Messages in this thread | | | Date | Wed, 11 Sep 2013 12:33:35 -0400 | From | Waiman Long <> | Subject | Re: [PATCH 1/2] seqlock: Add a new blocking reader type |
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On 09/11/2013 10:55 AM, Al Viro wrote: > On Wed, Sep 11, 2013 at 10:28:26AM -0400, Waiman Long wrote: >> The sequence lock (seqlock) was originally designed for the cases >> where the readers do not need to block the writers by making the >> readers retry the read operation when the data change. >> >> Since then, the use cases have been expanded to include situations >> where a thread does not need to change the data (effectively a reader) >> at all but have to take the writer lock because it can't tolerate >> changes to the protected structure. Some examples are the d_path() >> function and the getcwd() syscall in fs/dcache.c where the functions >> take the writer lock on rename_lock even though they don't need >> to change anything in the protected data structure at all. This is >> inefficient as a reader is now blocking other non-blocking readers >> by pretending to be a writer. >> >> This patch tries to eliminate this inefficiency by introducing a new >> type of blocking reader to the seqlock locking mechanism. This new >> blocking reader will not block other non-blocking readers, but will >> block other blocking readers and writers. > Umm... That's misleading - it doesn't _block_, it spins. Moroever, > seq_readbegin() also spins in presense of writer; the main property > of this one is that it keeps writers away.
I used "block" in the sense that it will stop a writer from moving forward. I will update the commit log to make that more clear. > Folks, any suggestions on better names? The semantics we are getting is
I will welcome any better name suggestion and will incorporate that in the patch.
-Longman
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