Re: [PATCH v2] The value may overflow

[Joel Fernandes]

On Tue, Sep 5, 2023 at 4:13 PM Paul E. McKenney <paulmck@xxxxxxxxxx> wrote:
>
> On Tue, Sep 05, 2023 at 03:36:40PM -0400, Mathieu Desnoyers wrote:
> > On 9/5/23 15:27, Paul E. McKenney wrote:
> > > On Tue, Sep 05, 2023 at 09:40:51AM -0700, Paul E. McKenney wrote:
> > > > On Tue, Sep 05, 2023 at 10:34:43AM -0400, Mathieu Desnoyers wrote:
> > > > > On 9/5/23 10:15, David Laight wrote:
> > > > > > ...
> > > > > > > That would instead be more than 512-16=496 CPUs, correct?  496 CPUs would
> > > > > > > only require a 31-bit shift, which should be OK, but 497 would require
> > > > > > > a 32-bit shift, which would result in sign extension.  If it turns out
> > > > > > > that sign extension is OK, then we should get in trouble at 513 CPUs,
> > > > > > > which would result in a 33-bit shift (and is that even defined in C?).
> > > > > >
> > > > > > Not quite right :-)
> > > > > >
> > > > > > (1 << 31) is int and negative, that gets sign extended before
> > > > > > being converted to 'unsigned long' - so has the top 33 bits set.
> > > >
> > > > Good point, thank you for the correction.
> > > >
> > > > > > (1 << 32) is undefined, the current x86 cpu ignore the high
> > > > > > shift bits so it is (1 << 0).
> > > >
> > > > Which would cause it to attempt to invoke SRCU callbacks on the
> > > > lowest-numbered CPU associated with that srcu_node structure.
> > > >
> > > > > Yes, I was about to reply the same thing. A shift of 31 is buggy,
> > > > > because shifting 1 << 31 raises the sign bit, which sets the top 33
> > > > > bits when cast to unsigned long. A shift of 1 << 32 is undefined,
> > > > > with for instance x86 choosing to ignore the top bit.
> > > > >
> > > > > But in order to have a 1 << 31 shift from this expression:
> > > > >
> > > > >                  sdp->grpmask = 1 << (cpu - sdp->mynode->grplo);
> > > > >
> > > > > I *think* we need the group to have 32 cpus or more (indexed within
> > > > > the group from grplo to grplo + 31 (both inclusive)).
> > > > >
> > > > > So as soon as we have one group with 32 cpus, the problem should show
> > > > > up. With FANOUT_LEAF=16, we can have 15 groups of 31 cpus and 1
> > > > > group of 32 cpus, for:
> > > > >
> > > > >    15*31 + 32 = 497 cpus.
> > > > >
> > > > > AFAIU, this would be the minimum value for smp_processor_id()+1 which
> > > > > triggers this issue.
> > > >
> > > > By default, there are 16 CPUs per leaf srcu_node structure.  Each leaf
> > > > srcu_node structure takes up one bit in the parent srcu_node structures'
> > > > srcu_data_have_cbs[] array.  Up to 30 bits is OK, 31 bits is questionable,
> > > > and 32 bits and larger erroneous.
> > > >
> > > > This is the situation as I see it (assuming dense CPU numbering):
> > > >
> > > >   # Leaf srcu_node                Largest
> > > >   structures      #CPUs           CPU #           Status
> > > >
> > > >   0-30            0-480           479             Good
> > > >   31              481-496         495             Questionable
> > > >   32-             497-            496+            Bad.
> > > >
> > > > Tree SRCU differs from Tree RCU in its operation, so this bug should
> > > > not hold up SRCU grace periods.  It might instead cause SRCU callbacks
> > > > to be ignored (which would admittedly look quite similar to the user).
> > > >
> > > > My attempts to cheat the numbering system ran up against the limited
> > > > height of the srcu_node tree.
> > > >
> > > > But there is still the question of why this has not been seen in the
> > > > wild, given that there really are systems with more than 479 CPUs
> > > > out there.  One possibility is the call to srcu_schedule_cbs_sdp()
> > > > from srcu_funnel_gp_start().  But it does not seem likely that this
> > > > would happen all that often, as it requires back-to-back grace periods
> > > > and then some.
> > > >
> > > > Maybe people with large systems boot with srcutree.convert_to_big=0?
> > > >
> > > > Adding Laurent for his thoughts.
> > > >
> > > > Aha!
> > > >
> > > > Here is what makes it work, given David's description of 1<<32:
> > > >
> > > > static void srcu_schedule_cbs_snp(struct srcu_struct *ssp, struct srcu_node *snp,
> > > >                             unsigned long mask, unsigned long delay)
> > > > {
> > > >   int cpu;
> > > >
> > > >   for (cpu = snp->grplo; cpu <= snp->grphi; cpu++) {
> > > >           if (!(mask & (1 << (cpu - snp->grplo))))
> > > >                   continue;
> > > >           srcu_schedule_cbs_sdp(per_cpu_ptr(ssp->sda, cpu), delay);
> > > >   }
> > > > }
> > > >
> > > > As long as at least one bit is set in the result of 1<<N, and as long
> > > > as the compiler always does the same thing for a given N, then this loop
> > > > will make the right thing happen.
> > > >
> > > > But even that is not relied upon, because the calling code looks like
> > > > this:
> > > >
> > > >                   spin_lock_irq_rcu_node(snp);
> > > >                   cbs = false;
> > > >                   last_lvl = snp >= sup->level[rcu_num_lvls - 1];
> > > >                   if (last_lvl)
> > > >                           cbs = ss_state < SRCU_SIZE_BIG || snp->srcu_have_cbs[idx] == gpseq;
> > > >                   snp->srcu_have_cbs[idx] = gpseq;
> > > >                   rcu_seq_set_state(&snp->srcu_have_cbs[idx], 1);
> > > >                   sgsne = snp->srcu_gp_seq_needed_exp;
> > > >                   if (srcu_invl_snp_seq(sgsne) || ULONG_CMP_LT(sgsne, gpseq))
> > > >                           WRITE_ONCE(snp->srcu_gp_seq_needed_exp, gpseq);
> > > >                   if (ss_state < SRCU_SIZE_BIG)
> > > >                           mask = ~0;
> > > >                   else
> > > >                           mask = snp->srcu_data_have_cbs[idx];
> > > >                   snp->srcu_data_have_cbs[idx] = 0;
> > > >                   spin_unlock_irq_rcu_node(snp);
> > > >                   if (cbs)
> > > >                           srcu_schedule_cbs_snp(ssp, snp, mask, cbdelay);
> > > >
> > > > So that last_lvl check means that the srcu_schedule_cbs_snp() function
> > > > is invoked only for leaf srcu_node structures.  Which by default limit
> > > > the shift to 16.
> > > >
> > > > So this bug appears to have no effect for default RCU setups, even on very
> > > > large 64-bit systems, which is consistent with field experience.  Even if
> > > > this is the case, it still should be fixed, to avoid confusion if nothing
> > > > else.  Or just in case someone decides to set CONFIG_RCU_FANOUT_LEAF=32.
> > > > Which actually happened the other day due to someone trusting ChatGPT's
> > > > opinion about RCU Kconfig options...
> > >
> > > And I therefore queued Denis's v3 patch with an edited commit log.
> > > Of course, if anyone sees some other way that the bug could manifest
> > > other than in a 64-bit kernel built with CONFIG_RCU_FANOUT_LEAF greater
> > > than 30 on a system with at least 31 CPUs, please let me know so that
> > > I can adjust.
> > >
> > >                                                     Thanx, Paul
> > >
> > > ------------------------------------------------------------------------
> > >
> > > commit ed083b0e22f1396dee3599896249a3f218845298
> > > Author: Denis Arefev <arefev@xxxxxxxxx>
> > > Date:   Mon Sep 4 15:21:14 2023 +0300
> > >
> > >      Fix srcu_struct node grpmask overflow on 64-bit systems
> > >      The value of an arithmetic expression 1 << (cpu - sdp->mynode->grplo)
> >
> > AFAIU, the overflow resides in the "bitwise expression" and not
> > the arithmetic expression.
>
> Rather than quibble about exactly what constitutes arithmetic, I
> updated the first paragraph and added your Reviewed-by as shown
> below.  ;-)
>
> > Other than this, please add my
> >
> > Reviewed-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
>
> Thank you all!!!
>
> ------------------------------------------------------------------------
>
> commit 50477ff756ab99402b1523b7c6be8b5d790d05e7
> Author: Denis Arefev <arefev@xxxxxxxxx>
> Date:   Mon Sep 4 15:21:14 2023 +0300
>
>     Fix srcu_struct node grpmask overflow on 64-bit systems
>
>     The value of a bitwise expression 1 << (cpu - sdp->mynode->grplo)
>     is subject to overflow due to a failure to cast operands to a larger
>     data type before performing the bitwise operation.
>
>     The maximum result of this subtraction is defined by the RCU_FANOUT_LEAF
>     Kconfig option, which on 64-bit systems defaults to 16 (resulting in a
>     maximum shift of 15), but which can be set up as high as 64 (resulting
>     in a maximum shift of 63).  A value of 31 can result in sign extension,
>     resulting in 0xffffffff80000000 instead of the desired 0x80000000.
>     A value of 31 or greater triggers undefined behavior per the C standard.

Do you mean here "A value of 32 or greater"?

Only N >= 32 throws warning for:
unsigned long foo = (1 << N);

N=31 does undesirable sign extension but no warning.

thanks,

 - Joel