On Sun, Aug 29, 2021 at 11:22:50AM +0200, Fabio M. De Francesco wrote:
Convert greybus/uart.c from IDR to XArray. The abstract data type XArray is more memory-efficient, parallelisable, and cache friendly. It takes advantage of RCU to perform lookups without locking. Furthermore, IDR is deprecated because XArray has a better (cleaner and more consistent) API.
Where does it say that IDR is deprecated? Almost all drivers use IDR/IDA and its interfaces are straight-forward. In most cases we don't care about a possible slight increase in efficiency either, and so also in this case. Correctness is what matters and doing these conversions risks introducing regressions.
And I believe IDR use XArray internally these days anyway.
Signed-off-by: Fabio M. De Francesco fmdefrancesco@gmail.com
v3->v4: Remove mutex_lock/unlock around xa_load(). These locks seem to be unnecessary because there is a 1:1 correspondence between a specific minor and its gb_tty and there is no reference counting. I think that the RCU locks used inside xa_load() are sufficient to protect this API from returning an invalid gb_tty in case of concurrent access. Some more considerations on this topic are in the following message to linux-kernel list: https://lore.kernel.org/lkml/3554184.2JXonMZcNW@localhost.localdomain/
This just doesn't make sense (and a valid motivation would need to go in the commit message if there was one).
v2->v3: Fix some issues according to a review by Alex Elder elder@ieee.org v1->v2: Fix an issue found by the kernel test robot. It is due to passing to xa_*lock() the same old mutex that IDR used with the previous version of the code.
drivers/staging/greybus/uart.c | 32 ++++++++++++++------------------ 1 file changed, 14 insertions(+), 18 deletions(-)
diff --git a/drivers/staging/greybus/uart.c b/drivers/staging/greybus/uart.c index 73f01ed1e5b7..f66983adb51b 100644 --- a/drivers/staging/greybus/uart.c +++ b/drivers/staging/greybus/uart.c @@ -22,7 +22,7 @@ #include <linux/serial.h> #include <linux/tty_driver.h> #include <linux/tty_flip.h> -#include <linux/idr.h> +#include <linux/xarray.h> #include <linux/fs.h> #include <linux/kdev_t.h> #include <linux/kfifo.h> @@ -32,8 +32,9 @@ #include "gbphy.h" -#define GB_NUM_MINORS 16 /* 16 is more than enough */ -#define GB_NAME "ttyGB" +#define GB_NUM_MINORS 16 /* 16 is more than enough */ +#define GB_RANGE_MINORS XA_LIMIT(0, GB_NUM_MINORS) +#define GB_NAME "ttyGB" #define GB_UART_WRITE_FIFO_SIZE PAGE_SIZE #define GB_UART_WRITE_ROOM_MARGIN 1 /* leave some space in fifo */ @@ -67,8 +68,7 @@ struct gb_tty { }; static struct tty_driver *gb_tty_driver; -static DEFINE_IDR(tty_minors); -static DEFINE_MUTEX(table_lock); +static DEFINE_XARRAY(tty_minors); static int gb_uart_receive_data_handler(struct gb_operation *op) { @@ -341,8 +341,7 @@ static struct gb_tty *get_gb_by_minor(unsigned int minor) { struct gb_tty *gb_tty;
- mutex_lock(&table_lock);
- gb_tty = idr_find(&tty_minors, minor);
- gb_tty = xa_load(&tty_minors, minor); if (gb_tty) { mutex_lock(&gb_tty->mutex); if (gb_tty->disconnected) {
@@ -353,19 +352,18 @@ static struct gb_tty *get_gb_by_minor(unsigned int minor) mutex_unlock(&gb_tty->mutex); } }
- mutex_unlock(&table_lock);
You can't just drop the locking here since you'd introduce a potential use-after-free in case gb_tty is freed after the lookup but before the port reference is taken.
That said, this driver is already broken since it can currently free the gb_tty while there are references to the port. I'll try to fix it up...
return gb_tty; }
But as you may have gathered, I don't think doing these conversions is a good idea.
Johan