linux/include
Linus Torvalds 4f988f152e seqlock: add 'raw_seqcount_begin()' function
The normal read_seqcount_begin() function will wait for any current
writers to exit their critical region by looping until the sequence
count is even.

That "wait for sequence count to stabilize" is the right thing to do if
the read-locker will just retry the whole operation on contention: no
point in doing a potentially expensive reader sequence if we know at the
beginning that we'll just end up re-doing it all.

HOWEVER.  Some users don't actually retry the operation, but instead
will abort and do the operation with proper locking.  So the sequence
count case may be the optimistic quick case, but in the presense of
writers you may want to do full locking in order to guarantee forward
progress.  The prime example of this would be the RCU name lookup.

And in that case, you may well be better off without the "retry early",
and are in a rush to instead get to the failure handling.  Thus this
"raw" interface that just returns the sequence number without testing it
- it just forces the low bit to zero so that read_seqcount_retry() will
always fail such a "active concurrent writer" scenario.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
2012-05-04 15:13:54 -07:00
..
acpi Merge branch 'release' of git://git.kernel.org/pub/scm/linux/kernel/git/lenb/linux 2012-03-30 16:45:39 -07:00
asm-generic asm-generic: Allow overriding clock_t and add attributes to siginfo_t 2012-04-23 16:29:18 -07:00
crypto crypto: user - Fix lookup of algorithms with IV generator 2012-03-29 19:52:47 +08:00
drm Merge branch 'exynos-drm-fixes' of git://git.infradead.org/users/kmpark/linux-samsung into drm-intel-fixes 2012-04-12 17:42:01 +01:00
keys
linux seqlock: add 'raw_seqcount_begin()' function 2012-05-04 15:13:54 -07:00
math-emu
media The following text was taken from the original review request: 2012-03-24 10:41:37 -07:00
misc
mtd
net Merge branch 'master' of git://git.kernel.org/pub/scm/linux/kernel/git/linville/wireless into for-davem 2012-05-01 14:14:05 -04:00
pcmcia
rdma Merge branches 'cma', 'cxgb3', 'cxgb4', 'ehca', 'iser', 'mad', 'nes', 'qib', 'srp' and 'srpt' into for-next 2012-03-19 09:50:33 -07:00
rxrpc
scsi [SCSI] libsas, libata: fix start of life for a sas ata_port 2012-04-23 12:11:47 +01:00
sound ALSA: hda - hide HDMI/ELD printks unless snd.debug=2 2012-04-10 14:53:55 +02:00
target target: Use array_zalloc for device_list 2012-03-15 19:15:51 -07:00
trace Merge branch 'perf-urgent-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip 2012-04-04 10:04:42 -07:00
video Merge branch 'platforms' of git://git.linaro.org/people/rmk/linux-arm 2012-03-27 18:17:02 -07:00
xen Merge branch 'for-linus' of git://git.linaro.org/people/mszyprowski/linux-dma-mapping 2012-04-04 17:13:43 -07:00
Kbuild