Re: 3.8-rc regression with pps-ldisc due to 70ece7a731

George Spelvin
2013-02-04T04:19:01Z

Just FYI, here is the (ugly and appears to crash) sort of patch
I was contemplating.

You may consider this Signed-off-by: in the narrow technical sense that I
can certify the origin of the code, but obviously I do not consider it
a candidate for upstream merging. It is posted here in the hopes that
its sheer hideousness will inspire someone else to show that they can
do better.

diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index 19083ef..c149c70 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -73,7 +73,10 @@
#define ECHO_OP_SET_CANON_COL 0x81
#define ECHO_OP_ERASE_TAB 0x82

+struct pps_device; /* Used by drivers/pps/clients/pps-ldisc.c */
+
struct n_tty_data {
+ struct pps_device *pps; /* First so pps-ldisc doesn't have to know offset. */
unsigned int column;
unsigned long overrun_time;
int num_overrun;
@@ -1636,6 +1639,7 @@ static int n_tty_open(struct tty_struct *tty)
mutex_init(&ldata->output_lock);
mutex_init(&ldata->echo_lock);
spin_lock_init(&ldata->read_lock);
+ //tty->pps = NULL; /* Done by kzalloc */

/* These are ugly. Currently a malloc failure here can panic */
ldata->read_buf = kzalloc(N_TTY_BUF_SIZE, GFP_KERNEL);
@@ -1646,10 +1650,10 @@ static int n_tty_open(struct tty_struct *tty)
tty->disc_data = ldata;
reset_buffer_flags(tty);
tty_unthrottle(tty);
- ldata->column = 0;
+ //ldata->column = 0; /* Done by kzalloc */
n_tty_set_termios(tty, NULL);
tty->minimum_to_wake = 1;
- tty->closing = 0;
+ //tty->closing = 0; /* Done by kzalloc */

return 0;
err_free_bufs:
diff --git a/drivers/pps/clients/pps-ldisc.c b/drivers/pps/clients/pps-ldisc.c
index 79451f2..b5c6dd8 100644
--- a/drivers/pps/clients/pps-ldisc.c
+++ b/drivers/pps/clients/pps-ldisc.c
@@ -31,9 +31,16 @@
static void pps_tty_dcd_change(struct tty_struct *tty, unsigned int status,
struct pps_event_time *ts)
{
- struct pps_device *pps = (struct pps_device *)tty->disc_data;
+ struct pps_device *pps = *(struct pps_device **)tty->disc_data;

- BUG_ON(pps == NULL);
+ /* Because we set the pps field *after* the tty is opened, there's
+ * a race window during which this can happen.
+ */
+ if (pps == NULL) {
+ pr_err("Race condition triggered, pps = NULL");
+ return;
+ }
+// BUG_ON(pps == NULL);

/* Now do the PPS event report */
pps_event(pps, ts, status ? PPS_CAPTUREASSERT :
@@ -67,7 +74,6 @@ static int pps_tty_open(struct tty_struct *tty)
pr_err("cannot register PPS source \"%s\"\n", info.path);
return -ENOMEM;
}
- tty->disc_data = pps;

/* Should open N_TTY ldisc too */
ret = alias_n_tty_open(tty);
@@ -75,13 +81,13 @@ static int pps_tty_open(struct tty_struct *tty)
pr_err("cannot open tty ldisc \"%s\"\n", info.path);
goto err_unregister;
}
+ *(struct pps_device **)tty->disc_data = pps;

dev_info(pps->dev, "source \"%s\" added\n", info.path);

return 0;

err_unregister:
- tty->disc_data = NULL;
pps_unregister_source(pps);
return ret;
}
@@ -90,11 +96,11 @@ static void (*alias_n_tty_close)(struct tty_struct *tty);

static void pps_tty_close(struct tty_struct *tty)
{
- struct pps_device *pps = (struct pps_device *)tty->disc_data;
+ struct pps_device *pps = *(struct pps_device **)tty->disc_data;

+ *(struct pps_device **)tty->disc_data = NULL;
alias_n_tty_close(tty);

- tty->disc_data = NULL;
dev_info(pps->dev, "removed\n");
pps_unregister_source(pps);
}