Messages in this thread | | | Subject | Re: [GIT PULL] Please pull proc and exec work for 5.7-rc1 | From | Waiman Long <> | Date | Sat, 4 Apr 2020 22:42:42 -0400 |
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On 4/3/20 10:28 PM, Linus Torvalds wrote: > On Fri, Apr 3, 2020 at 7:02 PM Waiman Long <longman@redhat.com> wrote: >> So in term of priority, my current thinking is >> >> upgrading unfair reader > unfair reader > reader/writer >> >> A higher priority locker will block other lockers from acquiring the lock. > An alternative option might be to have readers normally be 100% normal > (ie with fairness wrt writers), and not really introduce any special > "unfair reader" lock. A regular down_read() caller will be handled normally. > Instead, all the unfairness would come into play only when the special > case - execve() - does it's special "lock for reading with intent to > upgrade". > > But when it enters that kind of "intent to upgrade" lock state, it > would not only block all subsequent writers, it would also guarantee > that all other readers can continue to go).
Yes, that shouldn't be hard to do. If that is what is required, we may only need a special upgrade function to drain the OSQ and then wake up all the readers in the wait queue. I will add a flags argument to that special upgrade function so that we may be able to select different behavior in the future.
The regular down_read_interruptible() can be used unless we want to designate only some readers are allowed to do upgrade by calling a special down_read() function. > > So then the new rwsem operations would be > > - read_with_write_intent_lock_interruptible() > > This is the beginning of "execve()", and waits for all writers to > exit, and puts the lock into "all readers can go" mode. > > You could think of it as a "I'm queuing myself for a write lock, > but I'm allowing readers to go ahead" state. > > - read_lock_to_write_upgrade() > > This is the "now this turns into a regular write lock". It needs to > wait for all other readers to exit, of course. > > - read_with_write_intent_unlock() > > This is the "I'm unqueuing myself, I aborted and will not become a > write lock after all" operation. > > NOTE! In this model, there may be multiple threads that do that > initial queuing thing. We only guarantee that only one of them will > get to the actual write lock stage, and the others will abort before > that happens. > > If that is a more natural state machine, then that should work fine > too. And it has some advantages, in that it keeps the readers normally > fair, and only turns them unfair when we get to that special > read-for-write stage. > > But whatever it most natural for the rwsem code. Entirely up to you.
To be symmetric with the existing downgrade_write() function, I will choose the name upgrade_read() for the upgrade function.
Will that work for you?
Cheers, Longman
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