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Linux 5.3 compat: retire rw_tryupgrade()
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The Linux kernel's rwsem's have never provided an interface to
allow a reader to be upgraded to a writer.  Historically, this
functionality has been implemented by a SPL wrapper function.
However, this approach depends on internal knowledge of the
rw_semaphore and is therefore rather brittle.

Since the ZFS code must always be able to fallback to rw_exit()
and rw_enter() when an rw_tryupgrade() fails; this functionality
isn't critical.  Furthermore, the only potentially performance
sensitive consumer is dmu_zfetch() and no decrease in performance
was observed with this change applied.  See the PR comments for
additional testing details.

Therefore, it is being retired to make the build more robust and
to simplify the rwlock implementation.

Reviewed-by: Tony Hutter <[email protected]>
Reviewed-by: Tomohiro Kusumi <[email protected]>
Signed-off-by: Brian Behlendorf <[email protected]>
Closes openzfs#9029
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@behlendorf @tonyhutter
behlendorf authored and tonyhutter committed Aug 13, 2019
1 parent bb9e8ba commit 9983469
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Showing 2 changed files with 7 additions and 154 deletions.
60 changes: 7 additions & 53 deletions include/spl/sys/rwlock.h
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Original file line number Diff line number Diff line change
Expand Up @@ -29,43 +29,6 @@
#include <linux/rwsem.h>
#include <linux/sched.h>

/* Linux kernel compatibility */
#if defined(CONFIG_PREEMPT_RT_FULL)
#define SPL_RWSEM_SINGLE_READER_VALUE (1)
#define SPL_RWSEM_SINGLE_WRITER_VALUE (0)
#elif defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
#define SPL_RWSEM_SINGLE_READER_VALUE (1)
#define SPL_RWSEM_SINGLE_WRITER_VALUE (-1)
#elif defined(RWSEM_ACTIVE_MASK)
#define SPL_RWSEM_SINGLE_READER_VALUE (RWSEM_ACTIVE_READ_BIAS)
#define SPL_RWSEM_SINGLE_WRITER_VALUE (RWSEM_ACTIVE_WRITE_BIAS)
#endif

/* Linux 3.16 changed activity to count for rwsem-spinlock */
#if defined(CONFIG_PREEMPT_RT_FULL)
#define RWSEM_COUNT(sem) sem->read_depth
#elif defined(HAVE_RWSEM_ACTIVITY)
#define RWSEM_COUNT(sem) sem->activity
/* Linux 4.8 changed count to an atomic_long_t for !rwsem-spinlock */
#elif defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
#define RWSEM_COUNT(sem) atomic_long_read(&(sem)->count)
#else
#define RWSEM_COUNT(sem) sem->count
#endif

#if defined(RWSEM_SPINLOCK_IS_RAW)
#define spl_rwsem_lock_irqsave(lk, fl) raw_spin_lock_irqsave(lk, fl)
#define spl_rwsem_unlock_irqrestore(lk, fl) \
raw_spin_unlock_irqrestore(lk, fl)
#define spl_rwsem_trylock_irqsave(lk, fl) raw_spin_trylock_irqsave(lk, fl)
#else
#define spl_rwsem_lock_irqsave(lk, fl) spin_lock_irqsave(lk, fl)
#define spl_rwsem_unlock_irqrestore(lk, fl) spin_unlock_irqrestore(lk, fl)
#define spl_rwsem_trylock_irqsave(lk, fl) spin_trylock_irqsave(lk, fl)
#endif /* RWSEM_SPINLOCK_IS_RAW */

#define spl_rwsem_is_locked(rwsem) rwsem_is_locked(rwsem)

typedef enum {
RW_DRIVER = 2,
RW_DEFAULT = 4,
Expand Down Expand Up @@ -133,7 +96,7 @@ spl_rw_lockdep_on_maybe(krwlock_t *rwp) \
static inline int
RW_LOCK_HELD(krwlock_t *rwp)
{
return (spl_rwsem_is_locked(SEM(rwp)));
return (rwsem_is_locked(SEM(rwp)));
}

static inline int
Expand Down Expand Up @@ -170,6 +133,12 @@ RW_READ_HELD(krwlock_t *rwp)
*/
#define rw_destroy(rwp) ((void) 0)

/*
* Upgrading a rwsem from a reader to a writer is not supported by the
* Linux kernel. The lock must be dropped and reacquired as a writer.
*/
#define rw_tryupgrade(rwp) RW_WRITE_HELD(rwp)

#define rw_tryenter(rwp, rw) \
({ \
int _rc_ = 0; \
Expand Down Expand Up @@ -228,24 +197,9 @@ RW_READ_HELD(krwlock_t *rwp)
spl_rw_lockdep_on_maybe(rwp); \
})

#define rw_tryupgrade(rwp) \
({ \
int _rc_ = 0; \
\
if (RW_WRITE_HELD(rwp)) { \
_rc_ = 1; \
} else { \
spl_rw_lockdep_off_maybe(rwp); \
if ((_rc_ = rwsem_tryupgrade(SEM(rwp)))) \
spl_rw_set_owner(rwp); \
spl_rw_lockdep_on_maybe(rwp); \
} \
_rc_; \
})
/* END CSTYLED */

int spl_rw_init(void);
void spl_rw_fini(void);
int rwsem_tryupgrade(struct rw_semaphore *rwsem);

#endif /* _SPL_RWLOCK_H */
101 changes: 0 additions & 101 deletions module/spl/spl-rwlock.c
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Original file line number Diff line number Diff line change
Expand Up @@ -24,106 +24,5 @@
* Solaris Porting Layer (SPL) Reader/Writer Lock Implementation.
*/

#include <sys/rwlock.h>
#include <linux/module.h>

#if defined(CONFIG_PREEMPT_RT_FULL)

#include <linux/rtmutex.h>
#define RT_MUTEX_OWNER_MASKALL 1UL

static int
__rwsem_tryupgrade(struct rw_semaphore *rwsem)
{
#if defined(READER_BIAS) && defined(WRITER_BIAS)
/*
* After the 4.9.20-rt16 kernel the realtime patch series lifted the
* single reader restriction. While this could be accommodated by
* adding additional compatibility code assume the rwsem can never
* be upgraded. All caller must already cleanly handle this case.
*/
return (0);
#else
ASSERT((struct task_struct *)
((unsigned long)rwsem->lock.owner & ~RT_MUTEX_OWNER_MASKALL) ==
current);

/*
* Prior to 4.9.20-rt16 kernel the realtime patch series, rwsem is
* implemented as a single mutex held by readers and writers alike.
* However, this implementation would prevent a thread from taking
* a read lock twice, as the mutex would already be locked on
* the second attempt. Therefore the implementation allows a
* single thread to take a rwsem as read lock multiple times
* tracking that nesting as read_depth counter.
*/
if (rwsem->read_depth <= 1) {
/*
* In case, the current thread has not taken the lock
* more than once as read lock, we can allow an
* upgrade to a write lock. rwsem_rt.h implements
* write locks as read_depth == 0.
*/
rwsem->read_depth = 0;
return (1);
}
return (0);
#endif
}
#elif defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
static int
__rwsem_tryupgrade(struct rw_semaphore *rwsem)
{
int ret = 0;
unsigned long flags;
spl_rwsem_lock_irqsave(&rwsem->wait_lock, flags);
if (RWSEM_COUNT(rwsem) == SPL_RWSEM_SINGLE_READER_VALUE &&
list_empty(&rwsem->wait_list)) {
ret = 1;
RWSEM_COUNT(rwsem) = SPL_RWSEM_SINGLE_WRITER_VALUE;
}
spl_rwsem_unlock_irqrestore(&rwsem->wait_lock, flags);
return (ret);
}
#elif defined(RWSEM_ACTIVE_MASK)
#if defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
static int
__rwsem_tryupgrade(struct rw_semaphore *rwsem)
{
long val;
val = atomic_long_cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
SPL_RWSEM_SINGLE_WRITER_VALUE);
return (val == SPL_RWSEM_SINGLE_READER_VALUE);
}
#else
static int
__rwsem_tryupgrade(struct rw_semaphore *rwsem)
{
typeof(rwsem->count) val;
val = cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
SPL_RWSEM_SINGLE_WRITER_VALUE);
return (val == SPL_RWSEM_SINGLE_READER_VALUE);
}
#endif
#else
static int
__rwsem_tryupgrade(struct rw_semaphore *rwsem)
{
return (0);
}
#endif

int
rwsem_tryupgrade(struct rw_semaphore *rwsem)
{
if (__rwsem_tryupgrade(rwsem)) {
rwsem_release(&rwsem->dep_map, 1, _RET_IP_);
rwsem_acquire(&rwsem->dep_map, 0, 1, _RET_IP_);
return (1);
}
return (0);
}
EXPORT_SYMBOL(rwsem_tryupgrade);

int spl_rw_init(void) { return 0; }
void spl_rw_fini(void) { }

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