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Implement a proper rw_tryupgrade

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Current rw_tryupgrade does rw_exit and then rw_tryenter(RW_RWITER), and then
does rw_enter(RW_READER) if it fails. This violate the assumption that
rw_tryupgrade should be atomic and could cause extra contention or even lock
inversion.

This patch we implement a proper rw_tryupgrade. For rwsem-spinlock, we take
the spinlock to check rwsem->count and rwsem->wait_list. For normal rwsem, we
use cmpxchg on rwsem->count to change the value from single reader to single
writer.

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Closes zfsonlinux/zfs#4692
Closes #554
This commit is contained in:
Chunwei Chen
2016-05-25 16:35:42 -07:00
committed by Brian Behlendorf
parent c60a51b640
commit f58040c0fc
5 changed files with 147 additions and 13 deletions
Download Patch File Download Diff File Expand all files Collapse all files
include/sys/rwlock.h
+4 -7
View File
@@ -223,13 +223,10 @@ RW_LOCK_HELD(krwlock_t *rwp)
if (RW_WRITE_HELD(rwp)) { \
_rc_ = 1; \
} else { \
rw_exit(rwp); \
if (rw_tryenter(rwp, RW_WRITER)) { \
_rc_ = 1; \
} else { \
rw_enter(rwp, RW_READER); \
_rc_ = 0; \
} \
spl_rw_lockdep_off_maybe(rwp); \
if ((_rc_ = rwsem_tryupgrade(SEM(rwp)))) \
spl_rw_set_owner(rwp); \
spl_rw_lockdep_on_maybe(rwp); \
} \
_rc_; \
})