Re: Is Recovery actually paused? - Mailing list pgsql-hackers

On Thu, Feb 4, 2021 at 10:19 PM Robert Haas <robertmhaas@gmail.com> wrote:
>
> On Thu, Feb 4, 2021 at 7:46 AM Dilip Kumar <dilipbalaut@gmail.com> wrote:
> > How can we do that this is not a 1 byte flag this is enum so I don't
> > think we can read any atomic state without a spin lock here.
>
> I think this discussion of atomics is confused. Let's talk about what
> atomic reads and writes mean. Imagine that you have a 64-bit value
> 0x0101010101010101. Somebody sets it to 0x0202020202020202. Imagine
> that just as they are doing that, someone else reads the value and
> gets 0x0202020201010101, because half of the value has been updated
> and the other half has not yet been updated yet. This kind of thing
> can actually happen on some platforms and what it means is that on
> those platforms 8-byte reads and writes are not atomic. The idea of an
> "atom" is that it can't be split into pieces but these reads and
> writes on some platforms are actually not "atomic" because they are
> split into two 4-byte pieces. But there's no such thing as a 1-byte
> read or write not being atomic. In theory you could imagine a computer
> where when you change 0x01 to 0x23 and read in the middle and see 0x21
> or 0x13 or something, but no actual computers behave that way, or at
> least no mainstream ones that anybody cares about. So the idea that
> you somehow need a lock to prevent this is just wrong.
>
> Concurrent programs also suffer from another problem which is
> reordering of operations, which can happen either as the program is
> compiled or as the program is executed by the CPU. The CPU can see you
> set a->x = 1 and a->y = 2 and decide to update y first and then x even
> though you wrote it the other way around in the program text. To
> prevent this, we have barrier operations; see README.barrier in the
> source tree for a longer explanation. Atomic operations like
> compare-and-exchange are also full barriers, so that they not only
> prevent the torn read/write problem described above, but also enforce
> order of operations more strictly.
>
> Now I don't know whether a lock is needed here or not. Maybe it is;
> perhaps for consistency with other code, perhaps because the lock
> acquire and release is serving the function of a barrier; or perhaps
> to guard against some other hazard. But saying that it's because
> reading or writing a 1-byte value might not be atomic does not sound
> correct.

I never told that reading /writing 1 byte is not atomic, of course,
they are.  I told that we can only guarantee that 1-byte read/write is
atomic but this variable is not a bool or 1-byte value and the enum
can take 32 bits on a 32-bit platform so we can not guarantee the
atomic read/write on some processor so we need a lock.

-- 
Regards,
Dilip Kumar
EnterpriseDB: http://www.enterprisedb.com