tty: fix data race on tty_buffer.commit
authorDmitry Vyukov <dvyukov@google.com>
Thu, 17 Sep 2015 15:17:10 +0000 (17:17 +0200)
committerGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Sun, 4 Oct 2015 18:03:40 +0000 (19:03 +0100)
commitfacd885c75067c2a88b72622dfb0fa4b9510da5e
tree836619d525c9ef7ee3827d6387b7b05ec2389e0d
parent9e6b7cd7e77d4ca43b57c726d9bfa86d06e0567f
tty: fix data race on tty_buffer.commit

Race on buffer data happens when newly committed data is
picked up by an old flush work in the following scenario:
__tty_buffer_request_room does a plain write of tail->commit,
no barriers were executed before that.
At this point flush_to_ldisc reads this new value of commit,
and reads buffer data, no barriers in between.
The committed buffer data is not necessary visible to flush_to_ldisc.

Similar bug happens when tty_schedule_flip commits data.

Update commit with smp_store_release and read commit with
smp_load_acquire, as it is commit that signals data readiness.
This is orthogonal to the existing synchronization on tty_buffer.next,
which is required to not dismiss a buffer with unconsumed data.

The data race was found with KernelThreadSanitizer (KTSAN).

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
drivers/tty/tty_buffer.c