[lttng-dev] [PATCH] Ensure that read-side functions meet 10-line LGPL criterion
Paul E. McKenney
paulmck at linux.vnet.ibm.com
Tue Sep 4 18:13:23 EDT 2012
On Mon, Sep 03, 2012 at 02:03:00PM -0400, Mathieu Desnoyers wrote:
> * Paul E. McKenney (paulmck at linux.vnet.ibm.com) wrote:
> > This commit ensures that all read-side functions meet the 10-line LGPL
> > criterion that permits them to be expanded directly into non-LGPL code,
> > without function-call instructions. It also documents this as the intent.
> >
> > Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
> >
> > diff --git a/urcu/static/urcu-bp.h b/urcu/static/urcu-bp.h
> > index e7b2eda..881b4a4 100644
> > --- a/urcu/static/urcu-bp.h
> > +++ b/urcu/static/urcu-bp.h
> > @@ -6,8 +6,8 @@
> > *
> > * Userspace RCU header.
> > *
> > - * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu.h for linking
> > - * dynamically with the userspace rcu library.
> > + * TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
> > + * RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
> > *
> > * Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
> > * Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
> > @@ -162,32 +162,48 @@ static inline int rcu_old_gp_ongoing(long *value)
> > ((v ^ rcu_gp_ctr) & RCU_GP_CTR_PHASE);
> > }
> >
> > +/*
> > + * Helper for _rcu_read_lock(). The format of rcu_gp_ctr (as well as
> > + * the per-thread rcu_reader.ctr) has the upper bits containing a count of
> > + * _rcu_read_lock() nesting, and a lower-order bit that contains either zero
> > + * or RCU_GP_CTR_PHASE. The smp_mb_slave() ensures that the accesses in
> > + * _rcu_read_lock() happen before the subsequent read-side critical section.
> > + */
> > +static inline void _rcu_read_lock_help(unsigned long tmp)
>
> could we rename the "_rcu_read_lock_help" to "_rcu_read_lock_update" ?
>
> I think it would fit better the role of this function in the algorithm.
>
> As Josh pointed out, "directloy" -> "directly" below,
>
> The rest looks good. I'll wait for an updated version.
Here you go!
Thanx, Paul
------------------------------------------------------------------------
Ensure that read-side functions meet 10-line LGPL criterion
This commit ensures that all read-side functions meet the 10-line LGPL
criterion that permits them to be expanded directly into non-LGPL code,
without function-call instructions. It also documents this as the intent.
[ paulmck: Spelling fixes called out by Josh Triplett and name
change called out by Mathieu Desnoyers (_rcu_read_lock_help() ->
_rcu_read_lock_update(). ]
Signed-off-by: Paul E. McKenney <paulmck at linux.vnet.ibm.com>
diff --git a/urcu/static/urcu-bp.h b/urcu/static/urcu-bp.h
index e7b2eda..a2f7368 100644
--- a/urcu/static/urcu-bp.h
+++ b/urcu/static/urcu-bp.h
@@ -6,8 +6,8 @@
*
* Userspace RCU header.
*
- * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu.h for linking
- * dynamically with the userspace rcu library.
+ * TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
+ * RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
*
* Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
* Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
@@ -162,32 +162,48 @@ static inline int rcu_old_gp_ongoing(long *value)
((v ^ rcu_gp_ctr) & RCU_GP_CTR_PHASE);
}
+/*
+ * Helper for _rcu_read_lock(). The format of rcu_gp_ctr (as well as
+ * the per-thread rcu_reader.ctr) has the upper bits containing a count of
+ * _rcu_read_lock() nesting, and a lower-order bit that contains either zero
+ * or RCU_GP_CTR_PHASE. The smp_mb_slave() ensures that the accesses in
+ * _rcu_read_lock() happen before the subsequent read-side critical section.
+ */
+static inline void _rcu_read_lock_update(unsigned long tmp)
+{
+ if (caa_likely(!(tmp & RCU_GP_CTR_NEST_MASK))) {
+ _CMM_STORE_SHARED(URCU_TLS(rcu_reader)->ctr, _CMM_LOAD_SHARED(rcu_gp_ctr));
+ cmm_smp_mb();
+ } else
+ _CMM_STORE_SHARED(URCU_TLS(rcu_reader)->ctr, tmp + RCU_GP_COUNT);
+}
+
+/*
+ * Enter an RCU read-side critical section.
+ *
+ * The first cmm_barrier() call ensures that the compiler does not reorder
+ * the body of _rcu_read_lock() with a mutex.
+ *
+ * This function and its helper are both less than 10 lines long. The
+ * intent is that this function meets the 10-line criterion in LGPL,
+ * allowing this function to be invoked directly from non-LGPL code.
+ */
static inline void _rcu_read_lock(void)
{
long tmp;
- /* Check if registered */
if (caa_unlikely(!URCU_TLS(rcu_reader)))
- rcu_bp_register();
-
+ rcu_bp_register(); /* If not yet registered. */
cmm_barrier(); /* Ensure the compiler does not reorder us with mutex */
tmp = URCU_TLS(rcu_reader)->ctr;
- /*
- * rcu_gp_ctr is
- * RCU_GP_COUNT | (~RCU_GP_CTR_PHASE or RCU_GP_CTR_PHASE)
- */
- if (caa_likely(!(tmp & RCU_GP_CTR_NEST_MASK))) {
- _CMM_STORE_SHARED(URCU_TLS(rcu_reader)->ctr, _CMM_LOAD_SHARED(rcu_gp_ctr));
- /*
- * Set active readers count for outermost nesting level before
- * accessing the pointer.
- */
- cmm_smp_mb();
- } else {
- _CMM_STORE_SHARED(URCU_TLS(rcu_reader)->ctr, tmp + RCU_GP_COUNT);
- }
+ _rcu_read_lock_update(tmp);
}
+/*
+ * Exit an RCU read-side critical section. This function is less than
+ * 10 lines of code, and is intended to be usable by non-LGPL code, as
+ * called out in LGPL.
+ */
static inline void _rcu_read_unlock(void)
{
/*
diff --git a/urcu/static/urcu-pointer.h b/urcu/static/urcu-pointer.h
index 48dc5bf..4361156 100644
--- a/urcu/static/urcu-pointer.h
+++ b/urcu/static/urcu-pointer.h
@@ -6,8 +6,8 @@
*
* Userspace RCU header. Operations on pointers.
*
- * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu-pointer.h for
- * linking dynamically with the userspace rcu library.
+ * TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
+ * RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
*
* Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
* Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
@@ -59,8 +59,11 @@ extern "C" {
* addition to forthcoming C++ standard.
*
* Should match rcu_assign_pointer() or rcu_xchg_pointer().
+ *
+ * This macro is less than 10 lines long. The intent is that this macro
+ * meets the 10-line criterion in LGPL, allowing this function to be
+ * expanded directly in non-LGPL code.
*/
-
#define _rcu_dereference(p) ({ \
__typeof__(p) _________p1 = CMM_LOAD_SHARED(p); \
cmm_smp_read_barrier_depends(); \
@@ -73,8 +76,11 @@ extern "C" {
* data structure, which can be safely freed after waiting for a quiescent state
* using synchronize_rcu(). If fails (unexpected value), returns old (which
* should not be freed !).
+ *
+ * This macro is less than 10 lines long. The intent is that this macro
+ * meets the 10-line criterion in LGPL, allowing this function to be
+ * expanded directly in non-LGPL code.
*/
-
#define _rcu_cmpxchg_pointer(p, old, _new) \
({ \
__typeof__(*p) _________pold = (old); \
@@ -89,8 +95,11 @@ extern "C" {
* _rcu_xchg_pointer - same as rcu_assign_pointer, but returns the previous
* pointer to the data structure, which can be safely freed after waiting for a
* quiescent state using synchronize_rcu().
+ *
+ * This macro is less than 10 lines long. The intent is that this macro
+ * meets the 10-line criterion in LGPL, allowing this function to be
+ * expanded directly in non-LGPL code.
*/
-
#define _rcu_xchg_pointer(p, v) \
({ \
__typeof__(*p) _________pv = (v); \
@@ -121,8 +130,11 @@ extern "C" {
* data structure before its publication.
*
* Should match rcu_dereference_pointer().
+ *
+ * This macro is less than 10 lines long. The intent is that this macro
+ * meets the 10-line criterion in LGPL, allowing this function to be
+ * expanded directly in non-LGPL code.
*/
-
#define _rcu_assign_pointer(p, v) _rcu_set_pointer(&(p), v)
#ifdef __cplusplus
diff --git a/urcu/static/urcu-qsbr.h b/urcu/static/urcu-qsbr.h
index 22908a4..5580092 100644
--- a/urcu/static/urcu-qsbr.h
+++ b/urcu/static/urcu-qsbr.h
@@ -6,8 +6,8 @@
*
* Userspace RCU QSBR header.
*
- * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu-qsbr.h for linking
- * dynamically with the userspace rcu QSBR library.
+ * TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
+ * RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
*
* Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
* Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
@@ -157,15 +157,36 @@ static inline int rcu_gp_ongoing(unsigned long *ctr)
return v && (v != rcu_gp_ctr);
}
+/*
+ * Enter an RCU read-side critical section.
+ *
+ * This function is less than 10 lines long. The intent is that this
+ * function meets the 10-line criterion for LGPL, allowing this function
+ * to be invoked directly from non-LGPL code.
+ */
static inline void _rcu_read_lock(void)
{
rcu_assert(URCU_TLS(rcu_reader).ctr);
}
+/*
+ * Exit an RCU read-side critical section.
+ *
+ * This function is less than 10 lines long. The intent is that this
+ * function meets the 10-line criterion for LGPL, allowing this function
+ * to be invoked directly from non-LGPL code.
+ */
static inline void _rcu_read_unlock(void)
{
}
+/*
+ * Inform RCU of a quiescent state.
+ *
+ * This function is less than 10 lines long. The intent is that this
+ * function meets the 10-line criterion for LGPL, allowing this function
+ * to be invoked directly from non-LGPL code.
+ */
static inline void _rcu_quiescent_state(void)
{
cmm_smp_mb();
@@ -175,6 +196,14 @@ static inline void _rcu_quiescent_state(void)
cmm_smp_mb();
}
+/*
+ * Take a thread offline, prohibiting it from entering further RCU
+ * read-side critical sections.
+ *
+ * This function is less than 10 lines long. The intent is that this
+ * function meets the 10-line criterion for LGPL, allowing this function
+ * to be invoked directly from non-LGPL code.
+ */
static inline void _rcu_thread_offline(void)
{
cmm_smp_mb();
@@ -184,6 +213,14 @@ static inline void _rcu_thread_offline(void)
cmm_barrier(); /* Ensure the compiler does not reorder us with mutex */
}
+/*
+ * Bring a thread online, allowing it to once again enter RCU
+ * read-side critical sections.
+ *
+ * This function is less than 10 lines long. The intent is that this
+ * function meets the 10-line criterion for LGPL, allowing this function
+ * to be invoked directly from non-LGPL code.
+ */
static inline void _rcu_thread_online(void)
{
cmm_barrier(); /* Ensure the compiler does not reorder us with mutex */
diff --git a/urcu/static/urcu.h b/urcu/static/urcu.h
index f27f8b6..f211dab 100644
--- a/urcu/static/urcu.h
+++ b/urcu/static/urcu.h
@@ -6,8 +6,8 @@
*
* Userspace RCU header.
*
- * TO BE INCLUDED ONLY IN LGPL-COMPATIBLE CODE. See urcu.h for linking
- * dynamically with the userspace rcu library.
+ * TO BE INCLUDED ONLY IN CODE THAT IS TO BE RECOMPILED ON EACH LIBURCU
+ * RELEASE. See urcu.h for linking dynamically with the userspace rcu library.
*
* Copyright (c) 2009 Mathieu Desnoyers <mathieu.desnoyers at efficios.com>
* Copyright (c) 2009 Paul E. McKenney, IBM Corporation.
@@ -252,46 +252,71 @@ static inline int rcu_gp_ongoing(unsigned long *ctr)
((v ^ rcu_gp_ctr) & RCU_GP_CTR_PHASE);
}
-static inline void _rcu_read_lock(void)
+/*
+ * Helper for _rcu_read_lock(). The format of rcu_gp_ctr (as well as
+ * the per-thread rcu_reader.ctr) has the upper bits containing a count of
+ * _rcu_read_lock() nesting, and a lower-order bit that contains either zero
+ * or RCU_GP_CTR_PHASE. The smp_mb_slave() ensures that the accesses in
+ * _rcu_read_lock() happen before the subsequent read-side critical section.
+ */
+static inline void _rcu_read_lock_update(unsigned long tmp)
{
- unsigned long tmp;
-
- cmm_barrier(); /* Ensure the compiler does not reorder us with mutex */
- tmp = URCU_TLS(rcu_reader).ctr;
- /*
- * rcu_gp_ctr is
- * RCU_GP_COUNT | (~RCU_GP_CTR_PHASE or RCU_GP_CTR_PHASE)
- */
if (caa_likely(!(tmp & RCU_GP_CTR_NEST_MASK))) {
_CMM_STORE_SHARED(URCU_TLS(rcu_reader).ctr, _CMM_LOAD_SHARED(rcu_gp_ctr));
- /*
- * Set active readers count for outermost nesting level before
- * accessing the pointer. See smp_mb_master().
- */
smp_mb_slave(RCU_MB_GROUP);
- } else {
+ } else
_CMM_STORE_SHARED(URCU_TLS(rcu_reader).ctr, tmp + RCU_GP_COUNT);
- }
}
-static inline void _rcu_read_unlock(void)
+/*
+ * Enter an RCU read-side critical section.
+ *
+ * The first cmm_barrier() call ensures that the compiler does not reorder
+ * the body of _rcu_read_lock() with a mutex.
+ *
+ * This function and its helper are both less than 10 lines long. The
+ * intent is that this function meets the 10-line criterion in LGPL,
+ * allowing this function to be invoked directly from non-LGPL code.
+ */
+static inline void _rcu_read_lock(void)
{
unsigned long tmp;
+ cmm_barrier();
tmp = URCU_TLS(rcu_reader).ctr;
- /*
- * Finish using rcu before decrementing the pointer.
- * See smp_mb_master().
- */
+ _rcu_read_lock_update(tmp);
+}
+
+/*
+ * This is a helper function for _rcu_read_unlock().
+ *
+ * The first smp_mb_slave() call ensures that the critical section is
+ * seen to preced the store to rcu_reader.ctr.
+ * The second smp_mb_slave() call ensures that we write to rcu_reader.ctr
+ * before reading the update-side futex.
+ */
+static inline void _rcu_read_unlock_help(unsigned long tmp)
+{
if (caa_likely((tmp & RCU_GP_CTR_NEST_MASK) == RCU_GP_COUNT)) {
smp_mb_slave(RCU_MB_GROUP);
_CMM_STORE_SHARED(URCU_TLS(rcu_reader).ctr, URCU_TLS(rcu_reader).ctr - RCU_GP_COUNT);
- /* write URCU_TLS(rcu_reader).ctr before read futex */
smp_mb_slave(RCU_MB_GROUP);
wake_up_gp();
- } else {
+ } else
_CMM_STORE_SHARED(URCU_TLS(rcu_reader).ctr, URCU_TLS(rcu_reader).ctr - RCU_GP_COUNT);
- }
+}
+
+/*
+ * Exit an RCU read-side crtical section. Both this function and its
+ * helper are smaller than 10 lines of code, and are intended to be
+ * usable by non-LGPL code, as called out in LGPL.
+ */
+static inline void _rcu_read_unlock(void)
+{
+ unsigned long tmp;
+
+ tmp = URCU_TLS(rcu_reader).ctr;
+ _rcu_read_unlock_help(tmp);
cmm_barrier(); /* Ensure the compiler does not reorder us with mutex */
}
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