Messages in this thread | | | From | Lai Jiangshan <> | Date | Thu, 17 Nov 2016 23:07:02 +0800 | Subject | Re: [PATCH RFC tip/core/rcu] SRCU rewrite |
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On Thu, Nov 17, 2016 at 10:31 PM, Boqun Feng <boqun.feng@gmail.com> wrote: > On Thu, Nov 17, 2016 at 08:18:51PM +0800, Lai Jiangshan wrote: >> On Tue, Nov 15, 2016 at 10:37 PM, Paul E. McKenney >> <paulmck@linux.vnet.ibm.com> wrote: >> > On Tue, Nov 15, 2016 at 09:44:45AM +0800, Boqun Feng wrote: >> >> >> >> >> __srcu_read_lock() used to be called with preemption disabled. I guess >> >> the reason was because we have two percpu variables to increase. So with >> >> only one percpu right, could we remove the preempt_{dis,en}able() in >> >> srcu_read_lock() and use this_cpu_inc() here? >> > >> > Quite possibly... >> > >> > > Hello, Lai ;-) > >> it will be nicer if it is removed. >> >> The reason for the preemption-disabled was also because we >> have to disallow any preemption between the fetching of the idx >> and the increasement. so that we have at most NR_CPUS worth >> of readers using the old index that haven't incremented the counters. >> > > After reading the comment for a while, I actually got a question, maybe > I miss something ;-) > > Why "at most NR_CPUS worth of readers using the old index haven't > incremented the counters" could save us from overflow the counter? > > Please consider the following case in current implementation: > > > {sp->completed = 0} so idx = 1 in srcu_advance_batches(...) > > one thread A is currently in __srcu_read_lock() and using idx = 1 and > about to increase the percpu c[idx], and ULONG_MAX __srcu_read_lock()s > have been called and returned with idx = 1, please note I think this is > possible because I assume we may have some code like this: > > unsigned long i = 0; > for (; i < ULONG_MAX; i++) > srcu_read_lock(); // return the same idx 1;
this is the wrong usage of the api.
you might rewrite it as:
unsigned long index[2] = {0, 0}; unsigned long i = 0; for (; index[1] < ULONG_MAX; i++) index[srcu_read_lock()]++;
I think we should add document to disallow this kind of usage. a reader should eat 4bytes on the memory at least.
> > And none of the corresponding srcu_read_unlock() has been called; > > In this case, at the time thread A increases the percpu c[idx], that > will result in an overflow, right? So even one reader using old idx will > result in overflow. > > > I think we won't be hit by overflow is not because we have few readers > using old idx, it's because there are unlikely ULONG_MAX + 1 > __srcu_read_lock() called for the same idx, right? And the reason of > this is much complex: because we won't have a fair mount of threads in > the system, because no thread will nest srcu many levels, because there > won't be a lot readers using old idx. > > And this will still be true if we use new mechanism and shrink the > preemption disabled section, right? > > Regards, > Boqun > >> if we remove the preempt_{dis,en}able(). we must change the >> "NR_CPUS" in the comment into ULONG_MAX/4. (I assume >> one on-going reader needs at least need 4bytes at the stack). it is still safe. >> >> but we still need to think more if we want to remove the preempt_{dis,en}able(). >> >> Thanks >> Lai
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