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
| ||
|
Date: Tue, 16 Jan 2018 11:23:49 +0900 From: Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> To: Petr Mladek <pmladek@...e.com> Cc: Steven Rostedt <rostedt@...dmis.org>, Sergey Senozhatsky <sergey.senozhatsky@...il.com>, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>, Tejun Heo <tj@...nel.org>, akpm@...ux-foundation.org, linux-mm@...ck.org, Cong Wang <xiyou.wangcong@...il.com>, Dave Hansen <dave.hansen@...el.com>, Johannes Weiner <hannes@...xchg.org>, Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>, Vlastimil Babka <vbabka@...e.cz>, Peter Zijlstra <peterz@...radead.org>, Linus Torvalds <torvalds@...ux-foundation.org>, Jan Kara <jack@...e.cz>, Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>, rostedt@...e.goodmis.org, Byungchul Park <byungchul.park@....com>, Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup On (01/15/18 15:45), Petr Mladek wrote: [..] > > With the preempt_disable() there really isn't a delay. I agree, we > > shouldn't let printk preempt (unless we have CONFIG_PREEMPT_RT enabled, > > but that's another story). > > > > > > > > so very schematically, for hand-off it's something like > > > > > > if (... console_trylock_spinning()) // grabbed the ownership > > > > > > << ... preempted ... >> > > > > > > console_unlock(); > > > > Which I think we should stop, with the preempt_disable(). > > Adding the preempt_disable() basically means to revert the already > mentioned commit 6b97a20d3a7909daa06625 ("printk: set may_schedule > for some of console_trylock() callers"). > > I originally wanted to solve this separately to make it easier. But > the change looks fine to me. Therefore we reached a mutual agreement. > Sergey, do you want to send a patch or should I just put it at > the end of this patchset? you can add the patch. [..] > > I think adding the preempt_disable() would fix printk() but let non > > printk console_unlock() still preempt. > > I would personally remove cond_resched() from console_unlock() > completely. hmm, not so sure. I think it's there for !PREEMPT systems which have to print a lot of messages. the case I'm speaking about in particular is when we register a CON_PRINTBUFFER console and need to console_unlock() (flush) all of the messages we currently have in the logbuf. we better have that cond_resched() there, I think. > Sleeping in console_unlock() increases the chance that more messages > would need to be handled. And more importantly it reduces the chance > of a successful handover. > > As a result, the caller might spend there very long time, it might > be getting increasingly far behind. There is higher risk of lost > messages. Also the eventual taker might have too much to proceed > in preemption disabled context. yes. > Removing cond_resched() is in sync with printk() priorities. hmm, not sure. we have sleeping console_lock()->console_unlock() path for PREEMPT kernels, that cond_resched() makes the !PREEMPT kernels to have the same sleeping console_lock()->console_unlock(). printk()->console_unlock() seems to be a pretty independent thing, unfortunately (!), yet sleeping console_lock()->console_unlock() messes up with it a lot. > The highest one is to get the messages out. > > Finally, removing cond_resched() should make the behavior more > predictable (never preempted) but we are always preempted in PREEMPT kernels when the current console_sem owner acquired the lock via console_lock(), not via console_trylock(). cond_resched() does the same, but for !PREEMPT. -ss
Powered by blists - more mailing lists