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