linux-kernel - Re: RFC: one-bit mutexes (was: Re: [PATCH 2/3] Memory management livelock)

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <Pine.LNX.4.64.0810201612210.22845@hs20-bc2-1.build.redhat.com>
Date:	Mon, 20 Oct 2008 16:14:15 -0400 (EDT)
From:	Mikulas Patocka <mpatocka@...hat.com>
To:	Nick Piggin <nickpiggin@...oo.com.au>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, agk@...hat.com, mbroz@...hat.com,
	chris@...chsys.com
Subject: Re: RFC: one-bit mutexes (was: Re: [PATCH 2/3] Memory management
 livelock)

> > If you are concerned about the size of an inode, I can convert other
> > mutexes to bit mutexes: i_mutex and inotify_mutex.
> 
> I wouldn't worry for now. mutexes can be unlocked much faster than bit
> mutexes, especially in the fastpath. And due to slab, it would be
> unlikely to actually save any space.

Maybe inotify_mutex. You are right that i_mutex is so heavily contended 
that slowing it down to save few words wouldn't be good. Do you know about 
any inotify-intensive workload?

> > I could also create 
> > bit_spinlock (one-bit spinlock that uses test_and_set_bit) and save space
> > for address_space->tree_lock, address_space->i_mmap_lock,
> > address_space->private_lock, inode->i_lock.
> 
> We have that already. It is much much faster to unlock spinlocks than
> bit spinlocks in general (if you own the word exclusively, then it's
> not, but then you would be less likely to save space), and we can also
> do proper FIFO ticket locks with a larger word.

BTW. why do spinlocks on x86(64) have 32 bits and not 8 bits or 16 bits? 
Are atomic 32-bit instuctions faster?

Can x86(86) system have 256 CPUs?

Mikulas
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/