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 #include <linux/module.h>
  29
  30 #if defined(CONFIG_PREEMPT_RT_FULL)
  31
  32 #include <linux/rtmutex.h>
  33 #define RT_MUTEX_OWNER_MASKALL  1UL
  34
  35 static int
  36 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
  37 {
  38 #if defined(READER_BIAS) && defined(WRITER_BIAS)
  39         /*
  40          * After the 4.9.20-rt16 kernel the realtime patch series lifted the
  41          * single reader restriction.  While this could be accommodated by
  42          * adding additional compatibility code assume the rwsem can never
  43          * be upgraded.  All caller must already cleanly handle this case.
  44          */
  45         return (0);
  46 #else
  47         ASSERT((struct task_struct *)
  48             ((unsigned long)rwsem->lock.owner & ~RT_MUTEX_OWNER_MASKALL) ==
  49             current);
  50
  51         /*
  52          * Prior to 4.9.20-rt16 kernel the realtime patch series, rwsem is
  53          * implemented as a single mutex held by readers and writers alike.
  54          * However, this implementation would prevent a thread from taking
  55          * a read lock twice, as the mutex would already be locked on
  56          * the second attempt. Therefore the implementation allows a
  57          * single thread to take a rwsem as read lock multiple times
  58          * tracking that nesting as read_depth counter.
  59          */
  60         if (rwsem->read_depth <= 1) {
  61                 /*
  62                  * In case, the current thread has not taken the lock
  63                  * more than once as read lock, we can allow an
  64                  * upgrade to a write lock. rwsem_rt.h implements
  65                  * write locks as read_depth == 0.
  66                  */
  67                 rwsem->read_depth = 0;
  68                 return (1);
  69         }
  70         return (0);
  71 #endif
  72 }
  73 #elif defined(CONFIG_RWSEM_GENERIC_SPINLOCK)
  74 static int
  75 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
  76 {
  77         int ret = 0;
  78         unsigned long flags;
  79         spl_rwsem_lock_irqsave(&rwsem->wait_lock, flags);
  80         if (RWSEM_COUNT(rwsem) == SPL_RWSEM_SINGLE_READER_VALUE &&
  81             list_empty(&rwsem->wait_list)) {
  82                 ret = 1;
  83                 RWSEM_COUNT(rwsem) = SPL_RWSEM_SINGLE_WRITER_VALUE;
  84         }
  85         spl_rwsem_unlock_irqrestore(&rwsem->wait_lock, flags);
  86         return (ret);
  87 }
  88 #elif defined(HAVE_RWSEM_ATOMIC_LONG_COUNT)
  89 static int
  90 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
  91 {
  92         long val;
  93         val = atomic_long_cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
  94             SPL_RWSEM_SINGLE_WRITER_VALUE);
  95         return (val == SPL_RWSEM_SINGLE_READER_VALUE);
  96 }
  97 #else
  98 static int
  99 __rwsem_tryupgrade(struct rw_semaphore *rwsem)
 100 {
 101         typeof(rwsem->count) val;
 102         val = cmpxchg(&rwsem->count, SPL_RWSEM_SINGLE_READER_VALUE,
 103             SPL_RWSEM_SINGLE_WRITER_VALUE);
 104         return (val == SPL_RWSEM_SINGLE_READER_VALUE);
 105 }
 106 #endif
 107
 108 int
 109 rwsem_tryupgrade(struct rw_semaphore *rwsem)
 110 {
 111         if (__rwsem_tryupgrade(rwsem)) {
 112                 rwsem_release(&rwsem->dep_map, 1, _RET_IP_);
 113                 rwsem_acquire(&rwsem->dep_map, 0, 1, _RET_IP_);
 114 #ifdef CONFIG_RWSEM_SPIN_ON_OWNER
 115                 rwsem->owner = current;
 116 #endif
 117                 return (1);
 118         }
 119         return (0);
 120 }
 121 EXPORT_SYMBOL(rwsem_tryupgrade);
 122
 123 int spl_rw_init(void) { return 0; }
 124 void spl_rw_fini(void) { }