Messages in this thread | | | Date | Wed, 17 Feb 2021 09:54:14 -0500 (EST) | From | Mathieu Desnoyers <> | Subject | Re: Local execution of ipi_sync_rq_state() on sync_runqueues_membarrier_state() |
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----- On Feb 16, 2021, at 4:35 PM, Nadav Amit nadav.amit@gmail.com wrote:
> Hello Mathieu, > > While trying to find some unrelated by, something in > sync_runqueues_membarrier_state() caught my eye: > > > static int sync_runqueues_membarrier_state(struct mm_struct *mm) > { > if (atomic_read(&mm->mm_users) == 1 || num_online_cpus() == 1) { > this_cpu_write(runqueues.membarrier_state, membarrier_state); > > /* > * For single mm user, we can simply issue a memory barrier > * after setting MEMBARRIER_STATE_GLOBAL_EXPEDITED in the > * mm and in the current runqueue to guarantee that no memory > * access following registration is reordered before > * registration. > */ > smp_mb(); > return 0; > } > > [ snip ] > > smp_call_function_many(tmpmask, ipi_sync_rq_state, mm, 1); > > > And ipi_sync_rq_state() does: > > this_cpu_write(runqueues.membarrier_state, > atomic_read(&mm->membarrier_state)); > > > So my question: are you aware smp_call_function_many() would not run > ipi_sync_rq_state() on the local CPU?
Generally, yes, I am aware of it, but it appears that when I wrote that code, I missed that important fact. See
commit 227a4aadc75b ("sched/membarrier: Fix p->mm->membarrier_state racy load")
> Is that the intention of the code?
Clearly not! If we look at sync_runqueues_membarrier_state(), there is even a special-case for mm_users==1 || num online cpus == 1 where it writes the membarrier state into the current cpu runqueue. I'll prepare a fix, thanks a bunch for spotting this.
Mathieu
> > Thanks, > Nadav
-- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com
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