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Subject[PATCH] rwsem: fix missed wakeup due to reordering of load
Date
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>
---
kernel/locking/rwsem-xadd.c | 34 ++++++++++++++++++++++++++++++++++
1 file changed, 34 insertions(+)

diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c
index 34e727f..21c111a 100644
--- a/kernel/locking/rwsem-xadd.c
+++ b/kernel/locking/rwsem-xadd.c
@@ -585,6 +585,40 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem)
unsigned long flags;
DEFINE_WAKE_Q(wake_q);

+ /*
+ * 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() here is to make sure that the spinner state is
+ * consulted after sem->count is updated in up_write context.
+ */
+ 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
--
Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.,
is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
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