Messages in this thread | | | Date | Tue, 20 Dec 2022 22:34:19 -0500 | Subject | Re: [RFC 0/2] srcu: Remove pre-flip memory barrier | From | Mathieu Desnoyers <> |
| |
On 2022-12-20 17:57, Frederic Weisbecker wrote: > On Tue, Dec 20, 2022 at 02:01:30PM -0500, Mathieu Desnoyers wrote: >> On 2022-12-20 13:29, Joel Fernandes wrote: >>> >> >>> I do want to finish my memory barrier studies of SRCU over the holidays since I have been deep in the hole with that already. Back to the post flip memory barrier here since I think now even that might not be needed… >> >> I strongly suspect the memory barrier after flip is useless for the same >> reasons I mentioned explaining why the barrier before the flip is useless. >> >> However, we need to double-check that we have memory barriers at the >> beginning and end of synchronize_srcu, and between load of "unlock" counters >> and load of "lock" counters. >> >> Where is the barrier at the beginning of synchronize_srcu ? > > rcu_seq_snap() ?
The memory barrier in rcu_seq_snap is not at the very beginning of synchronize_srcu.
For example we have:
unsigned long get_state_synchronize_srcu(struct srcu_struct *ssp) { // Any prior manipulation of SRCU-protected data must happen // before the load from ->srcu_gp_seq. smp_mb(); return rcu_seq_snap(&ssp->srcu_gp_seq);
which happens to have an explicit barrier before issuing rcu_seq_snap().
So my question still stands: where is the memory barrier at the beginning of synchronize_srcu ?
The memory ordering constraint I am concerned about here is:
* [...] In addition, * each CPU having an SRCU read-side critical section that extends beyond * the return from synchronize_srcu() is guaranteed to have executed a * full memory barrier after the beginning of synchronize_srcu() and before * the beginning of that SRCU read-side critical section. [...]
So if we have a SRCU read-side critical section that begins after the beginning of synchronize_srcu, but before its first memory barrier, it would miss the guarantee that the full memory barrier is issued before the beginning of that SRCU read-side critical section. IOW, that memory barrier needs to be at the very beginning of the grace period.
I suspect that the memory barrier in srcu_read_lock() invoked by srcu_gp_start_if_needed may happen to be early enough, but I'm not sure, and it does not appear to be documented as such.
Thanks,
Mathieu
> >> >> Thanks, >> >> Mathieu >> >> -- >> Mathieu Desnoyers >> EfficiOS Inc. >> https://www.efficios.com >>
-- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com
| |