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List:       linux-kernel
Subject:    Re: [Patch] shm bug introduced with pagecache in 2.3.11
From:       Alexander Viro <viro () math ! psu ! edu>
Date:       1999-11-19 20:43:31
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On Fri, 19 Nov 1999, Linus Torvalds wrote:

> 
> 
> On Fri, 19 Nov 1999, Benjamin C.R. LaHaise wrote:
> > 
> > Sure.  Does the following sound?
> > 
> > 	void down_read(struct rw_semaphore *sem);
> > 	void down_write(struct rw_semaphore *sem);
> > 	void up_read(struct rw_semaphore *sem);
> > 	void up_write(struct rw_semaphore *sem);
> 
> Looks fine.
> 
> > Looking at existing semaphores and rwlocks, it looks like a quick and
> > painless beast to create.
> 
> Yeah, sure. 
> 
> Especially considering that the current semaphore code has been basically
> re-written three times to get it stable ;)
> 
> But yes, mixing judicious amounts of the current semaphore code together
> with the rw-spinlock code should result in something working.
> 
> "Just add brains".

IIRC, there was an implementation and analysis in Vahalia. Streamlining it
shouldn't be too hard (I have something like that in AFFS patches, but
there I didn't need {up,down}_write and did need upgrade/downgrade. IOW,
writrs always started as readers). IIRC, Vahalia uses two queues, counters
a-la rwlock_t has and counters of pending readers and writers. down_read() 
blocks if there are pending writers and up_write() either wakes one writer
or (if ->writers==0) all pending readers.


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