[PATCHv5 7/7] tty/ldsem: Decrement wait_readers on timeouted down_read()

From: Dmitry Safonov <dima@arista.com>
To: linux-kernel@vger.kernel.org
Cc: "Dmitry Safonov" <0x7f454c46@gmail.com>,
	"Dmitry Safonov" <dima@arista.com>,
	"Daniel Axtens" <dja@axtens.net>,
	"Dmitry Vyukov" <dvyukov@google.com>,
	"Mark Rutland" <mark.rutland@arm.com>,
	"Michael Neuling" <mikey@neuling.org>,
	"Mikulas Patocka" <mpatocka@redhat.com>,
	"Nathan March" <nathan@gt.net>, "Pasi Kärkkäinen" <pasik@iki.fi>,
	"Peter Hurley" <peter@hurleysoftware.com>,
	"Peter Zijlstra" <peterz@infradead.org>,
	"Rong, Chen" <rong.a.chen@intel.com>,
	"Sergey Senozhatsky" <sergey.senozhatsky.work@gmail.com>,
	"Tan Xiaojun" <tanxiaojun@huawei.com>,
	"Tetsuo Handa" <penguin-kernel@I-love.SAKURA.ne.jp>,
	"Greg Kroah-Hartman" <gregkh@linuxfoundation.org>,
	"Jiri Slaby" <jslaby@suse.com>
Subject: [PATCHv5 7/7] tty/ldsem: Decrement wait_readers on timeouted down_read()
Date: Tue, 18 Sep 2018 00:52:58 +0100	[thread overview]
Message-ID: <20180917235258.5719-8-dima@arista.com> (raw)
In-Reply-To: <20180917235258.5719-1-dima@arista.com>

It seems like when ldsem_down_read() fails with timeout, it misses
update for sem->wait_readers. By that reason, when writer finally
releases write end of the semaphore __ldsem_wake_readers() does adjust
sem->count with wrong value:
sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS)

I.e, if update comes with 1 missed wait_readers decrement, sem->count
will be 0x100000001 which means that there is active reader and it'll
make any further writer to fail in acquiring the semaphore.

It looks like, this is a dead-code, because ldsem_down_read() is never
called with timeout different than MAX_SCHEDULE_TIMEOUT, so it might be
worth to delete timeout parameter and error path fall-back..

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
---
 drivers/tty/tty_ldsem.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/drivers/tty/tty_ldsem.c b/drivers/tty/tty_ldsem.c
index d4d0dbf4a6d9..717292c1c0df 100644
--- a/drivers/tty/tty_ldsem.c
+++ b/drivers/tty/tty_ldsem.c
@@ -212,6 +212,7 @@ down_read_failed(struct ld_semaphore *sem, long count, long timeout)
 		raw_spin_lock_irq(&sem->wait_lock);
 		if (waiter.task) {
 			atomic_long_add_return(-LDSEM_WAIT_BIAS, &sem->count);
+			sem->wait_readers--;
 			list_del(&waiter.list);
 			raw_spin_unlock_irq(&sem->wait_lock);
 			put_task_struct(waiter.task);
-- 
2.13.6

