Date: Fri, 20 Aug 2010 03:49:23 +0200 From: Max Laier <max@love2party.net> To: freebsd-arch@freebsd.org Cc: ups@freebsd.org Subject: Re: rmlock(9) two additions Message-ID: <201008200349.23940.max@love2party.net> In-Reply-To: <201008160515.21412.max@love2party.net> References: <201008160515.21412.max@love2party.net>
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Hi again,
On Monday 16 August 2010 05:15:21 I wrote:
> I'd like to run two additions to rmlock(9) by you:
>
> 1) See the attached patch. It adds rm_try_rlock() which avoids taking the
> mutex if the lock is currently in a write episode. The only overhead to
> the hot path is an additional argument and a return value for _rm_rlock*.
> If you are worried about that, it can obviously be done in a separate code
> path, but I reckon it not worth the code crunch. Finally, there is one
> additional branch to check the "trylock" argument, but that's well past
> the hot path.
>
> 2) No code for this yet - testing the waters first. I'd like to add the
> ability to replace the mutex for writer synchronization with a general lock
> - especially to be able to use a sx(9) lock here.
the attached patch includes code for this as well and updates the man pages
accordingly. Instead of using a general lock, I just added a rm_init_flags()
flag called "RM_SLEEPABLE" that changes the lock to use an sx(9) lock instead
of the mutex. This is enough to support the use case I have in mind. See
below for details on that from my original post. There is a bit of "creative"
use of witness in this patch, but the result provides proper warnings in all
cases.
Again: Are there any objections or concerns against this? Otherwise I will
commit it soon.
Thanks,
Max
> The reason for #2 is the following use case in a debugging facility:
>
> "reader":
> if (rm_try_rlock()) {
> grab_per_cpu_buffer();
> fill_per_cpu_buffer();
> rm_runlock();
> }
>
> "writer" - better exclusive access thread:
> rm_wlock();
> collect_buffers_and_copy_out_to_userspace();
> rm_wunlock();
>
> This is much cleaner and possibly cheaper than the various hand rolled
> versions I've come across, that try to get the same synchronization with
> atomic operations. If we could sleep with the wlock held, we can also
> avoid copying the buffer contents, or swapping buffers.
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diff --git a/share/man/man9/Makefile b/share/man/man9/Makefile
index f8dec17..f05c4c6 100644
--- a/share/man/man9/Makefile
+++ b/share/man/man9/Makefile
@@ -985,6 +985,7 @@ MLINKS+=rman.9 rman_activate_resource.9 \
MLINKS+=rmlock.9 rm_destroy.9 \
rmlock.9 rm_init.9 \
rmlock.9 rm_rlock.9 \
+ rmlock.9 rm_try_rlock.9 \
rmlock.9 rm_runlock.9 \
rmlock.9 RM_SYSINIT.9 \
rmlock.9 rm_wlock.9 \
diff --git a/share/man/man9/locking.9 b/share/man/man9/locking.9
index 005f476..3728319 100644
--- a/share/man/man9/locking.9
+++ b/share/man/man9/locking.9
@@ -301,7 +301,7 @@ one of the synchronization primitives discussed:
.It mutex Ta \&ok Ta \&ok-1 Ta \&no Ta \&ok Ta \&ok Ta \&no-3
.It sx Ta \&ok Ta \&ok Ta \&ok-2 Ta \&ok Ta \&ok Ta \&ok-4
.It rwlock Ta \&ok Ta \&ok Ta \&no Ta \&ok-2 Ta \&ok Ta \&no-3
-.It rmlock Ta \&ok Ta \&ok Ta \&no Ta \&ok Ta \&ok-2 Ta \&no
+.It rmlock Ta \&ok Ta \&ok Ta \&ok-5 Ta \&ok Ta \&ok-2 Ta \&ok-5
.El
.Pp
.Em *1
@@ -326,6 +326,13 @@ Though one can sleep holding an sx lock, one can also use
.Fn sx_sleep
which will atomically release this primitive when going to sleep and
reacquire it on wakeup.
+.Pp
+.Em *5
+.Em Read-mostly
+locks can be initialized to support sleeping while holding a write lock.
+See
+.Xr rmlock 9
+for details.
.Ss Context mode table
The next table shows what can be used in different contexts.
At this time this is a rather easy to remember table.
diff --git a/share/man/man9/rmlock.9 b/share/man/man9/rmlock.9
index e99661d..28ac0a5 100644
--- a/share/man/man9/rmlock.9
+++ b/share/man/man9/rmlock.9
@@ -35,6 +35,7 @@
.Nm rm_init_flags ,
.Nm rm_destroy ,
.Nm rm_rlock ,
+.Nm rm_try_rlock ,
.Nm rm_wlock ,
.Nm rm_runlock ,
.Nm rm_wunlock ,
@@ -53,6 +54,8 @@
.Fn rm_destroy "struct rmlock *rm"
.Ft void
.Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
+.Ft int
+.Fn rm_try_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
.Ft void
.Fn rm_wlock "struct rmlock *rm"
.Ft void
@@ -84,14 +87,16 @@ Although reader/writer locks look very similar to
locks, their usage pattern is different.
Reader/writer locks can be treated as mutexes (see
.Xr mutex 9 )
-with shared/exclusive semantics.
+with shared/exclusive semantics unless initialized with
+.Dv RM_SLEEPABLE .
Unlike
.Xr sx 9 ,
an
.Nm
can be locked while holding a non-spin mutex, and an
.Nm
-cannot be held while sleeping.
+cannot be held while sleeping, again unless initialized with
+.Dv RM_SLEEPABLE .
The
.Nm
locks have full priority propagation like mutexes.
@@ -135,6 +140,13 @@ to ignore this lock.
.It Dv RM_RECURSE
Allow threads to recursively acquire exclusive locks for
.Fa rm .
+.It Dv RM_SLEEPABLE
+Allow writers to sleep while holding the lock.
+Readers must not sleep while holding the lock and can avoid to sleep on
+taking the lock by using
+.Fn rm_try_rlock
+instead of
+.Fn rm_rlock .
.El
.It Fn rm_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
Lock
@@ -161,6 +173,13 @@ access on
.Fa rm .
This is called
.Dq "recursing on a lock" .
+.It Fn rm_try_rlock "struct rmlock *rm" "struct rm_priotracker* tracker"
+Try to lock
+.Fa rm
+as a reader.
+.Fn rm_try_rlock
+will return 0 if the lock cannot be acquired immediately;
+otherwise the lock will be acquired and a non-zero value will be returned.
.It Fn rm_wlock "struct rmlock *rm"
Lock
.Fa rm
diff --git a/sys/kern/kern_rmlock.c b/sys/kern/kern_rmlock.c
index a6a622e..c1aca1d 100644
--- a/sys/kern/kern_rmlock.c
+++ b/sys/kern/kern_rmlock.c
@@ -187,6 +187,8 @@ rm_cleanIPI(void *arg)
}
}
+CTASSERT((RM_SLEEPABLE & LO_CLASSFLAGS) == RM_SLEEPABLE);
+
void
rm_init_flags(struct rmlock *rm, const char *name, int opts)
{
@@ -199,7 +201,11 @@ rm_init_flags(struct rmlock *rm, const char *name, int opts)
liflags |= LO_RECURSABLE;
rm->rm_noreadtoken = 1;
LIST_INIT(&rm->rm_activeReaders);
- mtx_init(&rm->rm_lock, name, "rmlock_mtx", MTX_NOWITNESS);
+ if (opts & RM_SLEEPABLE) {
+ liflags |= RM_SLEEPABLE;
+ sx_init_flags(&rm->rm_lock_sx, "rmlock_sx", SX_RECURSE);
+ } else
+ mtx_init(&rm->rm_lock_mtx, name, "rmlock_mtx", MTX_NOWITNESS);
lock_init(&rm->lock_object, &lock_class_rm, name, NULL, liflags);
}
@@ -214,7 +220,10 @@ void
rm_destroy(struct rmlock *rm)
{
- mtx_destroy(&rm->rm_lock);
+ if (rm->lock_object.lo_flags & RM_SLEEPABLE)
+ sx_destroy(&rm->rm_lock_sx);
+ else
+ mtx_destroy(&rm->rm_lock_mtx);
lock_destroy(&rm->lock_object);
}
@@ -222,7 +231,10 @@ int
rm_wowned(struct rmlock *rm)
{
- return (mtx_owned(&rm->rm_lock));
+ if (rm->lock_object.lo_flags & RM_SLEEPABLE)
+ return (sx_xlocked(&rm->rm_lock_sx));
+ else
+ return (mtx_owned(&rm->rm_lock_mtx));
}
void
@@ -241,8 +253,8 @@ rm_sysinit_flags(void *arg)
rm_init_flags(args->ra_rm, args->ra_desc, args->ra_opts);
}
-static void
-_rm_rlock_hard(struct rmlock *rm, struct rm_priotracker *tracker)
+static int
+_rm_rlock_hard(struct rmlock *rm, struct rm_priotracker *tracker, int trylock)
{
struct pcpu *pc;
struct rm_queue *queue;
@@ -254,7 +266,7 @@ _rm_rlock_hard(struct rmlock *rm, struct rm_priotracker *tracker)
/* Check if we just need to do a proper critical_exit. */
if (0 == rm->rm_noreadtoken) {
critical_exit();
- return;
+ return (1);
}
/* Remove our tracker from the per-cpu list. */
@@ -265,7 +277,7 @@ _rm_rlock_hard(struct rmlock *rm, struct rm_priotracker *tracker)
/* Just add back tracker - we hold the lock. */
rm_tracker_add(pc, tracker);
critical_exit();
- return;
+ return (1);
}
/*
@@ -289,7 +301,7 @@ _rm_rlock_hard(struct rmlock *rm, struct rm_priotracker *tracker)
mtx_unlock_spin(&rm_spinlock);
rm_tracker_add(pc, tracker);
critical_exit();
- return;
+ return (1);
}
}
}
@@ -297,7 +309,13 @@ _rm_rlock_hard(struct rmlock *rm, struct rm_priotracker *tracker)
sched_unpin();
critical_exit();
- mtx_lock(&rm->rm_lock);
+ if (trylock)
+ return (0);
+
+ if (rm->lock_object.lo_flags & RM_SLEEPABLE)
+ sx_xlock(&rm->rm_lock_sx);
+ else
+ mtx_lock(&rm->rm_lock_mtx);
rm->rm_noreadtoken = 0;
critical_enter();
@@ -306,11 +324,16 @@ _rm_rlock_hard(struct rmlock *rm, struct rm_priotracker *tracker)
sched_pin();
critical_exit();
- mtx_unlock(&rm->rm_lock);
+ if (rm->lock_object.lo_flags & RM_SLEEPABLE)
+ sx_xunlock(&rm->rm_lock_sx);
+ else
+ mtx_unlock(&rm->rm_lock_mtx);
+
+ return (1);
}
-void
-_rm_rlock(struct rmlock *rm, struct rm_priotracker *tracker)
+int
+_rm_rlock(struct rmlock *rm, struct rm_priotracker *tracker, int trylock)
{
struct thread *td = curthread;
struct pcpu *pc;
@@ -338,10 +361,10 @@ _rm_rlock(struct rmlock *rm, struct rm_priotracker *tracker)
* conditional jump.
*/
if (0 == (td->td_owepreempt | rm->rm_noreadtoken))
- return;
+ return (1);
/* We do not have a read token and need to acquire one. */
- _rm_rlock_hard(rm, tracker);
+ return _rm_rlock_hard(rm, tracker, trylock);
}
static void
@@ -401,7 +424,10 @@ _rm_wlock(struct rmlock *rm)
struct rm_priotracker *prio;
struct turnstile *ts;
- mtx_lock(&rm->rm_lock);
+ if (rm->lock_object.lo_flags & RM_SLEEPABLE)
+ sx_xlock(&rm->rm_lock_sx);
+ else
+ mtx_lock(&rm->rm_lock_mtx);
if (rm->rm_noreadtoken == 0) {
/* Get all read tokens back */
@@ -439,7 +465,10 @@ void
_rm_wunlock(struct rmlock *rm)
{
- mtx_unlock(&rm->rm_lock);
+ if (rm->lock_object.lo_flags & RM_SLEEPABLE)
+ sx_xunlock(&rm->rm_lock_sx);
+ else
+ mtx_unlock(&rm->rm_lock_mtx);
}
#ifdef LOCK_DEBUG
@@ -454,7 +483,11 @@ void _rm_wlock_debug(struct rmlock *rm, const char *file, int line)
LOCK_LOG_LOCK("RMWLOCK", &rm->lock_object, 0, 0, file, line);
- WITNESS_LOCK(&rm->lock_object, LOP_EXCLUSIVE, file, line);
+ if (rm->lock_object.lo_flags & RM_SLEEPABLE)
+ WITNESS_LOCK(&rm->rm_lock_sx.lock_object, LOP_EXCLUSIVE,
+ file, line);
+ else
+ WITNESS_LOCK(&rm->lock_object, LOP_EXCLUSIVE, file, line);
curthread->td_locks++;
@@ -465,25 +498,35 @@ _rm_wunlock_debug(struct rmlock *rm, const char *file, int line)
{
curthread->td_locks--;
- WITNESS_UNLOCK(&rm->lock_object, LOP_EXCLUSIVE, file, line);
+ if (rm->lock_object.lo_flags & RM_SLEEPABLE)
+ WITNESS_UNLOCK(&rm->rm_lock_sx.lock_object, LOP_EXCLUSIVE,
+ file, line);
+ else
+ WITNESS_UNLOCK(&rm->lock_object, LOP_EXCLUSIVE, file, line);
LOCK_LOG_LOCK("RMWUNLOCK", &rm->lock_object, 0, 0, file, line);
_rm_wunlock(rm);
}
-void
+int
_rm_rlock_debug(struct rmlock *rm, struct rm_priotracker *tracker,
- const char *file, int line)
+ int trylock, const char *file, int line)
{
-
+ if (!trylock && (rm->lock_object.lo_flags & RM_SLEEPABLE))
+ WITNESS_CHECKORDER(&rm->rm_lock_sx.lock_object, LOP_NEWORDER,
+ file, line, NULL);
WITNESS_CHECKORDER(&rm->lock_object, LOP_NEWORDER, file, line, NULL);
- _rm_rlock(rm, tracker);
+ if (_rm_rlock(rm, tracker, trylock)) {
+ LOCK_LOG_LOCK("RMRLOCK", &rm->lock_object, 0, 0, file, line);
- LOCK_LOG_LOCK("RMRLOCK", &rm->lock_object, 0, 0, file, line);
+ WITNESS_LOCK(&rm->lock_object, 0, file, line);
- WITNESS_LOCK(&rm->lock_object, 0, file, line);
+ curthread->td_locks++;
- curthread->td_locks++;
+ return (1);
+ }
+
+ return (0);
}
void
@@ -517,12 +560,12 @@ _rm_wunlock_debug(struct rmlock *rm, const char *file, int line)
_rm_wunlock(rm);
}
-void
+int
_rm_rlock_debug(struct rmlock *rm, struct rm_priotracker *tracker,
- const char *file, int line)
+ int trylock, const char *file, int line)
{
- _rm_rlock(rm, tracker);
+ return _rm_rlock(rm, tracker, trylock);
}
void
diff --git a/sys/sys/_rmlock.h b/sys/sys/_rmlock.h
index e5c68d5..5fb4447 100644
--- a/sys/sys/_rmlock.h
+++ b/sys/sys/_rmlock.h
@@ -47,9 +47,13 @@ struct rmlock {
struct lock_object lock_object;
volatile int rm_noreadtoken;
LIST_HEAD(,rm_priotracker) rm_activeReaders;
- struct mtx rm_lock;
-
+ union {
+ struct mtx _rm_lock_mtx;
+ struct sx _rm_lock_sx;
+ } _rm_lock;
};
+#define rm_lock_mtx _rm_lock._rm_lock_mtx
+#define rm_lock_sx _rm_lock._rm_lock_sx
struct rm_priotracker {
struct rm_queue rmp_cpuQueue; /* Must be first */
diff --git a/sys/sys/rmlock.h b/sys/sys/rmlock.h
index 9766f67..ef5776b 100644
--- a/sys/sys/rmlock.h
+++ b/sys/sys/rmlock.h
@@ -33,6 +33,7 @@
#define _SYS_RMLOCK_H_
#include <sys/mutex.h>
+#include <sys/sx.h>
#include <sys/_lock.h>
#include <sys/_rmlock.h>
@@ -43,6 +44,7 @@
*/
#define RM_NOWITNESS 0x00000001
#define RM_RECURSE 0x00000002
+#define RM_SLEEPABLE 0x00000004
void rm_init(struct rmlock *rm, const char *name);
void rm_init_flags(struct rmlock *rm, const char *name, int opts);
@@ -53,14 +55,15 @@ void rm_sysinit_flags(void *arg);
void _rm_wlock_debug(struct rmlock *rm, const char *file, int line);
void _rm_wunlock_debug(struct rmlock *rm, const char *file, int line);
-void _rm_rlock_debug(struct rmlock *rm, struct rm_priotracker *tracker,
- const char *file, int line);
+int _rm_rlock_debug(struct rmlock *rm, struct rm_priotracker *tracker,
+ int trylock, const char *file, int line);
void _rm_runlock_debug(struct rmlock *rm, struct rm_priotracker *tracker,
const char *file, int line);
void _rm_wlock(struct rmlock *rm);
void _rm_wunlock(struct rmlock *rm);
-void _rm_rlock(struct rmlock *rm, struct rm_priotracker *tracker);
+int _rm_rlock(struct rmlock *rm, struct rm_priotracker *tracker,
+ int trylock);
void _rm_runlock(struct rmlock *rm, struct rm_priotracker *tracker);
/*
@@ -74,14 +77,17 @@ void _rm_runlock(struct rmlock *rm, struct rm_priotracker *tracker);
#define rm_wlock(rm) _rm_wlock_debug((rm), LOCK_FILE, LOCK_LINE)
#define rm_wunlock(rm) _rm_wunlock_debug((rm), LOCK_FILE, LOCK_LINE)
#define rm_rlock(rm,tracker) \
- _rm_rlock_debug((rm),(tracker), LOCK_FILE, LOCK_LINE )
+ ((void)_rm_rlock_debug((rm),(tracker), 0, LOCK_FILE, LOCK_LINE ))
+#define rm_try_rlock(rm,tracker) \
+ _rm_rlock_debug((rm),(tracker), 1, LOCK_FILE, LOCK_LINE )
#define rm_runlock(rm,tracker) \
_rm_runlock_debug((rm), (tracker), LOCK_FILE, LOCK_LINE )
#else
-#define rm_wlock(rm) _rm_wlock((rm))
-#define rm_wunlock(rm) _rm_wunlock((rm))
-#define rm_rlock(rm,tracker) _rm_rlock((rm),(tracker))
-#define rm_runlock(rm,tracker) _rm_runlock((rm), (tracker))
+#define rm_wlock(rm) _rm_wlock((rm))
+#define rm_wunlock(rm) _rm_wunlock((rm))
+#define rm_rlock(rm,tracker) ((void)_rm_rlock((rm),(tracker), 0))
+#define rm_try_rlock(rm,tracker) _rm_rlock((rm),(tracker), 1)
+#define rm_runlock(rm,tracker) _rm_runlock((rm), (tracker))
#endif
struct rm_args {
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