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There are use cases where a rwsem can be acquired by one task, but released by another task. In thess cases, optimistic spinning may need to be disabled. One example will be the filesystem freeze/thaw code where the task that freezes the filesystem will acquire a write lock on a rwsem and then un-owns it before returning to userspace. Later on, another task will come along, acquire the ownership, thaw the filesystem and release the rwsem. Bit 0 of the owner field was used to designate that it is a reader owned rwsem. It is now repurposed to mean that the owner of the rwsem is not known. If only bit 0 is set, the rwsem is reader owned. If bit 0 and other bits are set, it is writer owned with an unknown owner. One such value for the latter case is (-1L). So we can set owner to 1 for reader-owned, -1 for writer-owned. The owner is unknown in both cases. To handle transfer of rwsem ownership, the higher level code should set the owner field to -1 to indicate a write-locked rwsem with unknown owner. Optimistic spinning will be disabled in this case. Once the higher level code figures who the new owner is, it can then set the owner field accordingly. Tested-by: Amir Goldstein <amir73il@gmail.com> Signed-off-by: Waiman Long <longman@redhat.com> Acked-by: Peter Zijlstra <peterz@infradead.org> Cc: Andrew Morton <akpm@linux-foundation.org> Cc: Davidlohr Bueso <dave@stgolabs.net> Cc: Jan Kara <jack@suse.cz> Cc: Linus Torvalds <torvalds@linux-foundation.org> Cc: Matthew Wilcox <willy@infradead.org> Cc: Oleg Nesterov <oleg@redhat.com> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Cc: Theodore Y. Ts'o <tytso@mit.edu> Cc: Thomas Gleixner <tglx@linutronix.de> Cc: Will Deacon <will.deacon@arm.com> Cc: linux-fsdevel@vger.kernel.org Link: http://lkml.kernel.org/r/1526420991-21213-2-git-send-email-longman@redhat.com Signed-off-by: Ingo Molnar <mingo@kernel.org>
88 lines
2.7 KiB
C
88 lines
2.7 KiB
C
/* SPDX-License-Identifier: GPL-2.0 */
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/*
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* The owner field of the rw_semaphore structure will be set to
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* RWSEM_READER_OWNED when a reader grabs the lock. A writer will clear
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* the owner field when it unlocks. A reader, on the other hand, will
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* not touch the owner field when it unlocks.
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*
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* In essence, the owner field now has the following 4 states:
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* 1) 0
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* - lock is free or the owner hasn't set the field yet
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* 2) RWSEM_READER_OWNED
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* - lock is currently or previously owned by readers (lock is free
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* or not set by owner yet)
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* 3) RWSEM_ANONYMOUSLY_OWNED bit set with some other bits set as well
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* - lock is owned by an anonymous writer, so spinning on the lock
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* owner should be disabled.
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* 4) Other non-zero value
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* - a writer owns the lock and other writers can spin on the lock owner.
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*/
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#define RWSEM_ANONYMOUSLY_OWNED (1UL << 0)
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#define RWSEM_READER_OWNED ((struct task_struct *)RWSEM_ANONYMOUSLY_OWNED)
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#ifdef CONFIG_DEBUG_RWSEMS
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# define DEBUG_RWSEMS_WARN_ON(c) DEBUG_LOCKS_WARN_ON(c)
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#else
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# define DEBUG_RWSEMS_WARN_ON(c)
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#endif
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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/*
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* All writes to owner are protected by WRITE_ONCE() to make sure that
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* store tearing can't happen as optimistic spinners may read and use
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* the owner value concurrently without lock. Read from owner, however,
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* may not need READ_ONCE() as long as the pointer value is only used
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* for comparison and isn't being dereferenced.
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*/
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static inline void rwsem_set_owner(struct rw_semaphore *sem)
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{
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WRITE_ONCE(sem->owner, current);
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}
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static inline void rwsem_clear_owner(struct rw_semaphore *sem)
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{
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WRITE_ONCE(sem->owner, NULL);
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}
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static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
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{
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/*
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* We check the owner value first to make sure that we will only
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* do a write to the rwsem cacheline when it is really necessary
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* to minimize cacheline contention.
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*/
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if (READ_ONCE(sem->owner) != RWSEM_READER_OWNED)
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WRITE_ONCE(sem->owner, RWSEM_READER_OWNED);
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}
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/*
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* Return true if the a rwsem waiter can spin on the rwsem's owner
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* and steal the lock, i.e. the lock is not anonymously owned.
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* N.B. !owner is considered spinnable.
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*/
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static inline bool is_rwsem_owner_spinnable(struct task_struct *owner)
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{
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return !((unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED);
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}
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/*
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* Return true if rwsem is owned by an anonymous writer or readers.
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*/
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static inline bool rwsem_has_anonymous_owner(struct task_struct *owner)
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{
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return (unsigned long)owner & RWSEM_ANONYMOUSLY_OWNED;
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}
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#else
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static inline void rwsem_set_owner(struct rw_semaphore *sem)
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{
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}
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static inline void rwsem_clear_owner(struct rw_semaphore *sem)
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{
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}
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static inline void rwsem_set_reader_owned(struct rw_semaphore *sem)
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{
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}
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#endif
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