Re: [Patch 0/10]: Cleanup online reservations for 2.6.9-rc2-mm4.
From: Stephen C. Tweedie (sct_at_redhat.com)
Date: 09/30/04
- Previous message: Saber Zrelli: "patch for divert mechanism needed"
- In reply to: Manfred Spraul: "Re: [Patch 0/10]: Cleanup online reservations for 2.6.9-rc2-mm4."
- Next in thread: Andreas Dilger: "Re: [Patch 0/10]: Cleanup online reservations for 2.6.9-rc2-mm4."
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
To: Manfred Spraul <manfred@colorfullife.com> Date: 30 Sep 2004 18:04:47 +0100
Hi,
On Thu, 2004-09-30 at 17:00, Manfred Spraul wrote:
> >Locking
> >is minimised: the impact on the hot path consists of nothing more than
> >an smp_rmb() before we test sb->s_groups_count. That's a noop on x86,
> >
> No, wrong way around:
> wmb() is empty. rmb() is either lfence or a locked dummy instruction.
Hmm. But I'm still not sure we can get away with anything
lighter-weight.
The basic construct we need to worry about is:
new_group_table = kmalloc(...);
memcpy(new_group_table, old_group_table);
new_group_table[new_group] = foo;
sbi->s_group_desc = new_group_table;
/* SMP WRITE BARRIER */
sbi->s_group_count = new_group_count;
on the writer side, and
ngroups = sbi->s_group_count;
/* SMP READ BARRIER */
for (i = 0; i < ngroups; i++)
gdp = sbi->s_group_desc[i];
The latter is the worry --- we're doing a read that depends immediately
on "i" and s_group_desc, but not on sbi->s_group_count. There *IS* a
comparison between i and s_group_count, though, so the dependency is
implicit.
I'm just not familiar enough with the architecture of weakly-ordered
platforms to know if we can get away with smp_read_barrier_depends() in
this particular case. If so, we can use that and be done with the extra
locked op on x86.
--Stephen
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
- Previous message: Saber Zrelli: "patch for divert mechanism needed"
- In reply to: Manfred Spraul: "Re: [Patch 0/10]: Cleanup online reservations for 2.6.9-rc2-mm4."
- Next in thread: Andreas Dilger: "Re: [Patch 0/10]: Cleanup online reservations for 2.6.9-rc2-mm4."
- Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]
Relevant Pages
|
|
