Обсуждение: [HACKERS] A misconception about the meaning of 'volatile' in GetNewTransactionId?

Hi hackers,

I was reading xact.c and noticed this block:
               /*                * Use volatile pointer to prevent code rearrangement;
other backends                * could be examining my subxids info concurrently,
and we don't want                * them to see an invalid intermediate state, such as
incrementing                * nxids before filling the array entry.  Note we are
assuming that                * TransactionId and int fetch/store are atomic.                */               volatile
PGPROC*myproc = MyProc;               volatile PGXACT *mypgxact = MyPgXact;
 
               if (!isSubXact)                       mypgxact->xid = xid;               else               {
          int                     nxids = mypgxact->nxids;
 
                       if (nxids < PGPROC_MAX_CACHED_SUBXIDS)                       {
myproc->subxids.xids[nxids]= xid;                               mypgxact->nxids = nxids + 1;
 

Isn't this insufficient on non-TSO systems like POWER and Arm?  It
uses volatile qualifiers as a compiler barrier, which is probably
enough for x86 and Sparc in TSO mode, but doesn't include a memory
barrier to prevent hardware reordering.

I think the thing to do here would be to forget about volatile, stick
pg_write_barrier() between those two writes, and stick
pg_read_barrier() between the reads in any code that might read nxids
and then scan xids concurrently, such as TransactionIdIsInProgress().

This is almost exactly the example from the section "Avoiding Memory
Order Bugs" in src/backend/storage/lmgr/README.barrier.

Thoughts?

-- 
Thomas Munro
http://www.enterprisedb.com

Thomas Munro <thomas.munro@enterprisedb.com> writes:
> I was reading xact.c and noticed this block:
> ...
> Isn't this insufficient on non-TSO systems like POWER and Arm?

Yeah, I think you're right.  That code probably predates our support
for memory barriers, so "volatile" was the best we could do at the
time --- but as you say, it doesn't fix hardware-level rearrangements.
        regards, tom lane

On Sun, Apr 30, 2017 at 1:19 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Thomas Munro <thomas.munro@enterprisedb.com> writes:
>> I was reading xact.c and noticed this block:
>> ...
>> Isn't this insufficient on non-TSO systems like POWER and Arm?
>
> Yeah, I think you're right.  That code probably predates our support
> for memory barriers, so "volatile" was the best we could do at the
> time --- but as you say, it doesn't fix hardware-level rearrangements.

Here is an experimental patch, for discussion only, to drop some
apparently useless volatile qualifiers and introduce a write barrier
when extending the array and a corresponding read barrier when
scanning or copying the array from other processes.

I wonder about this code that shrinks the array:

#define XidCacheRemove(i) \
        do { \
                MyProc->subxids.xids[i] =
MyProc->subxids.xids[MyPgXact->nxids - 1]; \
                MyPgXact->nxids--; \
        } while (0)

If a concurrent process saw the decremented nxids value before seeing
the effect of xids[i] = xids[final], then it would miss an arbitrary
running subtransaction (not the aborting subtransaction being removed
from the array, but whichever xid had the bad luck to be in final
position).  In the patch I added pg_write_barrier(), but I suspect
that that might be not really a problem because of higher level
interlocking that I'm missing, because this code makes no mention of
the problem and doesn't (ab)use volatile qualifiers like the code that
extends the array (so it has neither compiler barrier/volatile nor
memory barrier so could be broken even on TSO assumptions at the whim
of the compiler if my guess were right about that).

-- 
Thomas Munro
http://www.enterprisedb.com