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List:       linux-kernel
Subject:    Re: [Patch] shm bug introduced with pagecache in 2.3.11
From:       Manfred Spraul <manfreds () colorfullife ! com>
Date:       1999-11-19 20:31:54
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"Benjamin C.R. LaHaise" wrote:
> 
> On Fri, 19 Nov 1999, Linus Torvalds wrote:
> 
> > Is anybody willing to take a stab at creating a read-write semaphore?
> 
> 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);
> 
> Looking at existing semaphores and rwlocks, it looks like a quick and
> painless beast to create.
> 
There is a subtle problem: a rw semaphore is no real semaphore, it will
not support counts [at least if you base them on the rwlock asm code]:
semaphores allow a call to up() before the corresponding down(), eg
vfork() could do that [on SMP with an extremely long interrupt on the
line with the fork_out-label ;)

I would prefer if you use completely different function names to avoid
any confusion.
eg
acquire_shared()
acquire_exclusive()
release_shared()
release_exclusive()


--
	Manfred

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