[haiku-bugs] Re: [Haiku] #10786: Lack of non-volatile "atomic_*" functions breaks some GCC4 apps
- From: "pdziepak" <trac@xxxxxxxxxxxx>
- Date: Mon, 28 Apr 2014 19:53:33 -0000
#10786: Lack of non-volatile "atomic_*" functions breaks some GCC4 apps
----------------------------+----------------------------
Reporter: waddlesplash | Owner: nobody
Type: bug | Status: closed
Priority: normal | Milestone: R1
Component: System | Version: R1/Development
Resolution: invalid | Keywords:
Blocked By: | Blocking:
Has a Patch: 0 | Platform: All
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Comment (by pdziepak):
That `volatile` qualifier was pointless. `atomic_*()` functions do not
need it and it didn't help in any way when the variable has been accessed
directly (usually, in such situation either `volatile` is not enough and a
memory barrier is also needed or `volatile` is not needed at all).
Moreover, in some use cases it is actually harmful, e.g. consider
[http://cgit.haiku-os.org/haiku/tree/src/system/kernel/fs/unused_vnodes.h
this file] and how `sNextHotVnodeIndex` is used. That variable is accessed
concurrently by many threads each holding read lock on `sHotVnodesLock`.
It is also sometimes accessed with write lock on `sHotVnodesLock` and in
such case neither `atomic_*()` functions are needed nor `volatile`
qualifier.
In that commit I also broke binary interface of `atomic_set()` but as far
as I know that particular function is Haiku addition. Unfortunately, it
looks like `atomic_{add, and, or}()` have to stay the way they are now,
which is very bad since they are much more expensive than it is often
necessary.
--
Ticket URL: <https://dev.haiku-os.org/ticket/10786#comment:3>
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