When PREEMPT_RT_FULL does the spinlock -> rt_mutex substitution the PI chain code will (falsely) report a deadlock and BUG. The problem is that it hold hb->lock (now an rt_mutex) while doing task_blocks_on_rt_mutex on the futex's pi_state::rtmutex. This, when interleaved just right with futex_unlock_pi() leads it to believe to see an AB-BA deadlock. Task1 (holds rt_mutex, Task2 (does FUTEX_LOCK_PI) does FUTEX_UNLOCK_PI) lock hb->lock lock rt_mutex (as per start_proxy) lock hb->lock Which is a trivial AB-BA. It is not an actual deadlock, because it won't be holding hb->lock by the time it actually blocks on the rt_mutex, but the chainwalk code doesn't know that and it would be a nightmare to handle this gracefully. To avoid this problem, do the same as in futex_unlock_pi() and drop hb->lock after acquiring wait_lock. This still fully serializes against futex_unlock_pi(), since adding to the wait_list does the very same lock dance, and removing it holds both locks. Aside of solving the RT problem this makes the lock and unlock mechanism symetric and reduces the hb->lock held time. Reported-and-tested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de> Suggested-by: Thomas Gleixner <tglx@linutronix.de> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: juri.lelli@arm.com Cc: xlpang@redhat.com Cc: rostedt@goodmis.org Cc: mathieu.desnoyers@efficios.com Cc: jdesfossez@efficios.com Cc: dvhart@infradead.org Cc: bristot@redhat.com Link: http://lkml.kernel.org/r/20170322104152.161341537@infradead.org Signed-off-by: Thomas Gleixner <tglx@linutronix.de> |
||
---|---|---|
Documentation | ||
arch | ||
block | ||
certs | ||
crypto | ||
drivers | ||
firmware | ||
fs | ||
include | ||
init | ||
ipc | ||
kernel | ||
lib | ||
mm | ||
net | ||
samples | ||
scripts | ||
security | ||
sound | ||
tools | ||
usr | ||
virt | ||
.cocciconfig | ||
.get_maintainer.ignore | ||
.gitattributes | ||
.gitignore | ||
.mailmap | ||
COPYING | ||
CREDITS | ||
Kbuild | ||
Kconfig | ||
MAINTAINERS | ||
Makefile | ||
README |
README
Linux kernel ============ This file was moved to Documentation/admin-guide/README.rst Please notice that there are several guides for kernel developers and users. These guides can be rendered in a number of formats, like HTML and PDF. In order to build the documentation, use ``make htmldocs`` or ``make pdfdocs``. There are various text files in the Documentation/ subdirectory, several of them using the Restructured Text markup notation. See Documentation/00-INDEX for a list of what is contained in each file. Please read the Documentation/process/changes.rst file, as it contains the requirements for building and running the kernel, and information about the problems which may result by upgrading your kernel.