Messages in this thread | | | Date | Wed, 21 Nov 2018 09:18:19 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH 20/25] sched/kcpustat: Introduce vtime-aware kcpustat accessor |
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On Tue, Nov 20, 2018 at 11:40:22PM +0100, Frederic Weisbecker wrote: > On Tue, Nov 20, 2018 at 03:23:06PM +0100, Peter Zijlstra wrote: > > On Wed, Nov 14, 2018 at 03:46:04AM +0100, Frederic Weisbecker wrote: > > > > > +void kcpustat_cputime(struct kernel_cpustat *kcpustat, int cpu, > > > + u64 *user, u64 *nice, u64 *system, > > > + u64 *guest, u64 *guest_nice) > > > +{ > > > + struct task_struct *curr; > > > + struct vtime *vtime; > > > + int err; > > > + > > > + if (!vtime_accounting_enabled()) { > > > + kcpustat_cputime_raw(kcpustat, user, nice, > > > + system, guest, guest_nice); > > > + return; > > > + } > > > + > > > + rcu_read_lock(); > > > + > > > + do { > > > + curr = rcu_dereference(kcpustat->curr); > > > > Like I explained earlier; I don't think the above is correct. > > task_struct is itself not RCU protected. > > But there is at least one put_task_struct() that is enqueued as an RCU callback > on release_task(). That patchset (try to) make sure that kcpustat->curr can't > be assigned beyond that point. > > Or did I misunderstand something?
Yeah; release_task() is not the normal exit path. Oleg can probably remember how all that works, because I always get lost there :-/
In any case, have a look at task_rcu_dereference(), but that still does not explain the rcu_assign_pointer() stuff you use to set kcpustat->curr.
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