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Add rw_tryupgrade()

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This implementation of rw_tryupgrade() behaves slightly differently
from its counterparts on other platforms.  It drops the RW_READER lock
and then acquires the RW_WRITER lock leaving a small window where no
lock is held.  On other platforms the lock is never released during
the upgrade process.  This is necessary under Linux because the kernel
does not provide an upgrade function.

There are currently no callers in the ZFS code where this change in
behavior is a problem.  In fact, in most cases the code is already
written such that if the upgrade fails the RW_READER lock is dropped
and the caller blocks waiting to acquire the lock as RW_WRITER.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Matthew Thode <prometheanfire@gentoo.org>
Closes zfsonlinux/zfs#4388
Closes #534
This commit is contained in:
Brian Behlendorf
2016-03-09 14:20:48 -08:00
parent 47f9824781
commit a6ae97caed
3 changed files with 16 additions and 100 deletions
Download Patch File Download Diff File Expand all files Collapse all files
module/spl/spl-rwlock.c
-60
View File
@@ -32,65 +32,5 @@
#define DEBUG_SUBSYSTEM S_RWLOCK
#ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
/*
* From lib/rwsem-spinlock.c but modified such that the caller is
* responsible for acquiring and dropping the sem->wait_lock.
*/
struct rwsem_waiter {
struct list_head list;
struct task_struct *task;
unsigned int flags;
#define RWSEM_WAITING_FOR_READ 0x00000001
#define RWSEM_WAITING_FOR_WRITE 0x00000002
};
/* wake a single writer */
static struct rw_semaphore *
__rwsem_wake_one_writer_locked(struct rw_semaphore *sem)
{
struct rwsem_waiter *waiter;
struct task_struct *tsk;
sem->activity = -1;
waiter = list_entry(sem->wait_list.next, struct rwsem_waiter, list);
list_del(&waiter->list);
tsk = waiter->task;
smp_mb();
waiter->task = NULL;
wake_up_process(tsk);
put_task_struct(tsk);
return sem;
}
/* release a read lock on the semaphore */
void
__up_read_locked(struct rw_semaphore *sem)
{
if (--sem->activity == 0 && !list_empty(&sem->wait_list))
(void)__rwsem_wake_one_writer_locked(sem);
}
EXPORT_SYMBOL(__up_read_locked);
/* trylock for writing -- returns 1 if successful, 0 if contention */
int
__down_write_trylock_locked(struct rw_semaphore *sem)
{
int ret = 0;
if (sem->activity == 0 && list_empty(&sem->wait_list)) {
sem->activity = -1;
ret = 1;
}
return ret;
}
EXPORT_SYMBOL(__down_write_trylock_locked);
#endif
int spl_rw_init(void) { return 0; }
void spl_rw_fini(void) { }