Ulrich Drepper a écrit :
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
Eric Dumazet wrote:
3) if condvar is PRIVATE and mutex is SHARED, a FUTEX_WAKE_PRIVATE
should be done. (and loose the REQUEUE optim)
Yes we could add a special futex primitive for this special case. But I
cannot see how a program could use such a construct.
Very easily: mutexes are often created independently from the condvars
and they are used for many things. Maybe a program is even creating all
mutexes as shared to be ready for all situations. Normally doing this
is no big problem, the performance penalties are minimal.
4) if condvar is SHARED and mutex is private, we have a *problem*,
because the process doing the broadcast() can be in another mm. So a
requeue is not possible at all.
It is if we can specify the owner of the mutex. I.e., the PID.
well, then we have a refcounting issue on pid , or problem in pid reuse.
But yes, this case is extremely ugly.
The problem is that all these cases worked nice so far. They all had
the same good performance. Now we are severely penalizing code which
mismatches condvar and mutex shared attributes. There is a good reason
why we introduced FUTEX_CMP_REQUEUE, the benefits in certain programs is
huge.
This analysis seems unfair to me, after a quick reading of glibc code.
Right now, glibc cannot use FUTEX_CMP_REQUEUE if condvar is pshared.
/* Do not use requeue for pshared condvars. */
if (cond->__data.__mutex == (void *) ~0l)
goto wake_all;
So how introducing private futexes is penalizing this case ?
Fact is that if condvar is pshared, we *cannot* use CMP_REQUEUE since threads
doing the broadcast() can be in a separate process and virtual address of
mutex of waiting threads could point to unrelated memory.
Thanks
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
