Re: [PATCH v2] srcu: Fix WARN_ON() for timer_delete_sync() in cleanup_srcu_struct()

["Zqiang" <qiang.zhang@linux.dev>]

> 
> On Mon, Aug 10, 2026 at 06:55:22PM +0800, Zqiang wrote:
> 
> > 
> > The WARN_ON() for timer_delete_sync() means that the caller forgot to
> >  call srcu_barrier() before cleanup. however, it still can trigger even
> >  when srcu_barrier() was properly called.
> >  
> >  When a SRCU grace period ends and ss_state < SRCU_SIZE_BIG, the mask=~0
> >  causes delay_work timer be queued on every online CPU regardless of
> >  whether it's sdp->srcu_cblist has callbacks. srcu_barrier() only wait
> >  srcu_barrier_head's callbacks to complete, however, the srcu_barrier_head
> >  will not be inserted into an empty sdp->srcu_cblist. after that, when
> >  cleanup_srcu_struct() finds a pending delay_work timer on a CPU with no
> >  callbacks, triggering a false positive.
> >  
> >  This commit therefore add rcu_segcblist_n_cbs() to WARN_ON(), make the
> >  warning triggers only when the timer is still pending and there are actual
> >  outstanding callbacks.
> >  
> >  Fixes: 05c3e88488ed ("srcu: Queue sdp->work when the delay timer is successfully deleted")
> >  Reported-by: Breno Leitao <leitao@debian.org>
> >  Reported-by: kernel test robot <oliver.sang@intel.com> 
> >  Closes: https://lore.kernel.org/oe-lkp/202608060833.bce92909-lkp@intel.com/ 
> >  Tested-by: kernel test robot <oliver.sang@intel.com>
> >  Signed-off-by: Zqiang <qiang.zhang@linux.dev>
> >  ---
> >  kernel/rcu/srcutree.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> >  
> >  diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> >  index 35fface51d50..090dddfc7463 100644
> >  --- a/kernel/rcu/srcutree.c
> >  +++ b/kernel/rcu/srcutree.c
> >  @@ -753,8 +753,9 @@ void cleanup_srcu_struct(struct srcu_struct *ssp)
> >  
> >  // Call srcu_barrier() before this cleanup_srcu_struct()
> >  // to avoid triggering this WARN_ON().
> >  - if (WARN_ON(timer_delete_sync(&sdp->delay_work)) &&
> >  - rcu_cpu_beenfullyonline(sdp->cpu))
> >  + if (WARN_ON(timer_delete_sync(&sdp->delay_work) &&
> >  + rcu_segcblist_n_cbs(&sdp->srcu_cblist)) &&
> >  + rcu_cpu_beenfullyonline(sdp->cpu))
> > 
> This is quite hard to read, mainly with this indentation.
> 
> You probably want to rewrite the checks , otherwise the indentation
> should be something like, which is horrible (although correct?):
> 
>  if (WARN_ON(timer_delete_sync(&sdp->delay_work) &&
>  rcu_segcblist_n_cbs(&sdp->srcu_cblist)) &&
>  rcu_cpu_beenfullyonline(sdp->cpu))

Will update and resend (I still mark v2 because v1 has not been sent before).

Thanks
Zqiang

> 
> Would something like this work?
> 
>  if (rcu_cpu_beenfullyonline(sdp->cpu) &&
>  WARN_ON(timer_delete_sync(&sdp->delay_work) &&
>  rcu_segcblist_n_cbs(&sdp->srcu_cblist)))
>