Re: BUG #13487: GetBufferFromRing code bug

On Sun, Jul 5, 2015 at 3:12 AM, <jingwei_5107(at)qq(dot)com> wrote:
> The following bug has been logged on the website:
>
> Bug reference: 13487
> Logged by: will
> Email address: jingwei_5107(at)qq(dot)com
> PostgreSQL version: 9.4.4
> Operating system: It does not matter
> Description:
>
> In GetBufferFromRing method(/src/backend/storage/buffer/freelist.c),I think
> the following code:
> 506 buf = &BufferDescriptors[bufnum - 1];
> 507 LockBufHdr(buf);
> 508 if (buf->refcount == 0 && buf->usage_count <= 1)
> 509 {
> 510 strategy->current_was_in_ring = true;
> 511 return buf;
> 512 }
> 513 UnlockBufHdr(buf);
>
> should be changed to(Adding line 510):
>
> 506 buf = &BufferDescriptors[bufnum - 1];
> 507 LockBufHdr(buf);
> 508 if (buf->refcount == 0 && buf->usage_count <= 1)
> 509 {
> 510 UnlockBufHdr(buf);// add this line
> 511 strategy->current_was_in_ring = true;
> 512 return buf;
> 513 }
> 514 UnlockBufHdr(buf);
>
> It should Unlock the buf before returning buf, after exclusively accessing
> the buf's refcount&usage_count member.
>
> Am I right? Any reply is appreciated if I'm wrong.

From the function's documentation:

* The bufhdr spin lock is held on the returned buffer.

From the documentation of StrategyGetBuffer (which calls it and
returns the buffer if not NULL):

* To ensure that no one else can pin the buffer before we do, we must
* return the buffer with the buffer header spinlock still held.

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