Messages in this thread | | | Date | Tue, 30 Apr 2019 17:35:28 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH 24/28] locking/lockdep: Remove !dir in lock irq usage check |
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On Thu, Apr 25, 2019 at 10:03:36PM +0200, Peter Zijlstra wrote: > On Wed, Apr 24, 2019 at 06:19:30PM +0800, Yuyang Du wrote: > > In mark_lock_irq(), the following checks are performed: > > > > ---------------------------------- > > | -> | unsafe | read unsafe | > > |----------------------------------| > > | safe | F B | F* B* | > > |----------------------------------| > > | read safe | F? B* | - | > > ---------------------------------- > > > > Where: > > F: check_usage_forwards > > B: check_usage_backwards > > *: check enabled by STRICT_READ_CHECKS > > ?: check enabled by the !dir condition > > > > From checking point of view, the special F? case does not make sense, > > whereas it perhaps is made for peroformance concern. As later patch will > > address this issue, remove this exception, which makes the checks > > consistent later. > > > > With STRICT_READ_CHECKS = 1 which is default, there is no functional > > change. > > Oh man.. thinking required and it is way late.. anyway this whole read > stuff made me remember we had a patch set on readlocks last year. > > https://lkml.kernel.org/r/20180411135110.9217-1-boqun.feng@gmail.com > > I remember reviewing that a few times and then it dropped on the floor, > probably because Spectre crap or something sucked up all my time again :/
So if we look at Boqun's patches (as posted, I haven't looked at his github, but I'm assuming this hasn't changed with the 'Shared' state), we'll find he'll only does either 1 backward or 1 foward search (which is already an improvement over the current state).
His mark_lock_irq() looks like:
static int mark_lock_irq(struct task_struct *curr, struct *held_lock *this, enum lock_usage_bit new_bit) { int excl_bit = exclusive_bit(new_bit);
+ if (new_bit & 2) { + /* + * mark ENABLED has to look backwards -- to ensure no dependee + * has USED_IN state, which, again, would allow recursion + * deadlocks. + */ + if (!check_usage_backwards(curr, this, new_bit, excl_bit)) return 0; + } else { + /* + * mark USED_IN has to look forwards -- to ensure no dependency + * has ENABLED state, which would allow recursion deadlocks. + */ + if (!check_usage_forwards(curr, this, new_bit, excl_bit)) return 0; }
return 1; }
Where '& 2' would read '& LOCK_USAGE_DIR_MASK' in the current code.
Now, I'm thinking you're proposing to replace the backward search for USED_IN/safe with your reachable-safe state, which, if done on his 'strong' links, should still work.
That is; I _think_ the two patch-sets are not in conceptual conflict.
Of course; I could have missed something; I've just read both patchsets again, and it's a bit much :-)
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