4lDO2
44527a8340
So, when I first introduced io_uring, it was not compiled with the `multi_core` kernel feature, mainly to make development easier (I thought). However, since io_uring allows multiple simultaneous system calls, we cannot longer make the in-kernel contexts block, for example when receiving a message from a pipe, if there can be multiple such requests simultaneously. This has required me to change WaitCondition into allowing multiple simultaneous tasks; although, it introduces a potential race condition: since a future can only return Pending and not block directly before releasing the lock (condvar logic), we need some way to make sure that nothing happens after the context finds out that it has to wait, and the actual waiting. If a message is pushed in between, and the waker is called (Context::unblock), just before it was going to block itself, then we miss the message, and potentially cause a deadlock. Fortunately, in order to block and unblock contexts, we need to exclusively lock the context. So, what we can do to ensure that waking while running is no longer a no-op, is to introduce a "wake flag", which is set only if the context is currently running, and Runnable. But, this still caused all weird kinds of hard-to-debug problems, with arbitrary CPU exceptions and possibly memory corruption. The reason for this, is that the context switching logic uses really unsafe operations, which is why context switching (at the moment) requires an exclusive lock. Before this commit, it would modify the `running` field after the lock had been released, which obviously can cause a data race, when the regular context waker code that is run within a system call, locks the context but not the global switching lock. The solution was to make sure that the locks were held, all the way until the actual switching, which was done in assembly. There can still be a race condition here, since it modifies memory containing registers after the lock has been released, even if it may be behind &mut on another context, which can be UB, but it has not contributed to any actual bugs... yet. * I have not yet done that rigorous testing, but it appears to work well enough, and I have not encountered the bug after like 10 tries.
kernel
Redox OS Microkernel
Debugging the redox kernel
Running qemu with the -s flag will set up qemu to listen on port 1234 for
a gdb client to connect to it. To debug the redox kernel run.
make qemu gdb=yes
This will start a VM with and listen on port 1234 for a gdb or lldb client.
gdb
If you are going to use gdb, run the following to load debug symbols and connect to your running kernel.
(gdb) symbol-file build/kernel.sym
(gdb) target remote localhost:1234
lldb
If you are going to use lldb, run the following to start debugging.
(lldb) target create -s build/kernel.sym build/kernel
(lldb) gdb-remote localhost:1234
Debugging
After connecting to your kernel you can set some interesting breakpoints and continue
the process. See your debuggers man page for more information on useful commands to run.