futex: update futex commentary (b2d0994b) · Commits · 戴 / test

kernel/futex.c

+14 −19

Original line number	Diff line number	Diff line
		@@ -114,7 +114,9 @@ struct futex_q {
		};

		/*
		* Split the global futex_lock into every hash list lock.
		* Hash buckets are shared by all the futex_keys that hash to the same
		* location. Each key may have multiple futex_q structures, one for each task
		* waiting on a futex.
		*/
		struct futex_hash_bucket {
		spinlock_t lock;
		@@ -189,8 +191,7 @@ static void drop_futex_key_refs(union futex_key *key)
		/**
		* get_futex_key - Get parameters which are the keys for a futex.
		* @uaddr: virtual address of the futex
		* @shared: NULL for a PROCESS_PRIVATE futex,
		* &current->mm->mmap_sem for a PROCESS_SHARED futex
		* @fshared: 0 for a PROCESS_PRIVATE futex, 1 for PROCESS_SHARED
		* @key: address where result is stored.
		*
		* Returns a negative error code or 0
		@@ -200,9 +201,7 @@ static void drop_futex_key_refs(union futex_key *key)
		* offset_within_page). For private mappings, it's (uaddr, current->mm).
		* We can usually work out the index without swapping in the page.
		*
		* fshared is NULL for PROCESS_PRIVATE futexes
		* For other futexes, it points to &current->mm->mmap_sem and
		* caller must have taken the reader lock. but NOT any spinlocks.
		* lock_page() might sleep, the caller should not hold a spinlock.
		*/
		static int get_futex_key(u32 __user uaddr, int fshared, union futex_key key)
		{
		@@ -589,10 +588,9 @@ static void wake_futex(struct futex_q *q)
		* The waiting task can free the futex_q as soon as this is written,
		* without taking any locks. This must come last.
		*
		* A memory barrier is required here to prevent the following store
		* to lock_ptr from getting ahead of the wakeup. Clearing the lock
		* at the end of wake_up_all() does not prevent this store from
		* moving.
		* A memory barrier is required here to prevent the following store to
		* lock_ptr from getting ahead of the wakeup. Clearing the lock at the
		* end of wake_up() does not prevent this store from moving.
		*/
		smp_wmb();
		q->lock_ptr = NULL;
		@@ -693,8 +691,7 @@ double_lock_hb(struct futex_hash_bucket hb1, struct futex_hash_bucket hb2)
		}

		/*
		* Wake up all waiters hashed on the physical page that is mapped
		* to this virtual address:
		* Wake up waiters matching bitset queued on this futex (uaddr).
		*/
		static int futex_wake(u32 __user *uaddr, int fshared, int nr_wake, u32 bitset)
		{
		@@ -1076,11 +1073,9 @@ static int fixup_pi_state_owner(u32 __user uaddr, struct futex_q q,
		* in the user space variable. This must be atomic as we have
		* to preserve the owner died bit here.
		*
		* Note: We write the user space value _before_ changing the
		* pi_state because we can fault here. Imagine swapped out
		* pages or a fork, which was running right before we acquired
		* mmap_sem, that marked all the anonymous memory readonly for
		* cow.
		* Note: We write the user space value _before_ changing the pi_state
		* because we can fault here. Imagine swapped out pages or a fork
		* that marked all the anonymous memory readonly for cow.
		*
		* Modifying pi_state _before_ the user space value would
		* leave the pi_state in an inconsistent state when we fault
		@@ -1188,7 +1183,7 @@ retry:
		hb = queue_lock(&q);

		/*
		* Access the page AFTER the futex is queued.
		* Access the page AFTER the hash-bucket is locked.
		* Order is important:
		*
		* Userspace waiter: val = var; if (cond(val)) futex_wait(&var, val);
		@@ -1204,7 +1199,7 @@ retry:
		* a wakeup when *uaddr != val on entry to the syscall. This is
		* rare, but normal.
		*
		* for shared futexes, we hold the mmap semaphore, so the mapping
		* For shared futexes, we hold the mmap semaphore, so the mapping
		* cannot have changed since we looked it up in get_futex_key.
		*/
		ret = get_futex_value_locked(&uval, uaddr);

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