You can always check for a dead process if you save the
pid of the process which holds the lock. Alas, this
is a slow operation, involving calling kill(pid, 0),
two process switches and a return. As you
do relinquish the processor, that's not actually
evil, but is it something that conceivably could slow
you as much as the fcntl (;-))
From some old measurements on my Sun, I **think** it's
faster than **my** fcntl... I really need to remeasure
with Solaris 10.
--dave
Daniel J Blueman wrote:
On 22/04/06, David Collier-Brown <[EMAIL PROTECTED]> wrote:
Linux indeed hs some fancy locks for this, which were tested
with TDBs (see
http://scholar.google.com/url?sa=U&q=http://www.realitydiluted.com/nptl-uclibc/docs/futex.pdf)
but there is no discussion of what happens when we lose a
process which is holding a mmap'd lock. That was explicitly
not addressed in the paper, from the Ottawa Linux Symposium.
This may have been addressed by now, but I don't see any
references to it.
Fcntl locks explicitly release if the locking process had
died, so deadlock isn't a big issue. For that reason they're
preferred, even though they're far slower than spinlocks
on Solaris (that's a Solaris bug, by the way).
--dave
I was looking at futexes, but I found that current glibc packages
don't seem to provide a user-space header file (glibc seems to use
futexes internally). On top of this, we still need atomic operations,
which glibc doesn't provide to user-space.
glibc's pthreads use futexes (see PTHREAD_MUTEX_FAST_NP) and this
seems to work across processes sharing the lock, however the same
problem is there, that when a process holding the lock gets killed,
the lock isn't released. The recent robust futex work should deal with
this, but will be some time before it hits kernel + glibc.
Does anyone know if sysV IPC semaphores have any mechanism for recovering locks?
One mechanism to handle the process-crashing-holding-lock corner case
is to have lock acquisition timeout after eg 10s, and check if the pid
of the locker is still there - if not, the lock is dropped by 1.
Any comments on why this is a bad/unworkable idea with eg sysV semaphores?
Volker Lendecke wrote:
On Sat, Apr 22, 2006 at 06:15:31PM +0100, Daniel J Blueman wrote:
As a test, I changed the opening of the locking.tdb and brlock.tdb
files to use the TDB_INTERNAL flag, avoiding use of the fcntl(F_SETLK)
syscall for locking individual database records. Performance was a lot
snappier, with quite a bit less system time used.
What is the scope of implementing shared memory TDBs, where locking
could be done simply on structures?
How would you implement cross-process locking without
any syscalls? Maybe Linux has some fancy stuff here, but for
most unixes we need to coordinate the mmap'ed area for the
tdbs fcntl locks.
Volker
--
Daniel J Blueman
--
David Collier-Brown, | Always do right. This will gratify
System Programmer and Author | some people and astonish the rest
[EMAIL PROTECTED] | -- Mark Twain
(416) 223-5943
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