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    SubjectRe: RCU callbacks and TREE_PREEMPT_RCU
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    hi,

    On Thu, 2009-09-17 at 15:21 -0700, Paul E. McKenney wrote:
    > On Thu, Sep 17, 2009 at 10:29:02AM +0200, Eric Sesterhenn wrote:
    > > On Wed, 2009-09-16 at 16:26 -0700, Paul E. McKenney wrote:
    > > > On Thu, Sep 17, 2009 at 01:19:46AM +0200, Eric Sesterhenn wrote:
    > > > > On Wed, 2009-09-16 at 08:57 -0700, Paul E. McKenney wrote:
    > > > > > On Wed, Sep 16, 2009 at 08:47:16AM -0700, Paul E. McKenney wrote:
    > > > > > > On Wed, Sep 16, 2009 at 04:34:15PM +0100, Catalin Marinas wrote:
    > > > > > > > On Wed, 2009-09-16 at 08:29 -0700, Paul E. McKenney wrote:
    > > > > > > > > On Wed, Sep 16, 2009 at 03:17:21PM +0100, Catalin Marinas wrote:
    > > > > > > > > > When TREE_PREEMPT_RCU is enabled, the rcu list traversing above fails
    > > > > > > > > > with access to 0x6b6b6b6b but it is fine with TREE_PREEMPT_RCU=n and
    > > > > > > > > > TREE_RCU=y. During clean-up, kmemleak objects should no longer be freed
    > > > > > > > > > by other means since kmemleak was disabled and all callbacks are
    > > > > > > > > > ignored. The system is a 900Mhz P3, 256MB RAM, CONFIG_SMP=n.
    > > > > > > > > >
    > > > > > > > > > Is there something I'm doing wrong in kmemleak or a bug with RCU
    > > > > > > > > > preemption? The kernel oops looks like this:
    > > > > > > > >
    > > > > > > > > From your description and the code above, I must suspect a bug with
    > > > > > > > > RCU preemption. A new one, as the only bugs I am currently chasing
    > > > > > > > > involve NR_CPUS>32 (>64 on 64-bit systems).
    > > > > > > > >
    > > > > > > > > CONFIG_SMP=n implies NR_CPUS==1 in your build, correct?
    > > > > > > >
    > > > > > > > CONFIG_NR_CPUS=1.
    > > > > > >
    > > > > > > I was afraid of that. ;-)
    > > > > >
    > > > > > PS to previous -- there -is- a bug in mainline for TREE_PREEMPT_RCU for
    > > > > > single-CPU operation, but it is with synchronize_rcu() rather than
    > > > > > call_rcu(). The fix is in tip/core/urgent, commit #366b04ca. Or see
    > > > > > the following patch.
    > > > > >
    > > > > > So, could you please give the following patch a try?
    > > > >
    > > > > Sadly this does not fix the issue, is there any further information I
    > > > > can provide to you?
    > > >
    > > > :-(
    > > >
    > > > Would you be willing to give the attached diagnostic patch a go?
    > > >
    > > > Thanx, Paul
    > >
    > > It does not apply cleanly against current -git
    > > (rcu_preempt_check_blocked_tasks is missing in my rcutree_plugin.h for
    > > example) I tried to apply it by hand as good as possible, and will test
    > > it today.
    > >
    > > root@whiterabbit:/usr/src/linux# patch -p1 < ~/RCU_callbacks_and_TREE_PREEMPT_RCU-debug
    > > patching file kernel/rcutree.c
    > > Hunk #1 FAILED at 623.
    > > Hunk #2 FAILED at 657.
    > > Hunk #3 succeeded at 722 (offset 19 lines).
    > > Hunk #4 succeeded at 740 (offset 19 lines).
    > > Hunk #5 succeeded at 765 (offset 19 lines).
    > > Hunk #6 succeeded at 877 (offset 19 lines).
    > > Hunk #7 succeeded at 886 (offset 19 lines).
    > > 2 out of 7 hunks FAILED -- saving rejects to file kernel/rcutree.c.rej
    > > patching file kernel/rcutree_plugin.h
    > > Hunk #1 FAILED at 206.
    > > Hunk #2 succeeded at 206 (offset -10 lines).
    > > Hunk #3 FAILED at 270.
    > > Hunk #4 succeeded at 283 (offset -22 lines).
    > > Hunk #5 succeeded at 296 (offset -22 lines).
    > > Hunk #6 succeeded at 473 (offset -23 lines).
    > > 2 out of 6 hunks FAILED -- saving rejects to file
    > > kernel/rcutree_plugin.h.rej
    >
    > Sigh!!! I lost track of what was in mainline vs. -tip. You certainly
    > need the following patch from -tip as well.
    >
    > Please accept apologies for my confusion!!!

    no problem, it still did not apply cleanly, but i was able to get a
    working kernel and cant reproduce the issue with all 3 patches applied.

    Thanks, Eric

    > ------------------------------------------------------------------------
    >
    > Commit-ID: de078d875cc7fc709f7818f26d38389c04369826
    > Gitweb: http://git.kernel.org/tip/de078d875cc7fc709f7818f26d38389c04369826
    > Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > AuthorDate: Tue, 8 Sep 2009 15:54:36 -0700
    > Committer: Ingo Molnar <mingo@elte.hu>
    > CommitDate: Fri, 18 Sep 2009 00:04:54 +0200
    >
    > rcu: Need to update rnp->gpnum if preemptable RCU is to be reliable
    >
    > Without this patch, tasks preempted in RCU read-side critical
    > sections can fail to block the grace period, given that
    > rnp->gpnum is used to determine which rnp->blocked_tasks[]
    > element the preempted task is enqueued on.
    >
    > Before the patch, rnp->gpnum is always zero, so preempted tasks
    > are always enqueued on rnp->blocked_tasks[0], which is correct
    > only when the current CPU has not checked into the current
    > grace period and the grace-period number is even, or,
    > similarly, if the current CPU -has- checked into the current
    > grace period and the grace-period number is odd.
    >
    > Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    > Acked-by: Steven Rostedt <rostedt@goodmis.org>
    > Cc: laijs@cn.fujitsu.com
    > Cc: dipankar@in.ibm.com
    > Cc: akpm@linux-foundation.org
    > Cc: mathieu.desnoyers@polymtl.ca
    > Cc: josht@linux.vnet.ibm.com
    > Cc: dvhltc@us.ibm.com
    > Cc: niv@us.ibm.com
    > Cc: peterz@infradead.org
    > LKML-Reference: <12524504771622-git-send-email->
    > Signed-off-by: Ingo Molnar <mingo@elte.hu>
    >
    >
    > ---
    > kernel/rcutree.c | 6 +++++-
    > 1 files changed, 5 insertions(+), 1 deletions(-)
    >
    > diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    > index 6b11b07..c634a92 100644
    > --- a/kernel/rcutree.c
    > +++ b/kernel/rcutree.c
    > @@ -632,6 +632,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
    > /* Special-case the common single-level case. */
    > if (NUM_RCU_NODES == 1) {
    > rnp->qsmask = rnp->qsmaskinit;
    > + rnp->gpnum = rsp->gpnum;
    > rsp->signaled = RCU_SIGNAL_INIT; /* force_quiescent_state OK. */
    > spin_unlock_irqrestore(&rnp->lock, flags);
    > return;
    > @@ -657,8 +658,10 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
    > */
    >
    > rnp_end = rsp->level[NUM_RCU_LVLS - 1];
    > - for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++)
    > + for (rnp_cur = &rsp->node[0]; rnp_cur < rnp_end; rnp_cur++) {
    > rnp_cur->qsmask = rnp_cur->qsmaskinit;
    > + rnp->gpnum = rsp->gpnum;
    > + }
    >
    > /*
    > * Now set up the leaf nodes. Here we must be careful. First,
    > @@ -679,6 +682,7 @@ rcu_start_gp(struct rcu_state *rsp, unsigned long flags)
    > for (; rnp_cur < rnp_end; rnp_cur++) {
    > spin_lock(&rnp_cur->lock); /* irqs already disabled. */
    > rnp_cur->qsmask = rnp_cur->qsmaskinit;
    > + rnp->gpnum = rsp->gpnum;
    > spin_unlock(&rnp_cur->lock); /* irqs already disabled. */
    > }
    >
    > --
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