commit 0bc3bcab25532535b793e767a763b3e73e249d79
author Nick Maniscalco <maniscalco@google.com> Sat May 11 01:04:23 2024 +0000
committer CQ Bot <fuchsia-internal-scoped@luci-project-accounts.iam.gserviceaccount.com> Sat May 11 01:04:23 2024 +0000
tree 683dc21631984dea3a0a3ccf40594190a5f86977
parent 48691ba26761e2b8bf1f0318853133219e0dd366

[kernel][dlog] Use a Timer to allow printing from within the scheduler

This change resolves an issue that prevented printf from working
within the scheduler, and more generally, whenever there was an active
chainlock transaction.

printf bottoms out in DLog::Write, which places a record in a buffer
and then calls Event::Signal to unblock the notifier thread.  Prior to
this change, if DLog::Write was called from a context with an existing
active chainlock transaction (e.g. from within the scheduler), calling
Event::Signal would trigger an assert (can't start a new transaction
when you've already got one in progress).

To resolve the issue, we update DLog::Write to check for an active
chainlock transaction.  If there is none, it calls Event::Signal as
usual.  If there one, DLog::Write will instead set a Timer that will
immediately fire once the current CPU has re-enabled interrupts after
finishing the current transaction (or during the period in which
interrupts are re-enabled during backoff).  The Timer callback simply
calls Event::Signal on Write's behalf.

~DLog now cancels the Timer to ensure there is no use-after-free.

As a small optimization, DLog tracks whether there is a pending Timer
(pending_deferred_signal_).  Of course, this tracking is not 100%
accurate as the Timer callback could be racing with the write.
However, because the observation is made in a critical section guarded
by the same lock that the dumper thread must ultimately acquire, there
is no risk of a "lost wakeup".

Fixed: 339440805
Fixed: 331847876
Change-Id: Iad837950e9513547b986fd71fc045b3e407d58c5
Reviewed-on: https://fuchsia-review.googlesource.com/c/fuchsia/+/1045567
Reviewed-by: John Grossman <johngro@google.com>
Fuchsia-Auto-Submit: Nick Maniscalco <maniscalco@google.com>
Commit-Queue: Auto-Submit <auto-submit@fuchsia-infra.iam.gserviceaccount.com>