PTHREAD_MUTEX_ROBUST for tdb?

[Volker Lendecke]

Hi!

For years I have had a low-prio thread sitting in my mind
about getting rid of the fcntl calls for tdb access. A
couple of years ago I've discovered PTHREAD_PROCESS_SHARED
which make pthread mutexes cross-process. They are not
robust though, if a lock holder dies the mutex won't be
unlocked automatically. Now I was pointed out that in SUSV4
we have

http://pubs.opengroup.org/onlinepubs/9699919799/functions/pthread_mutexattr_getrobust.html

which gives seems to do exactly what we want. I've written a
tiny non-error checking test program. In one window, run

./mutex /tmp/mutexfile init

and in another windows run

./mutex /tmp/mutexfile

and kill -9 the first instance. The second one will be woken
up with EOWNERDEAD.

We get an additional benefit over fcntl locks: We can tell
whether a lock holder dies hard while holding a lock, so we
at least know when the tdb is potentially hosed.

To me this looks VERY promising. The first caller does not
do any syscalls on my Ubuntu Linux box to lock the mutex,
the second caller sits in a futex call.

Comments?

Volker

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