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authorPrateek Sood <prsood@codeaurora.org>2017-09-07 20:00:58 +0530
committerIngo Molnar <mingo@kernel.org>2017-09-29 10:10:20 +0200
commit9c29c31830a4eca724e137a9339137204bbb31be (patch)
tree633605dec4ffbc387640e12136eba911ea1a4cc4 /kernel
parentc74aef2d06a9f59cece89093eecc552933cba72a (diff)
download96b-common-9c29c31830a4eca724e137a9339137204bbb31be.tar.gz
locking/rwsem-xadd: Fix missed wakeup due to reordering of load
If a spinner is present, there is a chance that the load of rwsem_has_spinner() in rwsem_wake() can be reordered with respect to decrement of rwsem count in __up_write() leading to wakeup being missed: spinning writer up_write caller --------------- ----------------------- [S] osq_unlock() [L] osq spin_lock(wait_lock) sem->count=0xFFFFFFFF00000001 +0xFFFFFFFF00000000 count=sem->count MB sem->count=0xFFFFFFFE00000001 -0xFFFFFFFF00000001 spin_trylock(wait_lock) return rwsem_try_write_lock(count) spin_unlock(wait_lock) schedule() Reordering of atomic_long_sub_return_release() in __up_write() and rwsem_has_spinner() in rwsem_wake() can cause missing of wakeup in up_write() context. In spinning writer, sem->count and local variable count is 0XFFFFFFFE00000001. It would result in rwsem_try_write_lock() failing to acquire rwsem and spinning writer going to sleep in rwsem_down_write_failed(). The smp_rmb() will make sure that the spinner state is consulted after sem->count is updated in up_write context. Signed-off-by: Prateek Sood <prsood@codeaurora.org> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Peter Zijlstra <peterz@infradead.org> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: dave@stgolabs.net Cc: longman@redhat.com Cc: parri.andrea@gmail.com Cc: sramana@codeaurora.org Link: http://lkml.kernel.org/r/1504794658-15397-1-git-send-email-prsood@codeaurora.org Signed-off-by: Ingo Molnar <mingo@kernel.org>
Diffstat (limited to 'kernel')
-rw-r--r--kernel/locking/rwsem-xadd.c27
1 files changed, 27 insertions, 0 deletions
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 02f660666ab8..1fefe6dcafd7 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -613,6 +613,33 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
DEFINE_WAKE_Q(wake_q);
/*
+ * __rwsem_down_write_failed_common(sem)
+ * rwsem_optimistic_spin(sem)
+ * osq_unlock(sem->osq)
+ * ...
+ * atomic_long_add_return(&sem->count)
+ *
+ * - VS -
+ *
+ * __up_write()
+ * if (atomic_long_sub_return_release(&sem->count) < 0)
+ * rwsem_wake(sem)
+ * osq_is_locked(&sem->osq)
+ *
+ * And __up_write() must observe !osq_is_locked() when it observes the
+ * atomic_long_add_return() in order to not miss a wakeup.
+ *
+ * This boils down to:
+ *
+ * [S.rel] X = 1 [RmW] r0 = (Y += 0)
+ * MB RMB
+ * [RmW] Y += 1 [L] r1 = X
+ *
+ * exists (r0=1 /\ r1=0)
+ */
+ smp_rmb();
+
+ /*
* If a spinner is present, it is not necessary to do the wakeup.
* Try to do wakeup only if the trylock succeeds to minimize
* spinlock contention which may introduce too much delay in the