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From: Joel Fernandes <firstname.lastname@example.org> To: "Paul E. McKenney" <email@example.com> Cc: Steven Rostedt <firstname.lastname@example.org>, email@example.com, Benjamin Herrenschmidt <firstname.lastname@example.org>, Ingo Molnar <email@example.com>, Jonathan Corbet <firstname.lastname@example.org>, Josh Triplett <email@example.com>, firstname.lastname@example.org, Lai Jiangshan <email@example.com>, firstname.lastname@example.org, email@example.com, Mathieu Desnoyers <firstname.lastname@example.org>, Michael Ellerman <email@example.com>, Miguel Ojeda <firstname.lastname@example.org>, Paul Mackerras <email@example.com>, firstname.lastname@example.org Subject: Re: [PATCH RFC 0/5] Remove some notrace RCU APIs Date: Sat, 25 May 2019 14:14:07 -0400 [thread overview] Message-ID: <20190525181407.GA220326@google.com> (raw) In-Reply-To: <20190525155035.GE28207@linux.ibm.com> On Sat, May 25, 2019 at 08:50:35AM -0700, Paul E. McKenney wrote: > On Sat, May 25, 2019 at 10:19:54AM -0400, Joel Fernandes wrote: > > On Sat, May 25, 2019 at 07:08:26AM -0400, Steven Rostedt wrote: > > > On Sat, 25 May 2019 04:14:44 -0400 > > > Joel Fernandes <email@example.com> wrote: > > > > > > > > I guess the difference between the _raw_notrace and just _raw variants > > > > > is that _notrace ones do a rcu_check_sparse(). Don't we want to keep > > > > > that check? > > > > > > > > This is true. > > > > > > > > Since the users of _raw_notrace are very few, is it worth keeping this API > > > > just for sparse checking? The API naming is also confusing. I was expecting > > > > _raw_notrace to do fewer checks than _raw, instead of more. Honestly, I just > > > > want to nuke _raw_notrace as done in this series and later we can introduce a > > > > sparse checking version of _raw if need-be. The other option could be to > > > > always do sparse checking for _raw however that used to be the case and got > > > > changed in http://lists.infradead.org/pipermail/linux-afs/2016-July/001016.html > > > > > > What if we just rename _raw to _raw_nocheck, and _raw_notrace to _raw ? > > > > That would also mean changing 160 usages of _raw to _raw_nocheck in the > > kernel :-/. > > > > The tracing usage of _raw_notrace is only like 2 or 3 users. Can we just call > > rcu_check_sparse directly in the calling code for those and eliminate the APIs? > > > > I wonder what Paul thinks about the matter as well. > > My thought is that it is likely that a goodly number of the current uses > of _raw should really be some form of _check, with lockdep expressions > spelled out. Not that working out what exactly those lockdep expressions > should be is necessarily a trivial undertaking. ;-) Yes, currently where I am a bit stuck is the rcu_dereference_raw() cannot possibly know what SRCU domain it is under, so lockdep cannot check if an SRCU lock is held without the user also passing along the SRCU domain. I am trying to change lockdep to see if it can check if *any* srcu domain lock is held (regardless of which one) and complain if none are. This is at least better than no check at all. However, I think it gets tricky for mutexes. If you have something like: mutex_lock(some_mutex); p = rcu_dereference_raw(gp); mutex_unlock(some_mutex); This might be a perfectly valid invocation of _raw, however my checks (patch is still cooking) trigger a lockdep warning becase _raw cannot know that this is Ok. lockdep thinks it is not in a reader section. This then gets into the territory of a new rcu_derference_raw_protected(gp, assert_held(some_mutex)) which sucks because its yet another API. To circumvent this issue, can we just have callers of rcu_dereference_raw ensure that they call rcu_read_lock() if they are protecting dereferences by a mutex? That would make things a lot easier and also may be Ok since rcu_read_lock is quite cheap. > That aside, if we are going to change the name of an API that is > used 160 places throughout the tree, we would need to have a pretty > good justification. Without such a justification, it will just look > like pointless churn to the various developers and maintainers on the > receiving end of the patches. Actually, the API name change is not something I want to do, it is Steven suggestion. My suggestion is let us just delete _raw_notrace and just use the _raw API for tracing, since _raw doesn't do any tracing anyway. Steve pointed that _raw_notrace does sparse checking unlike _raw, but I think that isn't an issue since _raw doesn't do such checking at the moment anyway.. (if possible check my cover letter again for details/motivation of this series). thanks! - Joel > Thanx, Paul > > > thanks, Steven! > > >
next prev parent reply other threads:[~2019-05-25 18:14 UTC|newest] Thread overview: 17+ messages / expand[flat|nested] mbox.gz Atom feed top 2019-05-24 23:49 [PATCH RFC 0/5] Remove some notrace RCU APIs Joel Fernandes (Google) 2019-05-24 23:49 ` [PATCH RFC 1/5] powerpc: Use regular rcu_dereference_raw API Joel Fernandes (Google) 2019-05-24 23:49 ` [PATCH RFC 2/5] trace: " Joel Fernandes (Google) 2019-05-24 23:49 ` [PATCH RFC 3/5] hashtable: Use the regular hlist_for_each_entry_rcu API Joel Fernandes (Google) 2019-05-24 23:49 ` [PATCH RFC 4/5] rculist: Remove hlist_for_each_entry_rcu_notrace since no users Joel Fernandes (Google) 2019-05-26 16:20 ` Miguel Ojeda 2019-05-24 23:49 ` [PATCH RFC 5/5] rcu: Remove rcu_dereference_raw_notrace " Joel Fernandes (Google) 2019-05-25 3:24 ` [PATCH RFC 0/5] Remove some notrace RCU APIs Steven Rostedt 2019-05-25 8:14 ` Joel Fernandes 2019-05-25 11:08 ` Steven Rostedt 2019-05-25 14:19 ` Joel Fernandes 2019-05-25 15:50 ` Paul E. McKenney 2019-05-25 18:14 ` Joel Fernandes [this message] 2019-05-25 18:18 ` Joel Fernandes 2019-05-28 12:24 ` Paul E. McKenney 2019-05-28 19:00 ` Joel Fernandes 2019-05-28 20:00 ` Paul E. McKenney
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