Messages in this thread | | | Date | Wed, 18 May 2016 14:29:42 -0400 | From | Waiman Long <> | Subject | Re: [PATCH v4 2/5] locking/rwsem: Protect all writes to owner by WRITE_ONCE |
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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. 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.
Cheers, Longman
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