Retire MUTEX_OWNER checks

To minimize the size of a kmutex_t a MUTEX_OWNER check was added.
It allowed the kmutex_t wrapper to leverage the mutex owner which was
already stored in the mutex for certain kernel configurations.

The upside to this was that it reduced the size of the kmutex_t wrapper
structure by the size of a task_struct pointer (4/8 bytes).  The
downside was that two mutex implementations needed to be maintained.
Depending on your exact kernel configuration the correct one would
be selected.

Over the years this solution worked but it could be fragile since it
depending heavily on assumed kernel mutex implementation details.  For
example the SPL_AC_MUTEX_OWNER_TASK_STRUCT configure check needed to
be added when the kernel changed how the owner was stored.  It also
made the code more complicated than it needed to be.

Therefore, in the name of simplicity and portability this optimization
is being retired.  It will slightly increase the memory requirements
for a kmutex_t but only very slightly.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@chase2k.com>
Issue #435

diff --git a/config/spl-build.m4 b/config/spl-build.m4
index d2cb68b6b4990a3d848688cc238ce57545bdb8db..4b55809ce09de09e74fa178af9c947d0b8416a09 100644 (file)
--- a/config/spl-build.m4
+++ b/config/spl-build.m4
@@ -25,8 +25,6 @@ AC_DEFUN([SPL_AC_CONFIG_KERNEL], [
        SPL_AC_SHRINKER_CALLBACK
        SPL_AC_CTL_NAME
        SPL_AC_PDE_DATA
-       SPL_AC_MUTEX_OWNER
-       SPL_AC_MUTEX_OWNER_TASK_STRUCT
        SPL_AC_SET_FS_PWD_WITH_CONST
        SPL_AC_2ARGS_VFS_UNLINK
        SPL_AC_4ARGS_VFS_RENAME
@@ -865,54 +863,6 @@ AC_DEFUN([SPL_AC_CTL_NAME], [
        ])
 ])
 
-dnl #
-dnl # 2.6.29 API change,
-dnl # Adaptive mutexs were introduced which track the mutex owner.  The
-dnl # mutex wrappers leverage this functionality to avoid tracking the
-dnl # owner multipe times.
-dnl #
-AC_DEFUN([SPL_AC_MUTEX_OWNER], [
-       AC_MSG_CHECKING([whether struct mutex has owner])
-       SPL_LINUX_TRY_COMPILE([
-               #include <linux/mutex.h>
-       ],[
-               struct mutex mtx __attribute__ ((unused));
-               mtx.owner = NULL;
-       ],[
-               AC_MSG_RESULT(yes)
-               AC_DEFINE(HAVE_MUTEX_OWNER, 1, [struct mutex has owner])
-       ],[
-               AC_MSG_RESULT(no)
-       ])
-])
-
-dnl #
-dnl # 2.6.39 API change,
-dnl # Owner type change.  A Linux mutex prior to 2.6.39 would store
-dnl # the owner as a thread_info pointer when CONFIG_DEBUG_MUTEXES
-dnl # was defined.  As of 2.6.39 this was changed to a task_struct
-dnl # pointer which frankly makes a lot more sense.
-dnl #
-AC_DEFUN([SPL_AC_MUTEX_OWNER_TASK_STRUCT], [
-       AC_MSG_CHECKING([whether struct mutex owner is a task_struct])
-       tmp_flags="$EXTRA_KCFLAGS"
-       EXTRA_KCFLAGS="-Werror"
-       SPL_LINUX_TRY_COMPILE([
-               #include <linux/mutex.h>
-               #include <linux/sched.h>
-       ],[
-               struct mutex mtx __attribute__ ((unused));
-               mtx.owner = current;
-       ],[
-               AC_MSG_RESULT(yes)
-               AC_DEFINE(HAVE_MUTEX_OWNER_TASK_STRUCT, 1,
-               [struct mutex owner is a task_struct])
-       ],[
-               AC_MSG_RESULT(no)
-       ])
-       EXTRA_KCFLAGS="$tmp_flags"
-])
-
 dnl #
 dnl # 3.10 API change,
 dnl # PDE is replaced by PDE_DATA

diff --git a/include/sys/mutex.h b/include/sys/mutex.h
index 43467223c6c795645033544782e43616ff2abd0a..2e45093455fe6c2f98592716b6e6394845ae53ee 100644 (file)
--- a/include/sys/mutex.h
+++ b/include/sys/mutex.h
@@ -35,81 +35,6 @@ typedef enum {
        MUTEX_ADAPTIVE  = 2
 } kmutex_type_t;
 
-#if defined(HAVE_MUTEX_OWNER) && defined(CONFIG_SMP) && \
-       !defined(CONFIG_DEBUG_MUTEXES)
-
-typedef struct {
-       struct mutex            m;
-       spinlock_t              m_lock; /* used for serializing mutex_exit */
-} kmutex_t;
-
-static inline kthread_t *
-mutex_owner(kmutex_t *mp)
-{
-#if defined(HAVE_MUTEX_OWNER_TASK_STRUCT)
-       return (ACCESS_ONCE(mp->m.owner));
-#else
-       struct thread_info *owner = ACCESS_ONCE(mp->m.owner);
-       if (owner)
-               return (owner->task);
-
-       return (NULL);
-#endif
-}
-
-#define        mutex_owned(mp)         (mutex_owner(mp) == current)
-#define        MUTEX_HELD(mp)          mutex_owned(mp)
-#define        MUTEX_NOT_HELD(mp)      (!MUTEX_HELD(mp))
-#undef mutex_init
-#define        mutex_init(mp, name, type, ibc)                         \
-{                                                              \
-       static struct lock_class_key __key;                     \
-       ASSERT(type == MUTEX_DEFAULT);                          \
-                                                               \
-       __mutex_init(&(mp)->m, #mp, &__key);                    \
-       spin_lock_init(&(mp)->m_lock);                          \
-}
-
-#undef mutex_destroy
-#define        mutex_destroy(mp)                                       \
-{                                                              \
-       VERIFY3P(mutex_owner(mp), ==, NULL);                    \
-}
-
-#define        mutex_tryenter(mp)      mutex_trylock(&(mp)->m)
-#define        mutex_enter(mp)                                         \
-{                                                              \
-       ASSERT3P(mutex_owner(mp), !=, current);                 \
-       mutex_lock(&(mp)->m);                                   \
-}
-/*
- * The reason for the spinlock:
- *
- * The Linux mutex is designed with a fast-path/slow-path design such that it
- * does not guarantee serialization upon itself, allowing a race where latter
- * acquirers finish mutex_unlock before former ones.
- *
- * The race renders it unsafe to be used for serializing the freeing of an
- * object in which the mutex is embedded, where the latter acquirer could go
- * on to free the object while the former one is still doing mutex_unlock and
- * causing memory corruption.
- *
- * However, there are many places in ZFS where the mutex is used for
- * serializing object freeing, and the code is shared among other OSes without
- * this issue. Thus, we need the spinlock to force the serialization on
- * mutex_exit().
- *
- * See http://lwn.net/Articles/575477/ for the information about the race.
- */
-#define        mutex_exit(mp)                                          \
-{                                                              \
-       spin_lock(&(mp)->m_lock);                               \
-       mutex_unlock(&(mp)->m);                                 \
-       spin_unlock(&(mp)->m_lock);                             \
-}
-
-#else /* HAVE_MUTEX_OWNER */
-
 typedef struct {
        struct mutex            m_mutex;
        spinlock_t              m_lock; /* used for serializing mutex_exit */
@@ -175,6 +100,25 @@ spl_mutex_clear_owner(kmutex_t *mp)
        spl_mutex_set_owner(mp);                                \
 }
 
+/*
+ * The reason for the spinlock:
+ *
+ * The Linux mutex is designed with a fast-path/slow-path design such that it
+ * does not guarantee serialization upon itself, allowing a race where latter
+ * acquirers finish mutex_unlock before former ones.
+ *
+ * The race renders it unsafe to be used for serializing the freeing of an
+ * object in which the mutex is embedded, where the latter acquirer could go
+ * on to free the object while the former one is still doing mutex_unlock and
+ * causing memory corruption.
+ *
+ * However, there are many places in ZFS where the mutex is used for
+ * serializing object freeing, and the code is shared among other OSes without
+ * this issue. Thus, we need the spinlock to force the serialization on
+ * mutex_exit().
+ *
+ * See http://lwn.net/Articles/575477/ for the information about the race.
+ */
 #define        mutex_exit(mp)                                          \
 {                                                              \
        spin_lock(&(mp)->m_lock);                               \
@@ -183,8 +127,6 @@ spl_mutex_clear_owner(kmutex_t *mp)
        spin_unlock(&(mp)->m_lock);                             \
 }
 
-#endif /* HAVE_MUTEX_OWNER */
-
 int spl_mutex_init(void);
 void spl_mutex_fini(void);