Commit 9198f6ed authored Mar 06, 2015 by Jason Low Committed by Ingo Molnar Mar 07, 2015

locking/rwsem: Fix lock optimistic spinning when owner is not running



Ming reported soft lockups occurring when running xfstest due to
the following tip:locking/core commit:

  b3fd4f03 ("locking/rwsem: Avoid deceiving lock spinners")

When doing optimistic spinning in rwsem, threads should stop
spinning when the lock owner is not running. While a thread is
spinning on owner, if the owner reschedules, owner->on_cpu
returns false and we stop spinning.

However, this commit essentially caused the check to get
ignored because when we break out of the spin loop due to
!on_cpu, we continue spinning if sem->owner != NULL.

This patch fixes this by making sure we stop spinning if the
owner is not running. Furthermore, just like with mutexes,
refactor the code such that we don't have separate checks for
owner_running(). This makes it more straightforward in terms of
why we exit the spin on owner loop and we would also avoid
needing to "guess" why we broke out of the loop to make this
more readable.

Reported-and-tested-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jason Low <jason.low2@hp.com>
Acked-by: Davidlohr Bueso <dave@stgolabs.net>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Dave Jones <davej@codemonkey.org.uk>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Michel Lespinasse <walken@google.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Sasha Levin <sasha.levin@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Link: http://lkml.kernel.org/r/1425714331.2475.388.camel@j-VirtualBox


Signed-off-by: Ingo Molnar <mingo@kernel.org>

parent 4d3199e4

kernel/locking/rwsem-xadd.c

+11 −20

Original line number	Diff line number	Diff line
		@@ -324,32 +324,23 @@ done:
		return ret;
		}

		static inline bool owner_running(struct rw_semaphore *sem,
		struct task_struct *owner)
		{
		if (sem->owner != owner)
		return false;

		/*
		* Ensure we emit the owner->on_cpu, dereference _after_ checking
		* sem->owner still matches owner, if that fails, owner might
		* point to free()d memory, if it still matches, the rcu_read_lock()
		* ensures the memory stays valid.
		*/
		barrier();

		return owner->on_cpu;
		}

		static noinline
		bool rwsem_spin_on_owner(struct rw_semaphore sem, struct task_struct owner)
		{
		long count;

		rcu_read_lock();
		while (owner_running(sem, owner)) {
		/* abort spinning when need_resched */
		if (need_resched()) {
		while (sem->owner == owner) {
		/*
		* Ensure we emit the owner->on_cpu, dereference _after_
		* checking sem->owner still matches owner, if that fails,
		* owner might point to free()d memory, if it still matches,
		* the rcu_read_lock() ensures the memory stays valid.
		*/
		barrier();

		/* abort spinning when need_resched or owner is not running */
		if (!owner->on_cpu \|\| need_resched()) {
		rcu_read_unlock();
		return false;
		}

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