On Sat, Aug 01, 2026 at 05:19:25AM +0000, Zqiang wrote:
> >
> > Hello Zqiang,
> >
> > On Thu, Jul 09, 2026 at 06:06:02PM +0800, Zqiang wrote:
> >
> > >
> > > In the cleanup_srcu_struct(), when iterating over per-cpu's srcu_data,
> > > the timer_delete_sync(&sdp->delay_work) is called to cancel the delay
> > > timer before flush_work(&sdp->work).
> > >
> > > However, if the timer_delete_sync() returns 1 means that it successfully
> > > deleted an pending timer before it had a chance to fire, also means that
> > > the sdp->work cannot be queued, the subsequent flush_work(&sdp->work)
> > > will returns immediately without waiting for anything, this causes SRCU
> > > callbacks to not be processed.
> > >
> > > Fix this by checking the return value of timer_delete_sync(), if it
> > > returns 1, explicitly queue sdp->work so that the following flush_work()
> > > can correctly wait for the work to complete.
> > >
> > > Signed-off-by: Zqiang <[email protected]>
> > > ---
> > > kernel/rcu/srcutree.c | 6 +++++-
> > > 1 file changed, 5 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c
> > > index 7c2f7cc131f7..02c322b7c6f1 100644
> > > --- a/kernel/rcu/srcutree.c
> > > +++ b/kernel/rcu/srcutree.c
> > > @@ -725,7 +725,11 @@ void cleanup_srcu_struct(struct srcu_struct *ssp)
> > > for_each_possible_cpu(cpu) {
> > > struct srcu_data *sdp = per_cpu_ptr(ssp->sda, cpu);
> > >
> > > - timer_delete_sync(&sdp->delay_work);
> > > + //In most scenarios, calling srcu_barrier before cleanup
> > > + //will not trigger WARN_ON().
> > > + if (WARN_ON(timer_delete_sync(&sdp->delay_work)) &&
> > > + rcu_cpu_beenfullyonline(sdp->cpu))
> > > + queue_work_on(sdp->cpu, rcu_gp_wq, &sdp->work);
> > >
> > I started seeing this on my tests, it is not trivial to decode this one,
> > but, I can try harder if _really_ needed.
> >
>
> The scenario I can think of is that we missed the call to srcu_barrier()
> before cleanup_srcu_struct():
>
> loop_add()
> ->blk_mq_alloc_tag_set
> init_srcu_struct(&set->tags_srcu)
>
> blk_mq_alloc_set_map_and_rqs() {
> ->__blk_mq_alloc_rq_maps()
> ->__blk_mq_alloc_map_and_rqs() return error
> goto out_unwind: __blk_mq_free_map_and_rqs()
> ->blk_mq_free_rq_map()
> ->blk_mq_free_tags()
> ->call_srcu(&set->tags_srcu, &tags->rcu_head,
> blk_mq_free_tags_callback);
> } return error
>
> goto out_free_mq_map:
> ....
> cleanup_srcu_struct(&set->tags_srcu)
> -> trigger WARN_ON(timer_delete_sync(&sdp->delay_work)
>
> Can you try the following patch?
I tried it, and it does not help -- the WARN still fires at the same rate. I
think the analysis points at the wrong call site.
Setup: linux-next-20260731 (arm64), 32 vCPU VM, HZ=1000, PROVE_LOCKING
and DEBUG_OBJECTS_TIMERS enabled, reproducer stress-ng --loop 32
--timeout 60s.
baseline 3 x WARN srcutree.c:706 in 60s
+ your blk-mq patch 4 x WARN srcutree.c:706 in 60s
(3 vs 4 is just jitter on a one-jiffy race, not a regression.)
The reason it cannot help is that the splat comes from
blk_mq_free_tag_set(), not from the blk_mq_alloc_tag_set() error path
your patch touches. All four splats in the patched run have the same
trace:
cleanup_srcu_struct+0x274/0x450 (P)
blk_mq_free_tag_set+0x1a4/0x1e0
loop_remove+0x2c/0x78
loop_control_ioctl+0x248/0x2a0
__arm64_sys_ioctl+0x9c0/0xb00
I also put a pr_warn() at out_cleanup_tags_srcu: to be sure -- it fired
zero times over the whole run, so loop_add() never takes that error path
in this workload.
Why do you wangt to have this
WARN_ON(timer_delete_sync(&sdp->delay_work)) ?
timer_delete_sync() != 0 means "a timer was armed", not "callbacks are
pending".
