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author | Waiman Long <longman@redhat.com> | 2018-05-15 17:49:50 -0400 |
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committer | Ingo Molnar <mingo@kernel.org> | 2018-05-16 11:45:15 +0200 |
commit | d7d760efad70c7a030725499bf9f342f04af24dd (patch) | |
tree | 6d4d57b3349f9df4e0dab493b9202dd1073117f8 /kernel/locking/rwsem-xadd.c | |
parent | 67b8d5c7081221efa252e111cd52532ec6d4266f (diff) | |
download | linux-d7d760efad70c7a030725499bf9f342f04af24dd.tar.gz linux-d7d760efad70c7a030725499bf9f342f04af24dd.tar.bz2 linux-d7d760efad70c7a030725499bf9f342f04af24dd.zip |
locking/rwsem: Add a new RWSEM_ANONYMOUSLY_OWNED flag
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>
Diffstat (limited to 'kernel/locking/rwsem-xadd.c')
-rw-r--r-- | kernel/locking/rwsem-xadd.c | 17 |
1 files changed, 7 insertions, 10 deletions
diff --git a/kernel/locking/rwsem-xadd.c b/kernel/locking/rwsem-xadd.c index e795908f3607..604d247ea8c3 100644 --- a/kernel/locking/rwsem-xadd.c +++ b/kernel/locking/rwsem-xadd.c @@ -357,11 +357,8 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem) rcu_read_lock(); owner = READ_ONCE(sem->owner); - if (!rwsem_owner_is_writer(owner)) { - /* - * Don't spin if the rwsem is readers owned. - */ - ret = !rwsem_owner_is_reader(owner); + if (!owner || !is_rwsem_owner_spinnable(owner)) { + ret = !owner; /* !owner is spinnable */ goto done; } @@ -382,11 +379,11 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem) { struct task_struct *owner = READ_ONCE(sem->owner); - if (!rwsem_owner_is_writer(owner)) - goto out; + if (!is_rwsem_owner_spinnable(owner)) + return false; rcu_read_lock(); - while (sem->owner == owner) { + while (owner && (READ_ONCE(sem->owner) == owner)) { /* * Ensure we emit the owner->on_cpu, dereference _after_ * checking sem->owner still matches owner, if that fails, @@ -408,12 +405,12 @@ static noinline bool rwsem_spin_on_owner(struct rw_semaphore *sem) cpu_relax(); } rcu_read_unlock(); -out: + /* * If there is a new owner or the owner is not set, we continue * spinning. */ - return !rwsem_owner_is_reader(READ_ONCE(sem->owner)); + return is_rwsem_owner_spinnable(READ_ONCE(sem->owner)); } static bool rwsem_optimistic_spin(struct rw_semaphore *sem) |