On Tue, Jun 30, 2015 at 03:25:45PM -0700, Paul E. McKenney wrote: > From: "Paul E. McKenney" <[email protected]> > > This commit creates rcu_exp_gp_seq_start() and rcu_exp_gp_seq_end() to > bracket an expedited grace period, rcu_exp_gp_seq_snap() to snapshot the > sequence counter, and rcu_exp_gp_seq_done() to check to see if a full > expedited grace period has elapsed since the snapshot. These will be > applied to synchronize_rcu_expedited(). These are defined in terms of > underlying rcu_seq_start(), rcu_seq_end(), rcu_seq_snap(), rcu_seq_done(), > which will be applied to _rcu_barrier().
It would be good to explain why you cannot use seqcount primitives. They're >.< close. > Signed-off-by: Paul E. McKenney <[email protected]> > --- > kernel/rcu/tree.c | 68 > +++++++++++++++++++++++++++++++++++++++++++++++-------- > 1 file changed, 58 insertions(+), 10 deletions(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index c58fd27b4a22..f96500e462fd 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -3307,6 +3307,60 @@ void cond_synchronize_sched(unsigned long oldstate) > } > EXPORT_SYMBOL_GPL(cond_synchronize_sched); > > +/* Adjust sequence number for start of update-side operation. */ > +static void rcu_seq_start(unsigned long *sp) > +{ > + WRITE_ONCE(*sp, *sp + 1); > + smp_mb(); /* Ensure update-side operation after counter increment. */ > + WARN_ON_ONCE(!(*sp & 0x1)); > +} That wants to be an ACQUIRE, right? > + > +/* Adjust sequence number for end of update-side operation. */ > +static void rcu_seq_end(unsigned long *sp) > +{ > + smp_mb(); /* Ensure update-side operation before counter increment. */ And that wants to be a RELEASE, right? > + WRITE_ONCE(*sp, *sp + 1); smp_store_release(); even if balanced against a full barrier, might be better here? > + WARN_ON_ONCE(*sp & 0x1); > +} And the only difference between these and raw_write_seqcount_{begin,end}() is the smp_wmb() vs your smp_mb(). Since seqcounts have a distinct read vs writer side, we really only care about limiting the stores. I suspect you really do care about reads between these 'sequence points'. A few words to that effect could explain the existence of these primitives. > +/* Take a snapshot of the update side's sequence number. */ > +static unsigned long rcu_seq_snap(unsigned long *sp) > +{ > + unsigned long s; > + > + smp_mb(); /* Caller's modifications seen first by other CPUs. */ > + s = (READ_ONCE(*sp) + 3) & ~0x1; > + smp_mb(); /* Above access must not bleed into critical section. */ smp_load_acquire() then? > + return s; > +} > + > +/* > + * Given a snapshot from rcu_seq_snap(), determine whether or not a > + * full update-side operation has occurred. > + */ > +static bool rcu_seq_done(unsigned long *sp, unsigned long s) > +{ > + return ULONG_CMP_GE(READ_ONCE(*sp), s); I'm always amused you're not wanting to rely on 2s complement for integer overflow. I _know_ its undefined behaviour in the C rule book, but the entire rest of the kernel hard assumes it. > +} > + > +/* Wrapper functions for expedited grace periods. */ > +static void rcu_exp_gp_seq_start(struct rcu_state *rsp) > +{ > + rcu_seq_start(&rsp->expedited_sequence); > +} > +static void rcu_exp_gp_seq_end(struct rcu_state *rsp) > +{ > + rcu_seq_end(&rsp->expedited_sequence); > +} > +static unsigned long rcu_exp_gp_seq_snap(struct rcu_state *rsp) > +{ > + return rcu_seq_snap(&rsp->expedited_sequence); > +} > +static bool rcu_exp_gp_seq_done(struct rcu_state *rsp, unsigned long s) > +{ > + return rcu_seq_done(&rsp->expedited_sequence, s); > +} This is wrappers for wrappers sake? Why? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
