Horton

Re: Atomic operations within spinlocks

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Andres Freund <andres@anarazel.de>
Cc: Robert Haas <robertmhaas@gmail.com>, pgsql-hackers@lists.postgresql.org
Date: 2020-06-06T02:52:47Z
Lists: pgsql-hackers 
Andres Freund <andres@anarazel.de> writes:
> On 2020-06-05 21:01:56 -0400, Tom Lane wrote:
>> At some point I think we'll have to give up --disable-spinlocks; it's
>> really of pretty marginal use (how often does anyone port PG to a new
>> CPU type?) and the number of weird interactions it adds in this area
>> seems like more than it's worth.

> Indeed. And any new architecture one would port PG to would have good
> enough compiler intrinsics to make that trivial. I still think it'd make
> sense to have a fallback implementation using compiler intrinsics...

> And I think we should just require 32bit atomics at the same time. Would
> probably kill gaur though.

Not only gaur.  A quick buildfarm survey finds these active members
reporting not having 32-bit atomics:

 anole         | 2020-06-05 11:20:17 | pgac_cv_gcc_atomic_int32_cas=no
 chipmunk      | 2020-05-29 22:27:56 | pgac_cv_gcc_atomic_int32_cas=no
 curculio      | 2020-06-05 22:30:06 | pgac_cv_gcc_atomic_int32_cas=no
 frogfish      | 2020-05-31 13:00:25 | pgac_cv_gcc_atomic_int32_cas=no
 gaur          | 2020-05-19 13:33:25 | pgac_cv_gcc_atomic_int32_cas=no
 gharial       | 2020-06-05 12:41:14 | pgac_cv_gcc_atomic_int32_cas=no
 hornet        | 2020-06-05 09:11:26 | pgac_cv_gcc_atomic_int32_cas=no
 hoverfly      | 2020-06-05 22:06:14 | pgac_cv_gcc_atomic_int32_cas=no
 locust        | 2020-06-05 10:14:29 | pgac_cv_gcc_atomic_int32_cas=no
 mandrill      | 2020-06-05 09:20:03 | pgac_cv_gcc_atomic_int32_cas=no
 prairiedog    | 2020-06-05 09:55:49 | pgac_cv_gcc_atomic_int32_cas=no

It looks to me like this is mostly about compiler support not the
hardware; that doesn't make it not a problem, though.  (I also
remain skeptical about the quality of the compiler intrinsics
on non-mainstream hardware.)

			regards, tom lane

Commits

  1. Convert SpinLock* macros to static inline functions.

  2. Clean up includes of s_lock.h.

  3. Fix deadlock danger when atomic ops are done under spinlock.

  4. Add basic spinlock tests to regression tests.

  5. spinlock emulation: Fix bug when more than INT_MAX spinlocks are initialized.

  6. Avoid potential spinlock in a signal handler as part of global barriers.

  7. Make pg_stat_wal_receiver consistent with the WAL receiver's shmem info

  8. Improve 64bit atomics support.