164c246571
Implement the wait queue cleanup algorithm for rqspinlock. There are three forms of waiters in the original queued spin lock algorithm. The first is the waiter which acquires the pending bit and spins on the lock word without forming a wait queue. The second is the head waiter that is the first waiter heading the wait queue. The third form is of all the non-head waiters queued behind the head, waiting to be signalled through their MCS node to overtake the responsibility of the head. In this commit, we are concerned with the second and third kind. First, we augment the waiting loop of the head of the wait queue with a timeout. When this timeout happens, all waiters part of the wait queue will abort their lock acquisition attempts. This happens in three steps. First, the head breaks out of its loop waiting for pending and locked bits to turn to 0, and non-head waiters break out of their MCS node spin (more on that later). Next, every waiter (head or non-head) attempts to check whether they are also the tail waiter, in such a case they attempt to zero out the tail word and allow a new queue to be built up for this lock. If they succeed, they have no one to signal next in the queue to stop spinning. Otherwise, they signal the MCS node of the next waiter to break out of its spin and try resetting the tail word back to 0. This goes on until the tail waiter is found. In case of races, the new tail will be responsible for performing the same task, as the old tail will then fail to reset the tail word and wait for its next pointer to be updated before it signals the new tail to do the same. We terminate the whole wait queue because of two main reasons. Firstly, we eschew per-waiter timeouts with one applied at the head of the wait queue. This allows everyone to break out faster once we've seen the owner / pending waiter not responding for the timeout duration from the head. Secondly, it avoids complicated synchronization, because when not leaving in FIFO order, prev's next pointer needs to be fixed up etc. Lastly, all of these waiters release the rqnode and return to the caller. This patch underscores the point that rqspinlock's timeout does not apply to each waiter individually, and cannot be relied upon as an upper bound. It is possible for the rqspinlock waiters to return early from a failed lock acquisition attempt as soon as stalls are detected. The head waiter cannot directly WRITE_ONCE the tail to zero, as it may race with a concurrent xchg and a non-head waiter linking its MCS node to the head's MCS node through 'prev->next' assignment. One notable thing is that we must use RES_DEF_TIMEOUT * 2 as our maximum duration for the waiting loop (for the wait queue head), since we may have both the owner and pending bit waiter ahead of us, and in the worst case, need to span their maximum permitted critical section lengths. Reviewed-by: Barret Rhoden <brho@google.com> Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com> Link: https://lore.kernel.org/r/20250316040541.108729-11-memxor@gmail.com Signed-off-by: Alexei Starovoitov <ast@kernel.org> |
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| LICENSES | ||
| arch | ||
| block | ||
| certs | ||
| crypto | ||
| drivers | ||
| fs | ||
| include | ||
| init | ||
| io_uring | ||
| ipc | ||
| kernel | ||
| lib | ||
| mm | ||
| net | ||
| rust | ||
| samples | ||
| scripts | ||
| security | ||
| sound | ||
| tools | ||
| usr | ||
| virt | ||
| .clang-format | ||
| .clippy.toml | ||
| .cocciconfig | ||
| .editorconfig | ||
| .get_maintainer.ignore | ||
| .gitattributes | ||
| .gitignore | ||
| .mailmap | ||
| .rustfmt.toml | ||
| COPYING | ||
| CREDITS | ||
| Kbuild | ||
| Kconfig | ||
| MAINTAINERS | ||
| Makefile | ||
| README | ||
README
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.