Commit-ID: 00f3c5a3df2c1e3dab14d0dd2b71f852d46be97f Gitweb: https://git.kernel.org/tip/00f3c5a3df2c1e3dab14d0dd2b71f852d46be97f Author: Waiman Long <long...@redhat.com> AuthorDate: Mon, 20 May 2019 16:59:07 -0400 Committer: Ingo Molnar <mi...@kernel.org> CommitDate: Mon, 17 Jun 2019 12:28:00 +0200
locking/rwsem: Always release wait_lock before waking up tasks With the use of wake_q, we can do task wakeups without holding the wait_lock. There is one exception in the rwsem code, though. It is when the writer in the slowpath detects that there are waiters ahead but the rwsem is not held by a writer. This can lead to a long wait_lock hold time especially when a large number of readers are to be woken up. Remediate this situation by releasing the wait_lock before waking up tasks and re-acquiring it afterward. The rwsem_try_write_lock() function is also modified to read the rwsem count directly to avoid stale count value. Suggested-by: Peter Zijlstra <pet...@infradead.org> Signed-off-by: Waiman Long <long...@redhat.com> Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org> Cc: Borislav Petkov <b...@alien8.de> Cc: Davidlohr Bueso <d...@stgolabs.net> Cc: H. Peter Anvin <h...@zytor.com> Cc: Linus Torvalds <torva...@linux-foundation.org> Cc: Thomas Gleixner <t...@linutronix.de> Cc: Tim Chen <tim.c.c...@linux.intel.com> Cc: Will Deacon <will.dea...@arm.com> Cc: huang ying <huang.ying.cari...@gmail.com> Link: https://lkml.kernel.org/r/20190520205918.22251-9-long...@redhat.com Signed-off-by: Ingo Molnar <mi...@kernel.org> --- include/linux/sched/wake_q.h | 5 +++++ kernel/locking/rwsem.c | 31 +++++++++++++++---------------- 2 files changed, 20 insertions(+), 16 deletions(-) diff --git a/include/linux/sched/wake_q.h b/include/linux/sched/wake_q.h index ad826d2a4557..26a2013ac39c 100644 --- a/include/linux/sched/wake_q.h +++ b/include/linux/sched/wake_q.h @@ -51,6 +51,11 @@ static inline void wake_q_init(struct wake_q_head *head) head->lastp = &head->first; } +static inline bool wake_q_empty(struct wake_q_head *head) +{ + return head->first == WAKE_Q_TAIL; +} + extern void wake_q_add(struct wake_q_head *head, struct task_struct *task); extern void wake_q_add_safe(struct wake_q_head *head, struct task_struct *task); extern void wake_up_q(struct wake_q_head *head); diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index decda9fb8c6d..5532304406f7 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -400,13 +400,14 @@ static void rwsem_mark_wake(struct rw_semaphore *sem, * If wstate is WRITER_HANDOFF, it will make sure that either the handoff * bit is set or the lock is acquired with handoff bit cleared. */ -static inline bool rwsem_try_write_lock(long count, struct rw_semaphore *sem, +static inline bool rwsem_try_write_lock(struct rw_semaphore *sem, enum writer_wait_state wstate) { - long new; + long count, new; lockdep_assert_held(&sem->wait_lock); + count = atomic_long_read(&sem->count); do { bool has_handoff = !!(count & RWSEM_FLAG_HANDOFF); @@ -751,26 +752,25 @@ rwsem_down_write_slowpath(struct rw_semaphore *sem, int state) ? RWSEM_WAKE_READERS : RWSEM_WAKE_ANY, &wake_q); - /* - * The wakeup is normally called _after_ the wait_lock - * is released, but given that we are proactively waking - * readers we can deal with the wake_q overhead as it is - * similar to releasing and taking the wait_lock again - * for attempting rwsem_try_write_lock(). - */ - wake_up_q(&wake_q); - - /* We need wake_q again below, reinitialize */ - wake_q_init(&wake_q); + if (!wake_q_empty(&wake_q)) { + /* + * We want to minimize wait_lock hold time especially + * when a large number of readers are to be woken up. + */ + raw_spin_unlock_irq(&sem->wait_lock); + wake_up_q(&wake_q); + wake_q_init(&wake_q); /* Used again, reinit */ + raw_spin_lock_irq(&sem->wait_lock); + } } else { - count = atomic_long_add_return(RWSEM_FLAG_WAITERS, &sem->count); + atomic_long_or(RWSEM_FLAG_WAITERS, &sem->count); } wait: /* wait until we successfully acquire the lock */ set_current_state(state); while (true) { - if (rwsem_try_write_lock(count, sem, wstate)) + if (rwsem_try_write_lock(sem, wstate)) break; raw_spin_unlock_irq(&sem->wait_lock); @@ -811,7 +811,6 @@ wait: } raw_spin_lock_irq(&sem->wait_lock); - count = atomic_long_read(&sem->count); } __set_current_state(TASK_RUNNING); list_del(&waiter.list);
