module/spl/spl-rwlock.c

   1 /*****************************************************************************\
   2  *  Copyright (C) 2007-2010 Lawrence Livermore National Security, LLC.
   3  *  Copyright (C) 2007 The Regents of the University of California.
   4  *  Produced at Lawrence Livermore National Laboratory (cf, DISCLAIMER).
   5  *  Written by Brian Behlendorf <behlendorf1@llnl.gov>.
   6  *  UCRL-CODE-235197
   7  *
   8  *  This file is part of the SPL, Solaris Porting Layer.
   9  *  For details, see <http://zfsonlinux.org/>.
  10  *
  11  *  The SPL is free software; you can redistribute it and/or modify it
  12  *  under the terms of the GNU General Public License as published by the
  13  *  Free Software Foundation; either version 2 of the License, or (at your
  14  *  option) any later version.
  15  *
  16  *  The SPL is distributed in the hope that it will be useful, but WITHOUT
  17  *  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  18  *  FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  19  *  for more details.
  20  *
  21  *  You should have received a copy of the GNU General Public License along
  22  *  with the SPL.  If not, see <http://www.gnu.org/licenses/>.
  23  *****************************************************************************
  24  *  Solaris Porting Layer (SPL) Reader/Writer Lock Implementation.
  25 \*****************************************************************************/
  26
  27 #include <sys/rwlock.h>
  28
  29 #ifdef DEBUG_SUBSYSTEM
  30 #undef DEBUG_SUBSYSTEM
  31 #endif
  32
  33 #define DEBUG_SUBSYSTEM S_RWLOCK
  34
  35 #if defined(CONFIG_PREEMPT_RT_FULL)
  36
  37 #include <linux/rtmutex.h>
  38
  39 static int
  40 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
  41 {
  42         ASSERT(rt_mutex_owner(&rwsem->lock) == current);
  43
  44         /*
  45          * Under the realtime patch series, rwsem is implemented as a
  46          * single mutex held by readers and writers alike. However,
  47          * this implementation would prevent a thread from taking a
  48          * read lock twice, as the mutex would already be locked on
  49          * the second attempt. Therefore the implementation allows a
  50          * single thread to take a rwsem as read lock multiple times
  51          * tracking that nesting as read_depth counter.
  52          */
  53         if (rwsem->read_depth <= 1) {
  54                 /*
  55                  * In case, the current thread has not taken the lock
  56                  * more than once as read lock, we can allow an
  57                  * upgrade to a write lock. rwsem_rt.h implements
  58                  * write locks as read_depth == 0.
  59                  */
  60                 rwsem->read_depth = 0;
  61                 return (1);
  62         }
  63         return (0);
  64 }
  65 #elif defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
  66 static int
  67 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
  68 {
  69         int ret = 0;
  70         unsigned long flags;
  71         spl_rwsem_lock_irqsave(&rwsem->wait_lock, flags);
  72         if (RWSEM_COUNT(rwsem) == SPL_RWSEM_SINGLE_READER_VALUE &&
  73             list_empty(&rwsem->wait_list)) {
  74                 ret = 1;
  75                 RWSEM_COUNT(rwsem) = SPL_RWSEM_SINGLE_WRITER_VALUE;
  76         }
  77         spl_rwsem_unlock_irqrestore(&rwsem->wait_lock, flags);
  78         return (ret);
  79 }
  80 #elif defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
  81 static int
  82 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
  83 {
  84         long val;
  85         val = atomic_long_cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
  86             SPL_RWSEM_SINGLE_WRITER_VALUE);
  87         return (val == SPL_RWSEM_SINGLE_READER_VALUE);
  88 }
  89 #else
  90 static int
  91 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
  92 {
  93         typeof (rwsem->count) val;
  94         val = cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
  95             SPL_RWSEM_SINGLE_WRITER_VALUE);
  96         return (val == SPL_RWSEM_SINGLE_READER_VALUE);
  97 }
  98 #endif
  99
 100 int
 101 rwsem_tryupgrade(struct rw_semaphore *rwsem)
 102 {
 103         if (__rwsem_tryupgrade(rwsem)) {
 104                 rwsem_release(&rwsem->dep_map, 1, _RET_IP_);
 105                 rwsem_acquire(&rwsem->dep_map, 0, 1, _RET_IP_);
 106 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 107                 rwsem->owner = current;
 108 #endif
 109                 return (1);
 110         }
 111         return (0);
 112 }
 113 EXPORT_SYMBOL(rwsem_tryupgrade);
 114
 115 int spl_rw_init(void) { return 0; }
 116 void spl_rw_fini(void) { }