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f58040c0fc
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
78 lines
2.5 KiB
C
78 lines
2.5 KiB
C
/*****************************************************************************\
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* Copyright (C) 2007-2010 Lawrence Livermore National Security, LLC.
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* Copyright (C) 2007 The Regents of the University of California.
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* Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
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* Written by Brian Behlendorf <behlendorf1@llnl.gov>.
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* UCRL-CODE-235197
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*
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* This file is part of the SPL, Solaris Porting Layer.
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* For details, see <http://zfsonlinux.org/>.
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*
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* The SPL is free software; you can redistribute it and/or modify it
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* under the terms of the GNU General Public License as published by the
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* Free Software Foundation; either version 2 of the License, or (at your
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* option) any later version.
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*
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* The SPL is distributed in the hope that it will be useful, but WITHOUT
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* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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* for more details.
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*
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* You should have received a copy of the GNU General Public License along
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* with the SPL. If not, see <http://www.gnu.org/licenses/>.
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*****************************************************************************
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* Solaris Porting Layer (SPL) Reader/Writer Lock Implementation.
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\*****************************************************************************/
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#include <sys/rwlock.h>
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#ifdef DEBUG_SUBSYSTEM
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#undef DEBUG_SUBSYSTEM
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#endif
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#define DEBUG_SUBSYSTEM S_RWLOCK
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#ifdef CONFIG_RWSEM_GENERIC_SPINLOCK
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static int
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__rwsem_tryupgrade(struct rw_semaphore *rwsem)
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{
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int ret = 0;
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unsigned long flags;
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spl_rwsem_lock_irqsave(&rwsem->wait_lock, flags);
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if (RWSEM_COUNT(rwsem) == SPL_RWSEM_SINGLE_READER_VALUE &&
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list_empty(&rwsem->wait_list)) {
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ret = 1;
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RWSEM_COUNT(rwsem) = SPL_RWSEM_SINGLE_WRITER_VALUE;
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}
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spl_rwsem_unlock_irqrestore(&rwsem->wait_lock, flags);
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return (ret);
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}
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#else
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static int
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__rwsem_tryupgrade(struct rw_semaphore *rwsem)
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{
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typeof (rwsem->count) val;
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val = cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
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SPL_RWSEM_SINGLE_WRITER_VALUE);
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return (val == SPL_RWSEM_SINGLE_READER_VALUE);
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}
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#endif
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int
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rwsem_tryupgrade(struct rw_semaphore *rwsem)
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{
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if (__rwsem_tryupgrade(rwsem)) {
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rwsem_release(&rwsem->dep_map, 1, _RET_IP_);
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rwsem_acquire(&rwsem->dep_map, 0, 1, _RET_IP_);
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#ifdef CONFIG_RWSEM_SPIN_ON_OWNER
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rwsem->owner = current;
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#endif
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return (1);
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}
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return (0);
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}
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EXPORT_SYMBOL(rwsem_tryupgrade);
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int spl_rw_init(void) { return 0; }
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void spl_rw_fini(void) { }
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