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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>,
        =?UTF-8?q?Pasi=20K=C3=A4rkk=C3=A4inen?= <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: [PATCHv6 7/7] tty/ldsem: Decrement wait_readers on timeouted down_read()
Date:   Thu,  1 Nov 2018 00:24:52 +0000
Message-Id: <20181101002452.5483-8-dima@arista.com>
In-Reply-To: <20181101002452.5483-1-dima@arista.com>
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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);
-- 
2.19.1