|From:||Mathieu Desnoyers <firstname.lastname@example.org>|
|To:||Pavel Machek <email@example.com>|
|Subject:||Userspace RCU 0.2.4|
|Date:||Mon, 19 Oct 2009 19:59:05 -0400|
|Cc:||Pierre Habouzit <firstname.lastname@example.org>, Steven Munroe <email@example.com>, firstname.lastname@example.org, email@example.com, Josh Triplett <firstname.lastname@example.org>, email@example.com, Jon Bernard <firstname.lastname@example.org>, "Paul E. McKenney" <email@example.com>, Jan Blunck <firstname.lastname@example.org>|
* Pavel Machek (email@example.com) wrote: > On Sun 2009-10-18 18:02:43, Mathieu Desnoyers wrote: > > * Pavel Machek (firstname.lastname@example.org) wrote: > > > On Thu 2009-10-15 13:40:54, Pierre-Marc Fournier wrote: > > > > Josh Triplett wrote: > > > > > > > > > > Even Debian has given up on real 386 systems at this point, primarily > > > > > because system libraries like glibc have; 486 and better represents the > > > > > bare minimum required at this point. I don't know of any distributions > > > > > supporting real 386 systems at this point, and doing so would represent > > > > > a major undertaking. > > > > > > > > > > > > > What about embedded systems? Anyone know if some 386 chips, perhaps even > > > > in smp configurations, are still in use in those? > > > > > > smp 386: definitely not. > > > > Hrm, so for UP 386, I wonder what's the best approach. > > > > One would be to encapsulate all write accesses to the RCU pointers. If > > we detect that the architecture lacks cmpxchg, _all_ update operations > > (rcu_assign_pointer, rcu_xchg_pointer and rcu_cmpxchg_pointer) would > > have to use the signal-disabled+mutex fall-back. > > > > Does it make sense ? > > Yep, but it sounds expensive. Another option is to ignore the issue > and see how many people still have 386s :-). Few embedded systems > may be affected, but... Well.. I just enhanced liburcu to fully support 386 SMP (even if opinions seems to vary regarding its usefulness...) ;) It adds _no_ overhead whatsoever if building for i486+ or x86 64. What I did is a complete "compatibility mode" for all uatomic_arch_x86.h atomic operations (it's my own user-space reimplementation of the Linux kernel atomic.h). It's in liburcu 0.2.4 (now released). How it works: config x86 64 or x86 32 > i386 : #define to map directly to atomic operations. config i386 : dynamically detect the cpu id, caches it in "cas_avail" variable. If cas_avail is -1 (unset) -> dynamically check, cache result. If cas_avail is 1 -> use atomic operations. If cas_avail is 0 -> use compatibility mode for _all_ uatomic write operations involving signal disabling and a mutex. Only uatomic_read is exempt from locking. So it should be safe to access RCU pointers through the rcu_cmpxchg/xchg/set/assign_pointer() and rcu_dereference() primitives. I tried to force using the compatibility mode by changing the condition in compat_arch_x86.c and building for i386 compatibility. It works fine and passes the test_uatomic test cases. Passes the rcutorture test too. Mathieu > > Pavel > -- > (english) http://www.livejournal.com/~pavelmachek > (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blo... > > _______________________________________________ > ltt-dev mailing list > email@example.com > http://lists.casi.polymtl.ca/cgi-bin/mailman/listinfo/ltt... > -- Mathieu Desnoyers OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to firstname.lastname@example.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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