Re: msleep() on recursivly locked mutexes

On Thursday 26 April 2007 23:50, Attilio Rao wrote:
2007/4/26, Julian Elischer <julian@xxxxxxxxxxxx>:
The reason that mutexes ever recurse in the first place is usually
because one piece of code calls itself (or a related piece of code) in a
blind manner.. in other words, it doesn't know it is doing so. The whole
concept of recursing mutexes is a bit gross. The concept of blindly
unwinding them is I think just asking for trouble.

Further the idea that holding a mutex "except for when we sleep" is a
generally bright idea is also a bit odd to me.. If you hold a mutex and
release it during sleep you probably should invalidate all assumptions
you made during the period before you slept as whatever you were

That is not always correct. If you run your code in a separate
thread/taskqueue, then you simply wait for this thread/taskqueue to complete
somewhere else. This is basically when I need to exit mutexes.

protecting has possibly been raped while you slept. I have seen too many
instances where people just called msleep and dropped the mutex they
held, picked it up again on wakeup, and then blithely continued on
without checking what happened while they were asleep.

Basically, the idea you cannot hold "blocking" locks (mutexes and
rwlocks) while sleeping, cames from the difference there is behind
turnstiles and sleepqueues.

Turnstiles are thought to serve syncronous events, for short period of
time (or rather short) while sleepqueues are thought to serve
asyncronous events, so that the path to be protected can be
definitively bigger. If you fit in the situation you have to call
first a blocking lock and later a sleeping lock, probabilly you are
just using a wrong locking strategy and you should really revisit it.

The suggestion is just for convenience. Usually you don't have a recursed
mutex to sleep on. It is just to catch some rare cases where you will end up
with a doubly locked mutex, which is not part of the ordinary code path. I
don't have such cases in the kernel of the new USB stack, but there are some
cases in the USB device drivers, which is due to some mutex locking moves.
Those I can fix.

My idea was that by allowing recursive mutexes to sleep, you will end up with
less panics in the end for the unwary code developer. You just protect your
code with mutexes and if they recurse calling synchronous USB functions, you
don't have to care.


As you mention, it is not always possible to drop the blocking lock
before to sleep since you can break your critical path and free the
way for races of various genre. Even unlocking Giant, that is
auto-magically done by sleeping primitives, can lead to very difficult
to discover races (I can remind one in tty code, old of some months,
that can be a good proof-of-concept for that).


--HPS
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