linux-kernel - Re: [PATCH 07/13] powerpc: Preemptible mmu

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 for Android: free password hash cracker in your pocket

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

Date:	Wed, 14 Apr 2010 15:34:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Andrea Arcangeli <aarcange@...hat.com>,
	Avi Kivity <avi@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Rik van Riel <riel@...hat.com>, Ingo Molnar <mingo@...e.hu>,
	akpm@...ux-foundation.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	David Miller <davem@...emloft.net>,
	Hugh Dickins <hugh.dickins@...cali.co.uk>,
	Mel Gorman <mel@....ul.ie>, Nick Piggin <npiggin@...e.de>
Subject: Re: [PATCH 07/13] powerpc: Preemptible mmu_gather

On Tue, 2010-04-13 at 11:23 +1000, Benjamin Herrenschmidt wrote:
> > +      * A comment here about on why we have RCU freed page tables might be
> > +      * interesting, also explaining why we don't need any sort of grace
> > +      * period for mm_users == 1, and have some home brewn smp_call_func()
> > +      * for single frees.
> 
> iirc, we are synchronizing with CPUs walking page tables in their hash
> or TLB miss code, which is lockless. The mm_users test is a -little- bit
> dubious indeed. It may have to be mm_users < 2 && mm ==
> current->active_mm, ie, we know for sure nobody else is currently
> walking those page tables ... 
> 
> Tho even than is fishy nowadays. We -can- walk page tables on behave of
> another process. In fact, we do it in the Cell SPU code for faulting
> page table entries as a result of SPEs taking faults for example. So I'm
> starting to suspect that this mm_users optimisation is bogus.
> 
> We -do- want to optimize out the case where there is no user left
> though, ie, the MM is dead. IE. The typical exit case.

Can't you fix that by having the SPE code take a reference on these
mm_structs they're playing with?

Poking at one without a ref seems fishy anyway.

--
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/