Messages in this thread | | | Subject | Re: [PATCHv3 6/6] tty/ldsem: Decrement wait_readers on timeouted down_read() | From | Dmitry Safonov <> | Date | Tue, 11 Sep 2018 14:01:58 +0100 |
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On Tue, 2018-09-11 at 14:02 +0200, Peter Zijlstra wrote: > On Tue, Sep 11, 2018 at 02:48:21AM +0100, Dmitry Safonov wrote: > > It seems like when ldsem_down_read() fails with timeout, it misses > > update for sem->wait_readers. By that reason, when writer finally > > releases write end of the semaphore __ldsem_wake_readers() does > > adjust > > sem->count with wrong value: > > sem->wait_readers * (LDSEM_ACTIVE_BIAS - LDSEM_WAIT_BIAS) > > > > I.e, if update comes with 1 missed wait_readers decrement, sem- > > >count > > will be 0x100000001 which means that there is active reader and > > it'll > > make any further writer to fail in acquiring the semaphore. > > > > It looks like, this is a dead-code, because ldsem_down_read() is > > never > > called with timeout different than MAX_SCHEDULE_TIMEOUT, so it > > might be > > worth to delete timeout parameter and error path fall-back.. > > You might want to think about ditching that ldsem thing entirely, and > use a regular rwsem ?
Yeah, but AFAICS, regular rwsem will need to have a timeout then (for write). So, I thought fixing this pile would be simpler than adding timeout and probably writer-priority to generic rwsem?
And I guess, we still will need fixes for stable for the bugs here..
I expect that timeouts are ABI, while the gain of adding priority may be measured. I'll give it a shot (adding timeout/priority for linux- next) to rwsem if you say it's acceptable.
-- Thanks, Dmitry
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