public inbox for [email protected]  

help / color / mirror / Atom feed

From: Aleksander Alekseev <[email protected]>
To: PostgreSQL Hackers <[email protected]>
Subject: "38.10.10. Shared Memory and LWLocks" may require a clarification
Date: Fri, 19 May 2023 18:49:11 +0300
Message-ID: <CAJ7c6TPKhFgL+54cdTD9yGpG4+sNcyJ+N1GvQqAxgWENAOa3VA@mail.gmail.com> (raw)

Hi,

While re-reading 38.10.10. Shared Memory and LWLocks [1] and the
corresponding code in pg_stat_statements.c I noticed that there are
several things that can puzzle the reader.

The documentation and the example suggest that LWLock* should be
stored within a structure in shared memory:

```
typedef struct pgssSharedState
{
    LWLock       *lock;
    /* ... etc ... */
} pgssSharedState;
```

... and initialized like this:

```
    LWLockAcquire(AddinShmemInitLock, LW_EXCLUSIVE);

    pgss = ShmemInitStruct("pg_stat_statements",
                           sizeof(pgssSharedState),
                           &found);

    if (!found)
    {
        pgss->lock = &(GetNamedLWLockTranche("pg_stat_statements"))->lock;
        /* ... */
    }

    /* ... */

    LWLockRelease(AddinShmemInitLock);
```

It is not clear why placing LWLock* in a local process memory would be a bug.

On top of that the documentation says:

"""
To avoid possible race-conditions, each backend should use the LWLock
AddinShmemInitLock when connecting to and initializing its allocation
of shared memory
"""

However it's not clear when a race-condition may happen. The rest of
the text gives an overall impression that the shmem_startup_hook will
be called by postmaster once (unless an extension places several hooks
in series). Thus there is no real need to ackquire AddinShmemInitLock
and it should be safe to store LWLock* in local process memory. This
memory will be inherited from postmaster by child processes and the
overall memory usage is going to be the same due to copy-on-write.

Perhaps we should clarify this.

Thoughts?

[1]: https://www.postgresql.org/docs/15/xfunc-c.html#XFUNC-SHARED-ADDIN

-- 
Best regards,
Aleksander Alekseev

view thread (2+ messages)

reply

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Reply to all the recipients using the --to and --cc options:
  reply via email

  To: [email protected]
  Cc: [email protected], [email protected]
  Subject: Re: "38.10.10. Shared Memory and LWLocks" may require a clarification
  In-Reply-To: <CAJ7c6TPKhFgL+54cdTD9yGpG4+sNcyJ+N1GvQqAxgWENAOa3VA@mail.gmail.com>

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

This inbox is served by agora; see mirroring instructions
for how to clone and mirror all data and code used for this inbox