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We have currently two global locks, a rwlock and a rwsem, that are used for managing linking the PCM streams. Due to these global locks, once when a linked stream is used, the lock granularity suffers a lot. This patch attempts to eliminate the former global lock for atomic ops. The latter rwsem needs remaining because of the loosy way of the loop calls in snd_pcm_action_nonatomic(), as well as for avoiding the deadlock at linking. However, these are used far rarely, actually only by two actions (prepare and reset), where both are no timing critical ones. So this can be still seen as a good improvement. The basic strategy to eliminate the rwlock is to assure group->lock at adding or removing a stream to / from the group. Since we already takes the group lock whenever taking the all substream locks under the group, this shouldn't be a big problem. The reference to group pointer in snd_pcm_substream object is protected by the stream lock itself. However, there are still pitfalls: a race window at re-locking and the lifecycle of group object. The former is a small race window for dereferencing the substream group object opened while snd_pcm_action() performs re-locking to avoid ABBA deadlocks. This includes the unlink of group during that window, too. And the latter is the kfree performed after all streams are removed from the group while it's still dereferenced. For addressing these corner cases, two new tricks are introduced: - After re-locking, the group assigned to the stream is checked again; if the group is changed, we retry the whole procedure. - Introduce a refcount to snd_pcm_group object, so that it's freed only when it's empty and really no one refers to it. (Some readers might wonder why not RCU for the latter. RCU in this case would cost more than refcounting, unfortunately. We take the group lock sooner or later, hence the performance improvement by RCU would be negligible. Meanwhile, because we need to deal with schedulable context depending on the pcm->nonatomic flag, it'll become dynamic RCU/SRCU switch, and the grace period may become too long.) Along with these changes, there are a significant amount of code refactoring. The complex group re-lock & ref code is factored out to snd_pcm_stream_group_ref() function, for example. Signed-off-by: Takashi Iwai <tiwai@suse.de> |
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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 Restructured Text 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.