On 10/3/24 10:38, Jens Axboe wrote:
On 10/3/24 8:31 AM, Dan Carpenter wrote:
On Thu, Oct 03, 2024 at 07:21:25AM -0600, Jens Axboe wrote:
On 10/3/24 6:03 AM, Dan Carpenter wrote:
3117 ioc_now(iocg->ioc, &now);
3118 weight_updated(iocg, &now);
3119 spin_unlock(&iocg->ioc->lock);
3120 }
3121 }
3122 spin_unlock_irq(&blkcg->lock);
3123
3124 return nbytes;
3125 }
3126
3127 blkg_conf_init(&ctx, buf);
3128
3129 ret = blkg_conf_prep(blkcg, &blkcg_policy_iocost, &ctx);
3130 if (ret)
3131 goto err;
3132
3133 iocg = blkg_to_iocg(ctx.blkg);
3134
3135 if (!strncmp(ctx.body, "default", 7)) {
3136 v = 0;
3137 } else {
3138 if (!sscanf(ctx.body, "%u", &v))
3139 goto einval;
3140 if (v < CGROUP_WEIGHT_MIN || v > CGROUP_WEIGHT_MAX)
3141 goto einval;
3142 }
3143
3144 spin_lock(&iocg->ioc->lock);
But why is this not spin_lock_irq()? I haven't analyzed this so maybe it's
fine.
That's a bug.
I could obviously write this patch but I feel stupid writing the
commit message. My level of understanding is Monkey See Monkey do.
Could you take care of this?
Sure - or let's add Tejun who knows this code better. Ah he's already
added. Tejun?
So somewhere we're taking a lock in the IRQ handler and this can lead
to a deadlock? I thought this would have been caught by lockdep?
It's nested inside blkcg->lock which is IRQ safe, that is enough. But
doing a quick scan of the file, the usage is definitely (widly)
inconsistent. Most times ioc->lock is grabbed disabling interrupts, but
there are also uses that doesn't disable interrupts, coming from things
like seq_file show paths which certainly look like they need it. lockdep
should certainly warn about this, only explanation I have is that nobody
bothered to do that :-)
The lockdep validator will only warn about this if a debug kernel with
lockdep enabled has run a workload that exercises all the relevant
locking sequences that can implicate a potential for deadlock.
Cheers,
Longman
