#15234: userland rw_lock races into a deadlock
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Reporter: X512 | Owner: waddlesplash
Type: bug | Status: assigned
Priority: normal | Milestone: Unscheduled
Component: System | Version: R1/Development
Resolution: | Keywords:
Blocked By: | Blocking:
Has a Patch: 0 | Platform: All
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Comment (by waddlesplash):
So, the cause of the KDL in #15211 was indeed a race between the userland
rw_lock's
[https://xref.plausible.coop/source/xref/haiku/src/system/libroot/os/locks/rw_lock.cpp#22
wait()] calling _user_thread_block, and a separate thread calling
_user_thread_unblock, and unblocking it on something else (in all recorded
case in that ticket, the thread mutex) and then general mayhem ensuing.
In [https://git.haiku-
os.org/haiku/commit/?id=2c588b031fd618f8ec37376906fbfc68f4eeea0b
hrev53345~1], I fixed the KDLs by only having _user_unblock_thread
actually unblock threads when they were blocked by an equivalent
_user_block_thread, and otherwise just set the
{{{user_thread->wait_status}}}. Before touching that, it of course
acquires the thread's lock; and similarly in
[https://xref.plausible.coop/source/xref/haiku/src/system/kernel/thread.cpp#3700
_user_block_thread], it acquires the thread's mutex before checking
wait_status, and it returns immediately if wait_status has already been
set; and otherwise it blocks.
As we only unlock the thread inside _user_block_thread after calling
{{{thread_prepare_to_block}}}, this should (as I wrote in that commit)
prevent the exact race seen here. But apparently it doesn't, and so there
is somehow a way we are entering thread_block() (or, more particularly,
thread_prepare_to_block()) with the wait_status already changed, and so we
of course never get thread_unblocked.
I've gone over the code multiple times and as far as I can see, all bases
should be covered by locking the thread.
--
Ticket URL: <https://dev.haiku-os.org/ticket/15234#comment:3>
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