Date: Fri, 25 Feb 2022 09:01:27 -0800
From: Andres Freund <andres@anarazel.de>
To: Yura Sokolov <y.sokolov@postgrespro.ru>
Cc: Kyotaro Horiguchi <horikyota.ntt@gmail.com>, michail.nikolaev@gmail.com,
	x4mmm@yandex-team.ru, pgsql-hackers@lists.postgresql.org
Subject: Re: BufferAlloc: don't take two simultaneous locks
Message-ID: <20220225170127.lxe66l2swx3empgu@alap3.anarazel.de>
References: <CANtu0ohTqkVYCFyHRM6uG=jO=j_9MkQMrh04YPVt57rL5sTykA@mail.gmail.com>
 <CANtu0ogqNyceN0WCS186OnBLhw71xzwGDUw+Yk3Qt4J9TcM_mg@mail.gmail.com>
 <3b108afd19fa52ed20c464a69f64d545e4a14772.camel@postgrespro.ru>
 <20220217.141647.512059035403445205.horikyota.ntt@gmail.com>
 <1a97e80353d6855e9217cd6e2052257190a98f2d.camel@postgrespro.ru>
 <20220225080455.3zzxwfpvbizzcsfo@alap3.anarazel.de>
 <c1715ad78880399140f7a1ff9b11b80fd9b5856f.camel@postgrespro.ru>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <c1715ad78880399140f7a1ff9b11b80fd9b5856f.camel@postgrespro.ru>
Archived-At: <https://www.postgresql.org/message-id/20220225170127.lxe66l2swx3empgu%40alap3.anarazel.de>
Precedence: bulk

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