Messages in this thread | | | Date | Wed, 13 Jan 2010 14:36:03 -0500 | From | Mathieu Desnoyers <> | Subject | Re: [RFC PATCH] introduce sys_membarrier(): process-wide memory barrier (v5) |
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* Peter Zijlstra (peterz@infradead.org) wrote: > On Tue, 2010-01-12 at 20:37 -0500, Mathieu Desnoyers wrote: > > + for_each_cpu(cpu, tmpmask) { > > + spin_lock_irq(&cpu_rq(cpu)->lock); > > + mm = cpu_curr(cpu)->mm; > > + spin_unlock_irq(&cpu_rq(cpu)->lock); > > + if (current->mm != mm) > > + cpumask_clear_cpu(cpu, tmpmask); > > + } > > Why not: > > rcu_read_lock(); > if (current->mm != cpu_curr(cpu)->mm) > cpumask_clear_cpu(cpu, tmpmask); > rcu_read_unlock(); > > the RCU read lock ensures the task_struct obtained remains valid, and it > avoids taking the rq->lock. >
If we go for a simple rcu_read_lock, I think that we need a smp_mb() after switch_to() updates the current task on the remote CPU, before it returns to user-space. Do we have this guarantee for all architectures ?
So what I'm looking for, overall, is:
schedule() ... switch_mm() smp_mb() clear mm_cpumask set mm_cpumask switch_to() update current task smp_mb()
If we have that, then the rcu_read_lock should work.
What the rq lock currently gives us is the guarantee that if the current thread changes on a remote CPU while we are not holding this lock, then a full scheduler execution is performed, which implies a memory barrier if we change the current thread (it does, right ?).
Thanks,
Mathieu
-- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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