Re: BufferAlloc: don't take two simultaneous locks

Hi,

On 2022-02-25 12:51:22 +0300, Yura Sokolov wrote:
> > > + * The usage_count starts out at 1 so that the buffer can survive one
> > > + * clock-sweep pass.
> > > + *
> > > + * We use direct atomic OR instead of Lock+Unlock since no other backend
> > > + * could be interested in the buffer. But StrategyGetBuffer,
> > > + * Flush*Buffers, Drop*Buffers are scanning all buffers and locks them to
> > > + * compare tag, and UnlockBufHdr does raw write to state. So we have to
> > > + * spin if we found buffer locked.
> >
> > So basically the first half of of the paragraph is wrong, because no, we
> > can't?
>
> Logically, there are no backends that could be interesting in the buffer.
> Physically they do LockBufHdr/UnlockBufHdr just to check they are not interesting.

Yea, but that's still being interested in the buffer...

> > > + * Note that we write tag unlocked. It is also safe since there is always
> > > + * check for BM_VALID when tag is compared.
> >
> >
> > > */
> > > buf->tag = newTag;
> > > - buf_state &= ~(BM_VALID | BM_DIRTY | BM_JUST_DIRTIED |
> > > - BM_CHECKPOINT_NEEDED | BM_IO_ERROR | BM_PERMANENT |
> > > - BUF_USAGECOUNT_MASK);
> > > if (relpersistence == RELPERSISTENCE_PERMANENT || forkNum == INIT_FORKNUM)
> > > - buf_state |= BM_TAG_VALID | BM_PERMANENT | BUF_USAGECOUNT_ONE;
> > > + new_bits = BM_TAG_VALID | BM_PERMANENT | BUF_USAGECOUNT_ONE;
> > > else
> > > - buf_state |= BM_TAG_VALID | BUF_USAGECOUNT_ONE;
> > > -
> > > - UnlockBufHdr(buf, buf_state);
> > > + new_bits = BM_TAG_VALID | BUF_USAGECOUNT_ONE;
> > >
> > > - if (oldPartitionLock != NULL)
> > > + buf_state = pg_atomic_fetch_or_u32(&buf->state, new_bits);
> > > + while (unlikely(buf_state & BM_LOCKED))
> >
> > I don't think it's safe to atomic in arbitrary bits. If somebody else has
> > locked the buffer header in this moment, it'll lead to completely bogus
> > results, because unlocking overwrites concurrently written contents (which
> > there shouldn't be any, but here there are)...
>
> That is why there is safety loop in the case buf->state were locked just
> after first optimistic atomic_fetch_or. 99.999% times this loop will not
> have a job. But in case other backend did lock buf->state, loop waits
> until it releases lock and retry atomic_fetch_or.

> > And or'ing contents in also doesn't make sense because we it doesn't work to
> > actually unset any contents?
>
> Sorry, I didn't understand sentence :((

You're OR'ing multiple bits into buf->state. LockBufHdr() only ORs in
BM_LOCKED. ORing BM_LOCKED is fine:
Either the buffer is not already locked, in which case it just sets the
BM_LOCKED bit, acquiring the lock. Or it doesn't change anything, because
BM_LOCKED already was set.

But OR'ing in multiple bits is *not* fine, because it'll actually change the
contents of ->state while the buffer header is locked.

> > Why don't you just use LockBufHdr/UnlockBufHdr?
>
> This pair makes two atomic writes to memory. Two writes are heavier than
> one write in this version (if optimistic case succeed).

UnlockBufHdr doesn't use a locked atomic op. It uses a write barrier and an
unlocked write.

Greetings,

Andres Freund