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From: Waiman Long <longman@redhat.com>
To: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@redhat.com>, Will Deacon <will.deacon@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>
Cc: linux-arch@vger.kernel.org, linux-xtensa@linux-xtensa.org,
	Davidlohr Bueso <dave@stgolabs.net>,
	linux-ia64@vger.kernel.org, Tim Chen <tim.c.chen@linux.intel.com>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-sh@vger.kernel.org, linux-hexagon@vger.kernel.org,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	linux-kernel@vger.kernel.org,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Borislav Petkov <bp@alien8.de>,
	linux-alpha@vger.kernel.org, sparclinux@vger.kernel.org,
	Waiman Long <longman@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	linuxppc-dev@lists.ozlabs.org,
	linux-arm-kernel@lists.infradead.org
Subject: [PATCH-tip 22/22] locking/rwsem: Ensure an RT task will not spin on reader
Date: Thu,  7 Feb 2019 14:07:26 -0500
Message-ID: <1549566446-27967-23-git-send-email-longman@redhat.com> (raw)
In-Reply-To: <1549566446-27967-1-git-send-email-longman@redhat.com>

An RT task can do optimistic spinning only if the lock holder is
actually running. If the state of the lock holder isn't known, there
is a possibility that high priority of the RT task may block forward
progress of the lock holder if they happen to reside on the same CPU.
This will lead to deadlock. So we have to make sure that an RT task
will not spin on a reader-owned rwsem.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 kernel/locking/rwsem-xadd.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 3cf2e84..ac1f6c4 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -349,12 +349,14 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
 
 /*
  * Return the folowing three values depending on the lock owner state.
+ *   2	when owner is currently NULL
  *   1	when owner changes and no reader is detected yet.
  *   0	when owner changes and the new owner may be a reader.
  *  -1	when optimistic spinning has to stop because either the owner stops
  *	running, is unknown, or its timeslice has been used up.
  */
 enum owner_state {
+	OWNER_NULL = 2,
 	OWNER_SPINNABLE = 1,
 	OWNER_READER = 0,
 	OWNER_NONSPINNABLE = -1,
@@ -405,7 +407,8 @@ static noinline enum owner_state rwsem_spin_on_owner(struct rw_semaphore *sem)
 	owner = rwsem_get_owner(sem);
 	if (!is_rwsem_owner_spinnable(owner))
 		return OWNER_NONSPINNABLE;
-	return is_rwsem_owner_reader(owner) ? OWNER_READER : OWNER_SPINNABLE;
+	return !owner ? OWNER_NULL :
+	       is_rwsem_owner_reader(owner) ? OWNER_READER : OWNER_SPINNABLE;
 }
 
 static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
@@ -442,12 +445,13 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem, const long wlock)
 			break;
 
 		/*
-		 * When there's no owner, we might have preempted between the
-		 * owner acquiring the lock and setting the owner field. If
-		 * we're an RT task that will live-lock because we won't let
-		 * the owner complete.
+		 * An RT task cannot do optimistic spinning if it cannot
+		 * be sure the lock holder is running. When there's no owner
+		 * or is reader-owned, an RT task has to stop spinning or
+		 * live-lock may happen if the current task and the lock
+		 * holder happen to run in the same CPU.
 		 */
-		if (!rwsem_get_owner(sem) &&
+		if ((owner_state != OWNER_SPINNABLE) &&
 		   (need_resched() || rt_task(current)))
 			break;
 
-- 
1.8.3.1


  parent reply index

Thread overview: 44+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-02-07 19:07 [PATCH-tip 00/22] locking/rwsem: Rework rwsem-xadd & enable new rwsem features Waiman Long
2019-02-07 19:07 ` [PATCH-tip 01/22] locking/qspinlock_stat: Introduce a generic lockevent counting APIs Waiman Long
2019-02-07 19:07 ` [PATCH-tip 02/22] locking/lock_events: Make lock_events available for all archs & other locks Waiman Long
2019-02-07 19:07 ` [PATCH-tip 03/22] locking/rwsem: Relocate rwsem_down_read_failed() Waiman Long
2019-02-07 19:07 ` [PATCH-tip 04/22] locking/rwsem: Remove arch specific rwsem files Waiman Long
2019-02-07 19:36   ` Peter Zijlstra
2019-02-07 19:43     ` Waiman Long
2019-02-07 19:48     ` Peter Zijlstra
2019-02-07 19:07 ` [PATCH-tip 05/22] locking/rwsem: Move owner setting code from rwsem.c to rwsem.h Waiman Long
2019-02-07 19:07 ` [PATCH-tip 06/22] locking/rwsem: Rename kernel/locking/rwsem.h Waiman Long
2019-02-07 19:07 ` [PATCH-tip 07/22] locking/rwsem: Move rwsem internal function declarations to rwsem-xadd.h Waiman Long
2019-02-07 19:07 ` [PATCH-tip 08/22] locking/rwsem: Add debug check for __down_read*() Waiman Long
2019-02-07 19:07 ` [PATCH-tip 09/22] locking/rwsem: Enhance DEBUG_RWSEMS_WARN_ON() macro Waiman Long
2019-02-07 19:07 ` [PATCH-tip 10/22] locking/rwsem: Enable lock event counting Waiman Long
2019-02-07 19:07 ` [PATCH-tip 11/22] locking/rwsem: Implement a new locking scheme Waiman Long
2019-02-07 19:07 ` [PATCH-tip 12/22] locking/rwsem: Implement lock handoff to prevent lock starvation Waiman Long
2019-02-07 19:07 ` [PATCH-tip 13/22] locking/rwsem: Remove rwsem_wake() wakeup optimization Waiman Long
2019-02-07 19:07 ` [PATCH-tip 14/22] locking/rwsem: Add more rwsem owner access helpers Waiman Long
2019-02-07 19:07 ` [PATCH-tip 15/22] locking/rwsem: Merge owner into count on x86-64 Waiman Long
2019-02-07 19:45   ` Peter Zijlstra
2019-02-07 19:55     ` Waiman Long
2019-02-07 20:08   ` Peter Zijlstra
2019-02-07 20:54     ` Waiman Long
2019-02-08 14:19       ` Waiman Long
2019-02-07 19:07 ` [PATCH-tip 16/22] locking/rwsem: Remove redundant computation of writer lock word Waiman Long
2019-02-07 19:07 ` [PATCH-tip 17/22] locking/rwsem: Recheck owner if it is not on cpu Waiman Long
2019-02-07 19:07 ` [PATCH-tip 18/22] locking/rwsem: Make rwsem_spin_on_owner() return a tri-state value Waiman Long
2019-02-07 19:07 ` [PATCH-tip 19/22] locking/rwsem: Enable readers spinning on writer Waiman Long
2019-02-07 19:07 ` [PATCH-tip 20/22] locking/rwsem: Enable count-based spinning on reader Waiman Long
2019-02-07 19:07 ` [PATCH-tip 21/22] locking/rwsem: Wake up all readers in wait queue Waiman Long
2019-02-07 19:07 ` Waiman Long [this message]
2019-02-07 19:51 ` [PATCH-tip 00/22] locking/rwsem: Rework rwsem-xadd & enable new rwsem features Davidlohr Bueso
2019-02-07 20:00   ` Waiman Long
2019-02-11  7:38     ` Ingo Molnar
2019-02-08 19:50 ` Linus Torvalds
2019-02-08 20:31   ` Waiman Long
2019-02-09  0:03     ` Linus Torvalds
2019-02-14 13:23     ` Davidlohr Bueso
2019-02-14 15:22       ` Waiman Long
2019-02-13  9:19 ` Chen Rong
2019-02-13 19:56   ` Linus Torvalds
2019-04-10  8:15     ` huang ying
2019-04-10 16:08       ` Waiman Long
2019-04-12  0:49         ` huang ying

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