[lttng-dev] [rp] [PATCH] Ensure that read-side functions meet 10-line LGPL criterion
Paul E. McKenney
paulmck at linux.vnet.ibm.com
Sun Sep 2 12:07:33 EDT 2012
On Sat, Sep 01, 2012 at 10:13:55PM -0700, Josh Triplett wrote:
> On Sat, Sep 01, 2012 at 05:59:11PM -0700, Paul E. McKenney 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>
>
> s/directloy/directly/g in the comments.
Good catch, fixed.
> Also, this seems inordinately silly. :)
>
> Assuming you don't plan to copy other LGPLed code into this library (or
> more specifically the header file), you might consider just adding an
> explicit exception at the top, saying that the inline functions in this
> file may be assumed to qualify for the relevant clause of the LGPL,
> regardless of their length. (You'd probably want to limit that
> exception to only the code in the header, not any other code in the
> library, so someone couldn't just copy the whole library into the
> headers.)
I believe that it is important to allow LGPL code to flow easily between
these headers and other LGPL projects. This commit represents a trivial
change, admittedly, but one that could save a large amount of bookkeeping
and license-compatibility hassle down the road.
Thanx, Paul
> > 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)
> > +{
> > + 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_help(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..0ddf6a1 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 directloy 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 directloy 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 directloy 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 directloy 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..fe3194f 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_help(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_help(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 */
> > }
> >
> >
> >
> > _______________________________________________
> > rp mailing list
> > rp at svcs.cs.pdx.edu
> > http://svcs.cs.pdx.edu/mailman/listinfo/rp
>
> _______________________________________________
> rp mailing list
> rp at svcs.cs.pdx.edu
> http://svcs.cs.pdx.edu/mailman/listinfo/rp
>
More information about the lttng-dev
mailing list