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Subject[RFC][PATCH] tty: remove the BKL in n_tty.c
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Date

By using a bkl tracer, I saw that the opost function in n_tty.c, used when we
write to the console, requests a lock_kernel(). And since I'm displaying the
trace during the tracing, I see this lock very often.

The opost and opost_block functions in drivers/char/n_tty.c use
the bkl to protect the field tty->column.
Actually this attribute seems to be only touched on these two functions and
also we are in user context.

A mutex seems to be sufficient to prevent from race conditions here.
I made some tests by writing on a tty console whith several threads and it
seems to be correct.

I don't know where to post this patch. I'm not sure if the kill-the-bkl is
still on the air on the -tip tree.

Any comments?

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
diff --git a/drivers/char/n_tty.c b/drivers/char/n_tty.c
index 708c2b1..55c963f 100644
--- a/drivers/char/n_tty.c
+++ b/drivers/char/n_tty.c
@@ -62,6 +62,13 @@
#define TTY_THRESHOLD_THROTTLE 128 /* now based on remaining room */
#define TTY_THRESHOLD_UNTHROTTLE 128

+
+/*
+ * This Mutex is used to replace a big kernel lock that protected
+ * the access to tty->column.
+ */
+static DEFINE_MUTEX(tty_column_mutex);
+
static inline unsigned char *alloc_buf(void)
{
gfp_t prio = in_interrupt() ? GFP_ATOMIC : GFP_KERNEL;
@@ -264,18 +271,19 @@ static inline int is_continuation(unsigned char c, struct tty_struct *tty)
* relevant in the world today. If you ever need them, add them here.
*
* Called from both the receive and transmit sides and can be called
- * re-entrantly. Relies on lock_kernel() for tty->column state.
+ * re-entrantly.
*/

static int opost(unsigned char c, struct tty_struct *tty)
{
int space, spaces;
+ int ret = 0;

space = tty_write_room(tty);
if (!space)
return -1;

- lock_kernel();
+ mutex_lock(&tty_column_mutex);
if (O_OPOST(tty)) {
switch (c) {
case '\n':
@@ -283,8 +291,8 @@ static int opost(unsigned char c, struct tty_struct *tty)
tty->column = 0;
if (O_ONLCR(tty)) {
if (space < 2) {
- unlock_kernel();
- return -1;
+ ret = -1;
+ goto out;
}
tty_put_char(tty, '\r');
tty->column = 0;
@@ -293,8 +301,7 @@ static int opost(unsigned char c, struct tty_struct *tty)
break;
case '\r':
if (O_ONOCR(tty) && tty->column == 0) {
- unlock_kernel();
- return 0;
+ goto out;
}
if (O_OCRNL(tty)) {
c = '\n';
@@ -308,13 +315,12 @@ static int opost(unsigned char c, struct tty_struct *tty)
spaces = 8 - (tty->column & 7);
if (O_TABDLY(tty) == XTABS) {
if (space < spaces) {
- unlock_kernel();
- return -1;
+ ret = -1;
+ goto out;
}
tty->column += spaces;
tty->ops->write(tty, " ", spaces);
- unlock_kernel();
- return 0;
+ goto out;
}
tty->column += spaces;
break;
@@ -331,8 +337,9 @@ static int opost(unsigned char c, struct tty_struct *tty)
}
}
tty_put_char(tty, c);
- unlock_kernel();
- return 0;
+out:
+ mutex_unlock(&tty_column_mutex);
+ return ret;
}

/**
@@ -346,8 +353,7 @@ static int opost(unsigned char c, struct tty_struct *tty)
* the simple cases normally found and helps to generate blocks of
* symbols for the console driver and thus improve performance.
*
- * Called from write_chan under the tty layer write lock. Relies
- * on lock_kernel for the tty->column state.
+ * Called from write_chan under the tty layer write lock.
*/

static ssize_t opost_block(struct tty_struct *tty,
@@ -363,7 +369,7 @@ static ssize_t opost_block(struct tty_struct *tty,
if (nr > space)
nr = space;

- lock_kernel();
+ mutex_lock(&tty_column_mutex);
for (i = 0, cp = buf; i < nr; i++, cp++) {
switch (*cp) {
case '\n':
@@ -398,7 +404,8 @@ break_out:
if (tty->ops->flush_chars)
tty->ops->flush_chars(tty);
i = tty->ops->write(tty, buf, i);
- unlock_kernel();
+
+ mutex_unlock(&tty_column_mutex);
return i;
}



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