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SubjectLinus' email account is full. - Fwd: Mail System Error - Returned Mail
Linus' email account appears to be full if we can believe this returned email:

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>Date: Fri, 08 Feb 2002 20:14:32 +0000
>To: nigel@nrg.org
>From: Anton Altaparmakov <aia21@cam.ac.uk>
>Subject: Re: [RFC] New locking primitive for 2.5
>Cc: Christoph Hellwig <hch@ns.caldera.de>,Ingo Molnar <mingo@elte.hu>,
> yodaiken <yodaiken@fsmlabs.com>,Martin Wirth <Martin.Wirth@dlr.de>,
> linux-kernel <linux-kernel@vger.kernel.org>,akpm <akpm@zip.com.au>,
> torvalds <torvalds@transmet.com>,rml <rml@tech9.net>
>In-Reply-To: <Pine.LNX.4.40.0202080838230.3883-100000@cosmic.nrg.org>
>References: <200202081231.g18CV7e31341@ns.caldera.de>
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>At 16:51 08/02/02, Nigel Gamble wrote:
>> > 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.
>
>Read-write semaphores have their use and the current Linux implementation
>(big reader/occasional writer) guarantees that the writer is not starved
>as incoming read locks are put to sleep as soon as a write lock request
>comes in, even if that is sleeping waiting for the old readlocks to be
>released (unless the readers are holding the semaphore forever in which
>case this is the programmers fault and not the rw semaphore
>implementations fault). [I should add I only am familliar with the i386
>implementation but I assume the others are the same.]
>
>The value of allowing multiple cpus to read the same data simultaneously
>by far offsets the priority problems IMVHO. At least the way I am using rw
>semaphores in ntfs it is. Readlocks are grabbed loads and loads of times
>to serialize meta data access in the page cache while writelocks are a
>minute number in comparison and because the data required to be accessed
>may not be cached in memory (page cache page is not read in, is swapped
>out, whatever) a disk access may be required which means a rw spin lock is
>no good. In fact ntfs would be the perfect candidate for automatic rw
>combi locks where the locking switches from spinning to sleeping if the
>code path reaches a disk access. I can't use a manually controlled lock as
>the page cache lookups are done via the mm/filemap.c access functions
>which are the only ones who can know if a disk access is required or not
>so ntfs would never know if it is going to sleep or not so unless the
>locks had autodetection of whether to spin or sleep they would be useless.
>
>I guess the point I am trying to make is that both rw semaphores and combi
>locks are not bad per se but, as all other locking mechanisms, they should
>be used in situations appropriate for their locktype and their
>implementation...
>
>Anton
>
>
>--
> "I've not lost my mind. It's backed up on tape somewhere." - Unknown
>--
>Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
>Linux NTFS Maintainer / WWW: http://linux-ntfs.sf.net/
>ICQ: 8561279 / WWW: http://www-stu.christs.cam.ac.uk/~aia21/
>

--
"I've not lost my mind. It's backed up on tape somewhere." - Unknown
--
Anton Altaparmakov <aia21 at cam.ac.uk> (replace at with @)
Linux NTFS Maintainer / WWW: http://linux-ntfs.sf.net/
ICQ: 8561279 / WWW: http://www-stu.christs.cam.ac.uk/~aia21/

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