Sorry for the delay, but I don't receive email notification of new top-level comments, and so I just now saw your comment.
Your suggestion is quite reasonable, and perhaps someday something similar will be implemented. However, none of the people using or thinking of using SRCU (at least none that I am aware of) need more than one callback per thread. One way that they can get the effect with the current API is to remove a number of items in one pass, use one synchronize_srcu() to wait, then free up the items.
I did submit a patch in late 2006 that reduced the number of synchronize_sched() calls in synchronize_srcu() at the expense of barriers on the read side: http://lkml.org/lkml/2006/11/17/359, but the speedup was insufficient, so Oleg Nesterov did QRCU instead. :-)
I would expect that SRCU might get more attention as it becomes more heavily used, and people have additional things that they need it to do. RCU has always been very usage-driven: http://doi.acm.org/10.1145/1400097.1400099
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