From b972216e27d1c853eced33f8638926636c606341 Mon Sep 17 00:00:00 2001 From: Peter Zijlstra Date: Wed, 6 Aug 2014 16:08:20 -0700 Subject: [PATCH] mmu_notifier: add call_srcu and sync function for listener to delay call and sync MIME-Version: 1.0 Content-Type: text/plain; charset=utf8 Content-Transfer-Encoding: 8bit When kernel device drivers or subsystems want to bind their lifespan to t= he lifespan of the mm_struct, they usually use one of the following methods: 1. Manually calling a function in the interested kernel module. The funct= ion call needs to be placed in mmput. This method was rejected by several ker= nel maintainers. 2. Registering to the mmu notifier release mechanism. The problem with the latter approach is that the mmu_notifier_release cal= lback is called from__mmu_notifier_release (called from exit_mmap). That functi= on iterates over the list of mmu notifiers and don't expect the release call= back function to remove itself from the list. Therefore, the callback function= in the kernel module can't release the mmu_notifier_object, which is actuall= y the kernel module's object itself. As a result, the destruction of the kernel module's object must to be done in a delayed fashion. This patch adds support for this delayed callback, by adding a new mmu_notifier_call_srcu function that receives a function ptr and calls th= at function with call_srcu. In that function, the kernel module releases its object. To use mmu_notifier_call_srcu, the calling module needs to call b= efore that a new function called mmu_notifier_unregister_no_release that as its= name implies, unregisters a notifier without calling its notifier release call= back. This patch also adds a function that will call barrier_srcu so those kern= el modules can sync with mmu_notifier. Signed-off-by: Peter Zijlstra Signed-off-by: Jérôme Glisse Signed-off-by: Oded Gabbay Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- include/linux/mmu_notifier.h | 6 ++++++ mm/mmu_notifier.c | 40 +++++++++++++++++++++++++++++++++++- 2 files changed, 45 insertions(+), 1 deletion(-) diff --git a/include/linux/mmu_notifier.h b/include/linux/mmu_notifier.h index deca87452528..27288692241e 100644 --- a/include/linux/mmu_notifier.h +++ b/include/linux/mmu_notifier.h @@ -170,6 +170,8 @@ extern int __mmu_notifier_register(struct mmu_notifier *mn, struct mm_struct *mm); extern void mmu_notifier_unregister(struct mmu_notifier *mn, struct mm_struct *mm); +extern void mmu_notifier_unregister_no_release(struct mmu_notifier *mn, + struct mm_struct *mm); extern void __mmu_notifier_mm_destroy(struct mm_struct *mm); extern void __mmu_notifier_release(struct mm_struct *mm); extern int __mmu_notifier_clear_flush_young(struct mm_struct *mm, @@ -288,6 +290,10 @@ static inline void mmu_notifier_mm_destroy(struct mm_struct *mm) set_pte_at(___mm, ___address, __ptep, ___pte); \ }) +extern void mmu_notifier_call_srcu(struct rcu_head *rcu, + void (*func)(struct rcu_head *rcu)); +extern void mmu_notifier_synchronize(void); + #else /* CONFIG_MMU_NOTIFIER */ static inline void mmu_notifier_release(struct mm_struct *mm) diff --git a/mm/mmu_notifier.c b/mm/mmu_notifier.c index 41cefdf0aadd..950813b1eb36 100644 --- a/mm/mmu_notifier.c +++ b/mm/mmu_notifier.c @@ -22,6 +22,25 @@ /* global SRCU for all MMs */ static struct srcu_struct srcu; +/* + * This function allows mmu_notifier::release callback to delay a call to + * a function that will free appropriate resources. The function must be + * quick and must not block. + */ +void mmu_notifier_call_srcu(struct rcu_head *rcu, + void (*func)(struct rcu_head *rcu)) +{ + call_srcu(&srcu, rcu, func); +} +EXPORT_SYMBOL_GPL(mmu_notifier_call_srcu); + +void mmu_notifier_synchronize(void) +{ + /* Wait for any running method to finish. */ + srcu_barrier(&srcu); +} +EXPORT_SYMBOL_GPL(mmu_notifier_synchronize); + /* * This function can't run concurrently against mmu_notifier_register * because mm->mm_users > 0 during mmu_notifier_register and exit_mmap @@ -53,7 +72,6 @@ void __mmu_notifier_release(struct mm_struct *mm) */ if (mn->ops->release) mn->ops->release(mn, mm); - srcu_read_unlock(&srcu, id); spin_lock(&mm->mmu_notifier_mm->lock); while (unlikely(!hlist_empty(&mm->mmu_notifier_mm->list))) { @@ -69,6 +87,7 @@ void __mmu_notifier_release(struct mm_struct *mm) hlist_del_init_rcu(&mn->hlist); } spin_unlock(&mm->mmu_notifier_mm->lock); + srcu_read_unlock(&srcu, id); /* * synchronize_srcu here prevents mmu_notifier_release from returning to @@ -325,6 +344,25 @@ void mmu_notifier_unregister(struct mmu_notifier *mn, struct mm_struct *mm) } EXPORT_SYMBOL_GPL(mmu_notifier_unregister); +/* + * Same as mmu_notifier_unregister but no callback and no srcu synchronization. + */ +void mmu_notifier_unregister_no_release(struct mmu_notifier *mn, + struct mm_struct *mm) +{ + spin_lock(&mm->mmu_notifier_mm->lock); + /* + * Can not use list_del_rcu() since __mmu_notifier_release + * can delete it before we hold the lock. + */ + hlist_del_init_rcu(&mn->hlist); + spin_unlock(&mm->mmu_notifier_mm->lock); + + BUG_ON(atomic_read(&mm->mm_count) <= 0); + mmdrop(mm); +} +EXPORT_SYMBOL_GPL(mmu_notifier_unregister_no_release); + static int __init mmu_notifier_init(void) { return init_srcu_struct(&srcu); -- 2.20.1