From c2878c131a985a117e993d97fe464326b77d2850 Mon Sep 17 00:00:00 2001 From: Prateek Sood Date: Wed, 5 Jul 2017 23:24:16 +0530 Subject: [PATCH] rwsem: 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 RMB 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. Change-Id: I96de9a65adedb35d1ee2c6c36dc7759c9b8f5d4d Signed-off-by: Prateek Sood --- kernel/locking/rwsem-xadd.c | 35 +++++++++++++++++++++++++++++++++++ 1 file changed, 35 insertions(+) diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index a4d4de05b2d1..75c950ede9c7 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -510,6 +510,41 @@ struct rw_semaphore *rwsem_wake(struct rw_semaphore *sem) { unsigned long flags; + /* + * 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 + * RMB + * 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