty_buffer_free(struct tty_struct *tty, struct tty_buffer *b)
{
/* Dumb strategy for now - should keep some stats */
tty->buf.memory_used -= b->size;
WARN_ON(tty->buf.memory_used < 0);
if (b->size >= 512)
kfree(b);
else {
b->next = tty->buf.free;
tty->buf.free = b;
}
}
/**
* __tty_buffer_flush - flush full tty buffers
* @tty: tty to flush
*
* flush all the buffers containing receive data. Caller must
* hold the buffer lock and must have ensured no parallel flush to
* ldisc is running.
*
* Locking: Caller must hold tty->buf.lock
*/
static void __tty_buffer_flush(struct tty_struct *tty)
{
struct tty_buffer *thead;
while ((thead = tty->buf.head) != NULL) {
tty->buf.head = thead->next;
tty_buffer_free(tty, thead);
}
tty->buf.tail = NULL;
}
/**
* tty_buffer_flush - flush full tty buffers
* @tty: tty to flush
*
* flush all the buffers containing receive data. If the buffer is
* being processed by flush_to_ldisc then we defer the processing
* to that function
*
* Locking: none
*/
void tty_buffer_flush(struct tty_struct *tty)
{
unsigned long flags;
spin_lock_irqsave(&tty->buf.lock, flags);
/* If the data is being pushed to the tty layer then we can't
process it here. Instead set a flag and the flush_to_ldisc
path will process the flush request before it exits */
if (test_bit(TTY_FLUSHING, &tty->flags)) {
set_bit(TTY_FLUSHPENDING, &tty->flags);
spin_unlock_irqrestore(&tty->buf.lock, flags);
wait_event(tty->read_wait,
test_bit(TTY_FLUSHPENDING, &tty->flags) == 0);
return;
} else
__tty_buffer_flush(tty);
spin_unlock_irqrestore(&tty->buf.lock, flags);
}
/**
* tty_buffer_find - find a free tty buffer
* @tty: tty owning the buffer
* @size: characters wanted
*
* Locate an existing suitable tty buffer or if we are lacking one then
* allocate a new one. We round our buffers off in 256 character chunks
* to get better allocation behaviour.
*
* Locking: Caller must hold tty->buf.lock
*/
static struct tty_buffer *tty_buffer_find(struct tty_struct *tty, size_t size)
{
struct tty_buffer **tbh = &tty->buf.free;
while ((*tbh) != NULL) {
struct tty_buffer *t = *tbh;
if (t->size >= size) {
*tbh = t->next;
t->next = NULL;
t->used = 0;
t->commit = 0;
t->read = 0;
tty->buf.memory_used += t->size;
return t;
}
tbh = &((*tbh)->next);
}
/* Round the buffer size out */
size = (size + 0xFF) & ~0xFF;
return tty_buffer_alloc(tty, size);
/* Should possibly check if this fails for the largest buffer we
have queued and recycle that ? */
}
/**
* tty_buffer_request_room - grow tty buffer if needed
* @tty: tty structure
* @size: size desired
*
* Make at least size bytes of linear space available for the tty
* buffer. If we fail return the size we managed to find.
*
* Locking: Takes tty->buf.lock
*/
int tty_buffer_request_room(struct tty_struct *tty, size_t size)
{
struct tty_buffer *b, *n;
int left;
unsigned long flags;
spin_lock_irqsave(&tty->buf.lock, flags);
/* OPTIMISATION: We could keep a per tty "zero" sized buffer to
remove this conditional if its worth it. This would be invisible
to the callers */
if ((b = tty->buf.tail) != NULL)
left = b->size - b->used;
else
left = 0;
if (left < size) {
/* This is the slow path - looking for new buffers to use */
if ((n = tty_buffer_find(tty, size)) != NULL) {
if (b != NULL) {
b->next = n;
b->commit = b->used;
} else
tty->buf.head = n;
tty->buf.tail = n;
} else
size = left;
}
spin_unlock_irqrestore(&tty->buf.lock, flags);
return size;
}
EXPORT_SYMBOL_GPL(tty_buffer_request_room);
/**
* tty_insert_flip_string_fixed_flag - Add characters to the tty buffer
* @tty: tty structure
* @chars: characters
* @flag: flag value for each character
* @size: size
*
* Queue a series of bytes to the tty buffering. All the characters
* passed are marked with the supplied flag. Returns the number added.
*
* Locking: Called functions may take tty->buf.lock
*/
int tty_insert_flip_string_fixed_flag(struct tty_struct *tty,
const unsigned char *chars, char flag, size_t size)
{
int copied = 0;
do {
int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
int space = tty_buffer_request_room(tty, goal);
struct tty_buffer *tb = tty->buf.tail;
/* If there is no space then tb may be NULL */
if (unlikely(space == 0))
break;
memcpy(tb->char_buf_ptr + tb->used, chars, space);
memset(tb->flag_buf_ptr + tb->used, flag, space);
tb->used += space;
copied += space;
chars += space;
/* There is a small chance that we need to split the data over
several buffers. If this is the case we must loop */
} while (unlikely(size > copied));
return copied;
}
EXPORT_SYMBOL(tty_insert_flip_string_fixed_flag);
/**
* tty_insert_flip_string_flags - Add characters to the tty buffer
* @tty: tty structure
* @chars: characters
* @flags: flag bytes
* @size: size
*
* Queue a series of bytes to the tty buffering. For each character
* the flags array indicates the status of the character. Returns the
* number added.
*
* Locking: Called functions may take tty->buf.lock
*/
int tty_insert_flip_string_flags(struct tty_struct *tty,
const unsigned char *chars, const char *flags, size_t size)
{
int copied = 0;
do {
int goal = min_t(size_t, size - copied, TTY_BUFFER_PAGE);
int space = tty_buffer_request_room(tty, goal);
struct tty_buffer *tb = tty->buf.tail;
/* If there is no space then tb may be NULL */
if (unlikely(space == 0))
break;
memcpy(tb->char_buf_ptr + tb->used, chars, space);
memcpy(tb->flag_buf_ptr + tb->used, flags, space);
tb->used += space;
copied += space;
chars += space;
flags += space;
/* There is a small chance that we need to split the data over
several buffers. If this is the case we must loop */
} while (unlikely(size > copied));
return copied;
}
EXPORT_SYMBOL(tty_insert_flip_string_flags);
/**
* tty_schedule_flip - push characters to ldisc
* @tty: tty to push from
*
* Takes any pending buffers and transfers their ownership to the
* ldisc side of the queue. It then schedules those characters for
* processing by the line discipline.
*
* Locking: Takes tty->buf.lock
*/
void tty_schedule_flip(struct tty_struct *tty)
{
unsigned long flags;
spin_lock_irqsave(&tty->buf.lock, flags);
if (tty->buf.tail != NULL)
tty->buf.tail->commit = tty->buf.tail->used;
spin_unlock_irqrestore(&tty->buf.lock, flags);
schedule_work(&tty->buf.work);
}
EXPORT_SYMBOL(tty_schedule_flip);
/**
* tty_prepare_flip_string - make room for characters
* @tty: tty
* @chars: return pointer for character write area
* @size: desired size
*
* Prepare a block of space in the buffer for data. Returns the length
* available and buffer pointer to the space which is now allocated and
* accounted for as ready for normal characters. This is used for drivers
* that need their own block copy routines into the buffer. There is no
* guarantee the buffer is a DMA target!
*
* Locking: May call functions taking tty->buf.lock
*/
int tty_prepare_flip_string(struct tty_struct *tty, unsigned char **chars,
size_t size)
{
int space = tty_buffer_request_room(tty, size);
if (likely(space)) {
struct tty_buffer *tb = tty->buf.tail;
*chars = tb->char_buf_ptr + tb->used;
memset(tb->flag_buf_ptr + tb->used, TTY_NORMAL, space);
tb->used += space;
}
return space;
}
EXPORT_SYMBOL_GPL(tty_prepare_flip_string);
/**
* tty_prepare_flip_string_flags - make room for characters
* @tty: tty
* @chars: return pointer for character write area
* @flags: return pointer for status flag write area
* @size: desired size
*
* Prepare a block of space in the buffer for data. Returns the length
* available and buffer pointer to the space which is now allocated and
* accounted for as ready for characters. This is used for drivers
* that need their own block copy routines into the buffer. There is no
* guarantee the buffer is a DMA target!
*
* Locking: May call functions taking tty->buf.lock
*/
int tty_prepare_flip_string_flags(struct tty_struct *tty,
unsigned char **chars, char **flags, size_t size)
{
int space = tty_buffer_request_room(tty, size);
if (likely(space)) {
struct tty_buffer *tb = tty->buf.tail;
*chars = tb->char_buf_ptr + tb->used;
*flags = tb->flag_buf_ptr + tb->used;
tb->used += space;
}
return space;
}
EXPORT_SYMBOL_GPL(tty_prepare_flip_string_flags);
/**
* flush_to_ldisc
* @work: tty structure passed from work queue.
*
* This routine is called out of the software interrupt to flush data
* from the buffer chain to the line discipline.
*
* Locking: holds tty->buf.lock to guard buffer list. Drops the lock
* while invoking the line discipline receive_buf method. The
* receive_buf method is single threaded for each tty instance.
*/
static void flush_to_ldisc(struct work_struct *work)
{
struct tty_struct *tty =
container_of(work, struct tty_struct, buf.work);
unsigned long flags;
struct tty_ldisc *disc;
disc = tty_ldisc_ref(tty);
if (disc == NULL) /* !TTY_LDISC */
return;
spin_lock_irqsave(&tty->buf.lock, flags);
if (!test_and_set_bit(TTY_FLUSHING, &tty->flags)) {
struct tty_buffer *head;
while ((head = tty->buf.head) != NULL) {
int count;
char *char_buf;
unsigned char *flag_buf;
count = head->commit - head->read;
if (!count) {
if (head->next == NULL)
break;
tty->buf.head = head->next;
tty_buffer_free(tty, head);
continue;
}
/* Ldisc or user is trying to flush the buffers
we are feeding to the ldisc, stop feeding the
line discipline as we want to empty the queue */
if (test_bit(TTY_FLUSHPENDING, &tty->flags))
break;
if (!tty->receive_room)
break;
if (count > tty->receive_room)
count = tty->receive_room;
char_buf = head->char_buf_ptr + head->read;
flag_buf = head->flag_buf_ptr + head->read;
head->read += count;
spin_unlock_irqrestore(&tty->buf.lock, flags);
disc->ops->receive_buf(tty, char_buf,
flag_buf, count);
spin_lock_irqsave(&tty->buf.lock, flags);
}
clear_bit(TTY_FLUSHING, &tty->flags);
}
/* We may have a deferred request to flush the input buffer,
if so pull the chain under the lock and empty the queue */
if (test_bit(TTY_FLUSHPENDING, &tty->flags)) {
__tty_buffer_flush(tty);
clear_bit(TTY_FLUSHPENDING, &tty->flags);
wake_up(&tty->read_wait);
}
spin_unlock_irqrestore(&tty->buf.lock, flags);
tty_ldisc_deref(disc);
}
/**
* tty_flush_to_ldisc
* @tty: tty to push
*
* Push the terminal flip buffers to the line discipline.
*
* Must not be called from IRQ context.
*/
void tty_flush_to_ldisc(struct tty_struct *tty)
{
flush_work(&tty->buf.work);
}
/**
* tty_flip_buffer_push - terminal
* @tty: tty to push
*
* Queue a push of the terminal flip buffers to the line discipline. This
* function must not be called from IRQ context if tty->low_latency is set.
*
* In the event of the queue being busy for flipping the work will be
* held off and retried later.
*
* Locking: tty buffer lock. Driver locks in low latency mode.
*/
void tty_flip_buffer_push(struct tty_struct *tty)
{
unsigned long flags;
spin_lock_irqsave(&tty->buf.lock, flags);
if (tty->buf.tail != NULL)
tty->buf.tail->commit = tty->buf.tail->used;
spin_unlock_irqrestore(&tty->buf.lock, flags);
if (tty->low_latency)
flush_to_ldisc(&tty->buf.work);
else
schedule_work(&tty->buf.work);
}
EXPORT_SYMBOL(tty_flip_buffer_push);
/**
* tty_buffer_init - prepare a tty buffer structure
* @tty: tty to initialise
*
* Set up the initial state of the buffer management for a tty device.
* Must be called before the other tty buffer functions are used.
*
* Locking: none
*/
void tty_buffer_init(struct tty_struct *tty)
{
spin_lock_init(&tty->buf.lock);
tty->buf.head = NULL;
tty->buf.tail = NULL;
tty->buf.free = NULL;
tty->buf.memory_used = 0;
INIT_WORK(&tty->buf.work, flush_to_ldisc);
}