From: Nikita Danilov <nikita@clusterfs.com>
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Date: Thu, 15 Dec 2005 18:52:48 +0300
To: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Andrew Morton <akpm@osdl.org>, tglx@linutronix.de, dhowells@redhat.com,
       pj@sgi.com, mingo@elte.hu, hch@infradead.org, torvalds@osdl.org,
       arjan@infradead.org, matthew@wil.cx, linux-kernel@vger.kernel.org,
       linux-arch@vger.kernel.org
Subject: Re: [PATCH 1/19] MUTEX: Introduce simple mutex implementation
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Alan Cox writes:
 > On Iau, 2005-12-15 at 16:41 +0300, Nikita Danilov wrote:
 > > But this change is about fixing bugs: mutex assumes that
 > > 
 > >  - only owner can unlock, and
 > > 
 > >  - owner cannot lock (immediate self-deadlock).
 > 
 > So add mutex_up/mutex_down that use the same semaphores but do extra
 > checks if lock debugging is enabled. All you need is an owner field for
 > debugging.

And to convert almost all calls to down/up to mutex_{down,up}. At which
point, it no longer makes sense to share the same data-type for
semaphore and mutex.

Also, (as was already mentioned several times) having separate data-type
for mutex makes code easier to understand, as it specifies intended
usage.

To avoid duplicating code, mutex can be implemented on top of semaphore,
like

struct mutex {
        struct semaphore sema;
#ifdef DEBUG_MUTEX
        void *owner;
#endif
};

or something similar.

 > 
 > Now generate a trace dump on up when up and to check for sleeping on a
 > lock you already hold (for both sem and mutex).

Sleeping on a semaphore "held" by the current thread is perfectly
reasonable usage of a generic counting semaphore, as it can be upped by
another thread.

 > 
 > Alan

Nikita.
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