This fixes a race condition between __tty_buffer_request_room and
flush_to_ldisc which leads to a loss of data in flip-buffer.
The problem was introduced while development of linux-3.11 by
e8437d7ecbc50198705331449367d401ebb3181f and
e9975fdec0138f1b2a85b9624e41660abd9865d4.
Originally it was found and reproduced on linux-3.12.15 and
linux-3.12.15-rt25, by sending 500 byte blocks with 115kbaud to the
target uart in a loop with 100 milliseconds delay.
In short:
1. The consumer flush_to_ldisc is on to remove the head tty_buffer.
2. The producer adds a number of bytes, so that a new tty_buffer must
be allocated and added by __tty_buffer_request_room.
3. The consumer removes the head tty_buffer element, without handling
newly committed data.
Detailed example:
* Initial buffer:
* Head, Tail -> 0: used=250; commit=250; read=240; next=NULL
* Consumer: ''flush_to_ldisc''
* consumed 10 Byte
* buffer:
* Head, Tail -> 0: used=250; commit=250; read=250; next=NULL
{{{
count = head->commit - head->read; // count = 0
if (!count) { // enter
// INTERRUPTED BY PRODUCER ->
if (head->next == NULL)
break;
buf->head = head->next;
tty_buffer_free(port, head);
continue;
}
}}}
* Producer: tty_insert_flip_... 10 bytes + tty_flip_buffer_push
* buffer:
* Head, Tail -> 0: used=250; commit=250; read=250; next=NULL
* added 6 bytes: head-element filled to maximum.
* buffer:
* Head, Tail -> 0: used=256; commit=250; read=250; next=NULL
* added 4 bytes: __tty_buffer_request_room is called
* buffer:
* Head -> 0: used=256; commit=256; read=250; next=1
* Tail -> 1: used=4; commit=0; read=250 next=NULL
* push (tty_flip_buffer_push)
* buffer:
* Head -> 0: used=256; commit=256; read=250; next=1
* Tail -> 1: used=4; commit=4; read=250 next=NULL
* Consumer
{{{
count = head->commit - head->read;
if (!count) {
// INTERRUPTED BY PRODUCER <-
if (head->next == NULL) // -> no break
break;
buf->head = head->next;
tty_buffer_free(port, head);
// ERROR: tty_buffer head freed -> 6 bytes lost
continue;
}
}}}
This patch reintroduces a spin_lock to protect this case. Perhaps later
a lock-less solution could be found.
Signed-off-by: Manfred Schlaegl <manfred.schlaegl@xxxxxx>
---
drivers/tty/tty_buffer.c | 16 ++++++++++++++--
include/linux/tty.h | 1 +
2 files changed, 15 insertions(+), 2 deletions(-)
diff --git a/drivers/tty/tty_buffer.c b/drivers/tty/tty_buffer.c
index c043136f..9deefcc 100644
--- a/drivers/tty/tty_buffer.c
+++ b/drivers/tty/tty_buffer.c
@@ -246,10 +246,15 @@ int tty_buffer_request_room(struct tty_port
*port, size_t size)
if (left < size) {
/* This is the slow path - looking for new buffers to use */
if ((n = tty_buffer_alloc(port, size)) != NULL) {
+ unsigned int flags;
+
buf->tail = n;
+
+ spin_lock_irqsave(&buf->flush_lock, flags);
b->commit = b->used;
- smp_mb();
b->next = n;
+ spin_unlock_irqrestore(&buf->flush_lock, flags);
+
} else
size = left;
}
@@ -451,6 +456,7 @@ static void flush_to_ldisc(struct work_struct *work)
mutex_lock(&buf->lock);
while (1) {
+ unsigned long flags;
struct tty_buffer *head = buf->head;
int count;
@@ -458,14 +464,19 @@ static void flush_to_ldisc(struct work_struct *work)
if (atomic_read(&buf->priority))
break;
+ spin_lock_irqsave(&buf->flush_lock, flags);
count = head->commit - head->read;
if (!count) {
- if (head->next == NULL)
+ if (head->next == NULL) {
+ spin_unlock_irqrestore(&buf->flush_lock, flags);
break;
+ }
buf->head = head->next;
+ spin_unlock_irqrestore(&buf->flush_lock, flags);
tty_buffer_free(port, head);
continue;
}
+ spin_unlock_irqrestore(&buf->flush_lock, flags);
count = receive_buf(tty, head, count);
if (!count)
@@ -529,6 +540,7 @@ void tty_buffer_init(struct tty_port *port)
struct tty_bufhead *buf = &port->buf;
mutex_init(&buf->lock);
+ spin_lock_init(&buf->flush_lock);
tty_buffer_reset(&buf->sentinel, 0);
buf->head = &buf->sentinel;
buf->tail = &buf->sentinel;
diff --git a/include/linux/tty.h b/include/linux/tty.h
index 97d660e..f22573d 100644
--- a/include/linux/tty.h
+++ b/include/linux/tty.h
@@ -57,6 +57,7 @@ struct tty_bufhead {
struct tty_buffer *head; /* Queue head */
struct work_struct work;
struct mutex lock;
+ spinlock_t flush_lock;
atomic_t priority;
struct tty_buffer sentinel;
struct llist_head free; /* Free queue head */
--
1.7.10.4
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