Ulrich Drepper a écrit :
There are no such situations anymore in an optimal userlevel
implementation. The last problem (in pthread_cond_signal) was fixed
by the addition of FUTEX_WAKE_OP. The userlevel code you're looking
at is simply not optimized for the modern kernels.
I think there is a misunderstanding here.
FUTEX_WAKE_OP is implemented to handle simultaneously more than one
futex in some specific situations (such as pthread_cond_signal).
The scenario I've described occurred in futex_wake, futex_wake_op and
futex_requeue and is _independent_ of the userlevel code.
All these functions call wake_futex, and then wake_up_all, with the
futex_hash_bucket lock still held.
If the woken thread is immediately scheduled (in wake_up_all), and only
in this case (because of a higher priority, etc), it will try to take
this lock too (because of the "if (lock_ptr != 0)" statement in
unqueue_me), causing two task-switches to take this lock for nothing.
Otherwise, it will not: lock_ptr is set to NULL just after the
wake_up_all call)
This scenario happens at least in pthread_cond_signal,
pthread_cond_broadcast and probably all pthread_*_unlock functions.
The patch I've proposed should, at least in theory, solve this. But I'm
not sure of the correctness...
--
Pierre P.
-
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]
