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Date: Fri, 20 May 2016 16:26:42 -0400 From: Waiman Long <waiman.long@....com> To: Jason Low <jason.low2@....com> CC: <jason.low2@...com>, Davidlohr Bueso <dave@...olabs.net>, Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, <linux-kernel@...r.kernel.org>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>, Scott J Norton <scott.norton@....com>, Douglas Hatch <doug.hatch@....com> Subject: Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE On 05/19/2016 06:21 PM, Jason Low wrote: > On Wed, 2016-05-18 at 12:58 -0700, Jason Low wrote: >> On Wed, 2016-05-18 at 14:29 -0400, Waiman Long wrote: >>> On 05/18/2016 01:21 PM, Jason Low wrote: >>>> On Wed, 2016-05-18 at 07:04 -0700, Davidlohr Bueso wrote: >>>>> On Tue, 17 May 2016, Waiman Long wrote: >>>>> >>>>>> Without using WRITE_ONCE(), the compiler can potentially break a >>>>>> write into multiple smaller ones (store tearing). So a read from the >>>>>> same data by another task concurrently may return a partial result. >>>>>> This can result in a kernel crash if the data is a memory address >>>>>> that is being dereferenced. >>>>>> >>>>>> This patch changes all write to rwsem->owner to use WRITE_ONCE() >>>>>> to make sure that store tearing will not happen. READ_ONCE() may >>>>>> not be needed for rwsem->owner as long as the value is only used for >>>>>> comparison and not dereferencing. >>>> It might be okay to leave out READ_ONCE() for reading rwsem->owner, but >>>> couldn't we include it to at least document that we're performing a >>>> "special" lockless read? >>>> >>> Using READ_ONCE() does have a bit of cost as it limits compiler >>> optimization. If we changes all access to rwsem->owner to READ_ONCE() >>> and WRITE_ONCE(), we may as well change its type to volatile and be done >>> with. >> Right, although there are still places like the init function where >> WRITE_ONCE isn't necessary. >> >>> I am not against doing that, but it feels a bit over-reach for me. >>> On the other hand, we may define a do-nothing macro that designates the >>> owner as a special variable for documentation purpose, but don't need >>> protection at that particular call site. >> It should be fine to use the standard READ_ONCE here, even if it's just >> for documentation, as it's probably not going to cost anything in >> practice. It would be better to avoid adding any special macros for this >> which may just add more complexity. > By the way, this potential "partial write" issue may also apply to > mutexes as well, so we should also make a similar change to > mutex_set_owner() and mutex_clear_owner(). > > Jason > Yes, I am aware of that. I just don't have the time to to do a mutex patch yet. As you have sent out a patch on that, this is now covered. Cheers, Longman
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