Date: Tue, 26 Jan 2010 13:42:24 -0700 (MST) From: "M. Warner Losh" <imp@bsdimp.com> To: xcllnt@mac.com Cc: src-committers@freebsd.org, jhb@freebsd.org, svn-src-all@freebsd.org, attilio@freebsd.org, marcel@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r202889 - head/sys/kern Message-ID: <20100126.134224.366306193153347169.imp@bsdimp.com> In-Reply-To: <3023270A-755A-4BCF-AC9A-C1F290052279@mac.com> References: <C6A8F7A7-F0A9-4F63-B61E-DDC5332DC495@mac.com> <20100126.130932.722022410132669562.imp@bsdimp.com> <3023270A-755A-4BCF-AC9A-C1F290052279@mac.com>
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In message: <3023270A-755A-4BCF-AC9A-C1F290052279@mac.com>
Marcel Moolenaar <xcllnt@mac.com> writes:
:
: On Jan 26, 2010, at 12:09 PM, M. Warner Losh wrote:
: > cpu_switch(struct thread *old, struct thread *new, struct mutext *mtx)
: > {
: > /* Save the registers to the pcb */
: > old->td_lock = mtx;
: > #if defined(SMP) && defined(SCHED_ULE)
: > /* s/long/int/ if sizeof(long) != sizeof(void *) */
: > /* as we have no 'void *' version of the atomics */
: > while (atomic_load_acq_long(&new->td_lock) == (long)&blocked_lock)
: > continue;
: > #endif
: > /* Switch to new context */
: > }
:
: Ok. So this is what ia64 has already, except for the atomic_load()
: in the while loop. Since td_lock is volatile, I don't think we need
: atomic_load(). To be explicit, ia64 has:
:
: old->td_lock = mtx;
: #if defined(SCHED_ULE) && defined(SMP)
: /* td_lock is volatile */
: while (new->td_lock == &blocked_lock)
: ;
: #endif
:
: Am I right, or am I missing a critical aspect of using atomic load?
The Atomic load acq also has a memory barrier after the item is
fetched from memory.
: > I also think that we should have that code somewhere for reference.
:
: Since ia64 has a C implementation of cpu_switch(), we could make
: that the reference implementation?
Most likely :)
Warner
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