2020-01-30 10:09:24

by Johan Hovold

Subject: [PATCH] USB: serial: relax unthrottle memory barrier

Commit a8d78d9f3856 ("USB: serial: clean up throttle handling")
converted the throttle handling to use atomic bitops. This means that we
can relax the smp_mb() in unthrottle() to smp_mb__after_atomic(), which
for example is a no-op on architectures like x86 that provide fully
ordered atomics.

Signed-off-by: Johan Hovold <[email protected]>
---
drivers/usb/serial/generic.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/drivers/usb/serial/generic.c b/drivers/usb/serial/generic.c
index 1be8bea372a2..546a1c2ce2f2 100644
--- a/drivers/usb/serial/generic.c
+++ b/drivers/usb/serial/generic.c
@@ -417,7 +417,7 @@ void usb_serial_generic_read_bulk_callback(struct urb *urb)
/*
* Make sure URB is marked as free before checking the throttled flag
* to avoid racing with unthrottle() on another CPU. Matches the
- * smp_mb() in unthrottle().
+ * smp_mb__after_atomic() in unthrottle().
*/
smp_mb__after_atomic();

@@ -489,7 +489,7 @@ void usb_serial_generic_unthrottle(struct tty_struct *tty)
* Matches the smp_mb__after_atomic() in
* usb_serial_generic_read_bulk_callback().
*/
- smp_mb();
+ smp_mb__after_atomic();

usb_serial_generic_submit_read_urbs(port, GFP_KERNEL);
}
--
2.24.1

2020-01-30 10:25:11

by Greg KH

Subject: Re: [PATCH] USB: serial: relax unthrottle memory barrier

On Thu, Jan 30, 2020 at 11:06:58AM +0100, Johan Hovold wrote:
> Commit a8d78d9f3856 ("USB: serial: clean up throttle handling")
> converted the throttle handling to use atomic bitops. This means that we
> can relax the smp_mb() in unthrottle() to smp_mb__after_atomic(), which
> for example is a no-op on architectures like x86 that provide fully
> ordered atomics.
>
> Signed-off-by: Johan Hovold <[email protected]>

Reviewed-by: Greg Kroah-Hartman <[email protected]>

2020-01-30 15:44:55

by Davidlohr Bueso

Subject: Re: [PATCH] USB: serial: relax unthrottle memory barrier

On Thu, 30 Jan 2020, Johan Hovold wrote:

>Commit a8d78d9f3856 ("USB: serial: clean up throttle handling")
>converted the throttle handling to use atomic bitops. This means that we
>can relax the smp_mb() in unthrottle() to smp_mb__after_atomic(), which
>for example is a no-op on architectures like x86 that provide fully
>ordered atomics.
>
>Signed-off-by: Johan Hovold <[email protected]>

Reviewed-by: Davidlohr Bueso <[email protected]>

2020-02-10 09:38:47

by Johan Hovold

Subject: Re: [PATCH] USB: serial: relax unthrottle memory barrier

On Thu, Jan 30, 2020 at 07:33:28AM -0800, Davidlohr Bueso wrote:
> On Thu, 30 Jan 2020, Johan Hovold wrote:
>
> >Commit a8d78d9f3856 ("USB: serial: clean up throttle handling")
> >converted the throttle handling to use atomic bitops. This means that we
> >can relax the smp_mb() in unthrottle() to smp_mb__after_atomic(), which
> >for example is a no-op on architectures like x86 that provide fully
> >ordered atomics.
> >
> >Signed-off-by: Johan Hovold <[email protected]>
>
> Reviewed-by: Davidlohr Bueso <[email protected]>

Thanks for the review. Now applied.

Johan