From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Alexander Gordeev <agordeev@redhat.com>
Cc: linux-kernel@vger.kernel.org, Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v2 1/9] rcu: Panic if RCU tree can not accommodate all CPUs
Date: Tue, 2 Jun 2015 05:37:40 -0700 [thread overview]
Message-ID: <20150602123740.GF5989@linux.vnet.ibm.com> (raw)
In-Reply-To: <20150602064527.GA3679@agordeev.usersys.redhat.com>
On Tue, Jun 02, 2015 at 07:45:27AM +0100, Alexander Gordeev wrote:
> On Mon, Jun 01, 2015 at 11:37:21AM -0700, Paul E. McKenney wrote:
> > On Fri, May 29, 2015 at 11:53:37AM +0200, Alexander Gordeev wrote:
> > > Currently a condition when RCU tree is unable to accommodate
> > > the configured number of CPUs is not permitted and causes
> > > a fall back to compile-time values. However, the code has no
> > > means to exceed the RCU tree capacity neither at compile-time
> > > nor in run-time. Therefore, if the condition is met in run-
> > > time then it indicates a serios problem elsewhere and should
> > > be handled with a panic.
> > >
> > > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > > Cc: Steven Rostedt <rostedt@goodmis.org>
> > > Signed-off-by: Alexander Gordeev <agordeev@redhat.com>
> > > ---
> > > kernel/rcu/tree.c | 15 +++++++++------
> > > 1 file changed, 9 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 2fce662..66a4230 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -4117,16 +4117,19 @@ static void __init rcu_init_geometry(void)
> > > rcu_capacity[i] = rcu_capacity[i - 1] * RCU_FANOUT;
> > >
> > > /*
> > > + * The tree must be able to accommodate the configured number of CPUs.
> > > + * If this limit is exceeded than we have a serious problem elsewhere.
> > > + *
> > > * The boot-time rcu_fanout_leaf parameter is only permitted
> > > * to increase the leaf-level fanout, not decrease it. Of course,
> > > * the leaf-level fanout cannot exceed the number of bits in
> > > - * the rcu_node masks. Finally, the tree must be able to accommodate
> > > - * the configured number of CPUs. Complain and fall back to the
> > > - * compile-time values if these limits are exceeded.
> > > + * the rcu_node masks. Complain and fall back to the compile-
> > > + * time values if these limits are exceeded.
> > > */
> > > - if (rcu_fanout_leaf < RCU_FANOUT_LEAF ||
> > > - rcu_fanout_leaf > sizeof(unsigned long) * 8 ||
> > > - n > rcu_capacity[MAX_RCU_LVLS]) {
> > > + if (n > rcu_capacity[MAX_RCU_LVLS])
> > > + panic("rcu_init_geometry: rcu_capacity[] is too small");
> >
> > The way this is set up, if the boot parameter (illegally) sets
> > rcu_fanout_lead smaller than RCU_FANOUT_LEAF, we might panic. It would
> > be far better to first check for rcu_fanout_leaf being out of bounds,
> > and only then have the possibility of panic(). That way, a typo in
> > the rcu_fanout_leaf boot paremeter is ignored, but with a splat.
> >
> > Or am I missing something here?
>
> I think you are quite right. But the bounds check is misplaced then.
> I would say, it should be placed before rcu_capacity[] seed, as it
> only deals with constants and has nothing with rcu_capacity[].
That makes sense as well.
> I will send the updated version.
Very good, looking forward to it!
By the way, on the specific configurations that I test, the patch
generates the same topology as previously, which is reassuring.
An exhaustive test is needed, of course.
Thanx, Paul
> > > + else if (rcu_fanout_leaf < RCU_FANOUT_LEAF ||
> > > + rcu_fanout_leaf > sizeof(unsigned long) * 8) {
> > > WARN_ON(1);
> > > return;
> > > }
> > > --
> > > 1.8.3.1
> > >
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at http://vger.kernel.org/majordomo-info.html
> > Please read the FAQ at http://www.tux.org/lkml/
>
> --
> Regards,
> Alexander Gordeev
> agordeev@redhat.com
>
next prev parent reply other threads:[~2015-06-02 12:37 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-05-29 9:53 [PATCH v2 0/9] rcu: Cleanup RCU tree initialization Alexander Gordeev
2015-05-29 9:53 ` [PATCH v2 1/9] rcu: Panic if RCU tree can not accommodate all CPUs Alexander Gordeev
2015-06-01 18:37 ` Paul E. McKenney
2015-06-02 6:45 ` Alexander Gordeev
2015-06-02 12:37 ` Paul E. McKenney [this message]
2015-05-29 9:53 ` [PATCH v2 2/9] rcu: Remove superfluous local variable in rcu_init_geometry() Alexander Gordeev
2015-05-29 9:53 ` [PATCH v2 3/9] rcu: Cleanup rcu_init_geometry() code and arithmetics Alexander Gordeev
2015-05-29 9:53 ` [PATCH v2 4/9] rcu: Simplify rcu_init_geometry() capacity arithmetics Alexander Gordeev
2015-05-29 9:53 ` [PATCH v2 5/9] rcu: Limit rcu_state::levelcnt[] to RCU_NUM_LVLS items Alexander Gordeev
2015-05-29 9:53 ` [PATCH v2 6/9] rcu: Limit rcu_capacity[] size " Alexander Gordeev
2015-05-29 9:53 ` [PATCH v2 7/9] rcu: Remove unnecessary fields from rcu_state structure Alexander Gordeev
2015-05-29 9:53 ` [PATCH v2 8/9] rcu: Limit count of static data to the number of RCU levels Alexander Gordeev
2015-05-29 9:53 ` [PATCH v2 9/9] rcu: Simplify arithmetic to calculate number of RCU nodes Alexander Gordeev
2015-06-01 18:58 ` [PATCH v2 0/9] rcu: Cleanup RCU tree initialization Paul E. McKenney
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20150602123740.GF5989@linux.vnet.ibm.com \
--to=paulmck@linux.vnet.ibm.com \
--cc=agordeev@redhat.com \
--cc=linux-kernel@vger.kernel.org \
--cc=rostedt@goodmis.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).