Messages in this thread | | | Date | Mon, 23 Oct 2017 15:46:22 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH] locking/spinlock/debug: snapshot lock fields |
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On Mon, Oct 23, 2017 at 01:29:10PM +0100, Mark Rutland wrote: > Currently, the lock debug code doesn't use {READ,WRITE}_ONCE() to access > lock fields, and thus may observe torn values given a suitably unhelpful > compiler. These could result in false positives and/or false negatives > for some sanity checks. > > Further, as we don't snapshot the values of various fields, these might > change between the time they are sanity checked and the time they are > logged, making debugging difficult. > > This patch ensures that lock fields are accessed with > {READ,WRITE}_ONCE(), and uses a snapshot of the lock to ensure that > logged values are the same as those used for the sanity checks. > > Signed-off-by: Mark Rutland <mark.rutland@arm.com> > Cc: Peter Zijlstra <peterz@infradead.org> > Cc: Ingo Molnar <mingo@redhat.com>
ACK, I'll try and remember to look at this if there's anything further we can remove from this file.
> +static inline raw_spinlock_t > +debug_spin_lock_snapshot(raw_spinlock_t *lockp) > +{ > + raw_spinlock_t lock; > + > + lock.raw_lock = READ_ONCE(lockp->raw_lock);
I think you want to make that an smp_load_acquire(), such that we preserve the release-acquire relation for the lock.
Since this happens _before_ the regular lock, but debug_spin_unlock() also happens before the regular unlock. So the unlock release is still placed right. But we've messed up our acquire.
> + lock.magic = READ_ONCE(lockp->magic); > + lock.owner_cpu = READ_ONCE(lockp->owner_cpu); > + lock.owner = READ_ONCE(lockp->owner); > + > + return lock; > +}
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