From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steven Rostedt Subject: [PATCH RT 0/2] rwsem-rt: Make rwsem rt closer to mainline Date: Tue, 08 Apr 2014 22:47:00 -0400 Message-ID: <20140409024700.702797305@goodmis.org> Cc: Thomas Gleixner , Carsten Emde , Sebastian Andrzej Siewior , John Kacur , Paul Gortmaker , Peter Zijlstra , "H. Peter Anvin" To: linux-kernel@vger.kernel.org, linux-rt-users Return-path: Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-rt-users.vger.kernel.org Looking at mainline's down_read() I noticed that reader locks of rwsems are not made to nest. In fact, they should not. Although, it may seem fine if a down_read() nests as multiple readers can have the lock, rwsems are fair locks. That is, if a writer were to block on a rwsem while readers have the lock, a new reader will also block. If a reader were to try to take the lock again while a writer was waiting, it would block, and cause a deadlock as it has the lock its trying to grab and wont let it go as the writer is waiting. I also found that the rt_mutex_init() is identical in the two places it is defined in rtmutex.h. Steven Rostedt (Red Hat) (2): rwsem-rt: Do not allow readers to nest rtmutex: Remove duplicate rt_mutex_init() ---- include/linux/rtmutex.h | 12 +++--------- include/linux/rwsem_rt.h | 1 - kernel/rt.c | 37 ++++++++----------------------------- 3 files changed, 11 insertions(+), 39 deletions(-)