Re: Extended Prefetching using Asynchronous IO - proposal and patch

On 2014-06-26 00:08:48 +0300, Heikki Linnakangas wrote:
> LWLocks are implemented with semaphores, so if you can increment a semaphore
> in the signal handler / callback thread, then in theory you should be able
> to release a LWLock.

I don't think that's a convincing argument even if semop et al were
signal safe. LWLocks also use spinlocks and it's a fairly bad idea to do
anything with the same spinlock both inside and outside a signal
handler.
I don't think we're going to get lwlock.c/LWLockRelease to work
reasonably from a signal handler without a lot of work.

> On Linux at least we use System V semaphores, which are (unsurpisingly) not
> listed as safe for using in a signal handler. See list Async-signal-safe
> functions in signal(7) man page. The function used to increment a POSIX
> semaphore, sem_post(), is in the list, however.

Heh, just wrote the same after reading the beginning of your email ;)

Greetings,

Andres Freund

-- Andres Freund                       http://www.2ndQuadrant.com/PostgreSQL Development, 24x7 Support, Training &
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