Re: [RFC] New locking primitive for 2.5

Nigel Gamble
2002-02-08T16:52:19Z

On Fri, 8 Feb 2002, Christoph Hellwig wrote:
> In article <Pine.LNX.4.33.0202072305480.2976-100000@localhost.localdomain> you wrote:
> > i think one example *could* be to turn inode->i_sem into a combi-lock. Eg.
> > generic_file_llseek() could use the spin variant.
>
> No. i_sem should be split into a spinlock for short-time accessed
> fields that get written to even if the file content is only read (i.e.
> atime) and a read-write semaphore.

Read-write semaphores should never be used. As others have pointed out,
they cause really intractable priority inversion problems (because a
high priority writer will often have to wait for an unbounded number of
lower priority readers, some of which may have called a blocking
function while holding the read lock).

Note that I'm not talking about read-write spinlocks, which are (or
should be) held for a short, bounded time and can't be held over a
blocking call, so they are not quite so problematic.

Nigel Gamble nigel@nrg.org
Mountain View, CA, USA. http://www.nrg.org/