linux-kernel - [PATCH] USB: serial: relax unthrottle memory barrier

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Message-Id: <20200130100658.683-1-johan@kernel.org>
Date:   Thu, 30 Jan 2020 11:06:58 +0100
From:   Johan Hovold <johan@...nel.org>
To:     Johan Hovold <johan@...nel.org>
Cc:     linux-usb@...r.kernel.org, linux-kernel@...r.kernel.org,
        Davidlohr Bueso <dave@...olabs.net>
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 <johan@...nel.org>
---
 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