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Re: pserialized reader/writer locks
Date: Sat, 26 Jul 2014 16:48:27 +0100
From: Mindaugas Rasiukevicius <rmind%netbsd.org@localhost>
I would expect a better problem statement even if it is a brain dump (one
sentence would have been enough). Are you trying to solve sleepable reader
problem? In such case, messing up pserialize(9) with somewhat read-write
lock semantics in ad-hoc way is a wrong approach. If you are building a
read-optimised *lock*, then it should be designed and implemented as such
mechanism rather than built as an ad-hoc wrapper.
The object is scalable nestable transactions: entering and exiting a
transaction should not usually incur interprocessor synchronization,
and threads in transactions may sleep.
The main application I had in mind is fstrans(9), where every vnode
operation on a given file system must make sure the file system
doesn't go away, and may sleep -- many readers. Infrequently, a file
system may be unmounted or suspended, which will require waiting for
all pending transactions to complete -- infrequent, expensive writer.
fstrans(9) basically encodes the logic of what I called an rrwlock --
I wrote the rrwlock code when I sat down to disentangle what it does
and understand it. On doing so I realized it's the same as what zfs
does with ZFS_ENTER/ZFS_EXIT (and later found that zfs has another
abstraction also called rrwlock with ~the same semantics as mine, but
zfs's rrwlock is not scalable -- readers and writers both always
require interprocessor synchronization).
Basically, there are two approaches:
a) Implement read-optimised / read-mostly lock. Years ago ad@ wrote an
implementation of rdlock(9). It was not published, but he added a BSD
license so I guess it is okay to post here:
That makes a single reader cheap; doesn't make multiple readers
Alternatively, there is FreeBSD's rmlock(9):
That doesn't allow sleeping during a transaction.
b) Implement grace-period based synchronisation mechanism which allows
readers to sleep, such as SRCU. There are some dangers here:
However, it can provide less expensive writer side and more granular
garbage collection of objects. I think it is worth to consider this way.
Less expensive writer side is not the point here.
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