Re: sinval synchronization considered harmful

On Thu, Jul 21, 2011 at 6:44 PM, Dan Ports <drkp@csail.mit.edu> wrote:
> If you're suggesting that hardware memory barriers aren't going to be
> needed to implement lock-free code on x86, that isn't true. Because a
> read can be reordered with respect to a write to a different memory
> location, you can still have problems. So you do still need memory
> barriers, just fewer of them.
>
> Dekker's algorithm is the classic example: two threads each set a flag
> and then check whether the other thread's flag is set. In any
> sequential execution, at least one should see the other's flag set, but
> on the x86 that doesn't always happen. One thread's read might be
> reordered before its write.

In the case of sinval, what we need to do for SIGetDataEntries() is,
approximately, a bunch of loads, followed by a store to one of the
locations we loaded (which no one else can have written meanwhile).
So I think that's OK.

In SIInsertDataEntries(), what we need to do is, approximately, take a
lwlock, load from a location which can only be written while holding
the lwlock, do a bunch of stores, ending with a store to that first
location, and release the lwlock.  I think that's OK, too.

>> 2. Machines with weak memory ordering.  On this category of machines
>> (which includes PowerPC, Dec Alpha, and maybe some others), the CPU
>> reorders memory accesses arbitrarily unless you explicitly issue
>> instructions that enforce synchronization.  You still need to keep the
>> compiler from moving things around, too.  Alpha is particularly
>> pernicious, because something like a->b can fetch the pointed-to value
>> before loading the pointer itself.  This is otherwise known as "we
>> have basically no cache coherency circuits on this chip at all".  On
>> these machines, you need to issue an explicit memory barrier
>> instruction at each sequence point, or just acquire and release a
>> spinlock.
>
> The Alpha is pretty much unique (thankfully!) in allowing dependent
> reads to be reordered. That makes it even weaker than the typical
> weak-ordering machine. Since reading a pointer and then dereferencing
> it is a pretty reasonable thing to do regularly in RCU code, you
> probably don't want to emit barriers in between on architectures where
> it's not actually necessary. That argues for another operation that's
> defined to be a barrier (mb) on the Alpha but a no-op elsewhere.
> Certainly the Linux kernel found it useful to do so
> (read_barrier_depends)
>
> Alternatively, one might question how important it is to support the
> Alpha these days...

Well, currently, we do, so we probably don't want to drop support for
that without some careful thought.  I searched the archive and found
someone trying to compile 8.3.something on Alpha just a few years ago,
so it's apparently not totally dead yet.

--
Robert Haas
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