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Revision 4f988f152ee087831ea5c1c77cda4454cacc052c authored by Linus Torvalds on 04 May 2012, 22:13:54 UTC, committed by Linus Torvalds on 04 May 2012, 22:13:54 UTC 
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>
1 parent 2f62427
Raw File
coccicheck 
#!/bin/sh

SPATCH="`which ${SPATCH:=spatch}`"

if [ "$C" = "1" -o "$C" = "2" ]; then
    ONLINE=1

# This requires Coccinelle >= 0.2.3
#    FLAGS="-ignore_unknown_options -very_quiet"
#    OPTIONS=$*

# Workaround for Coccinelle < 0.2.3
	FLAGS="-I $srctree/include -very_quiet"
	shift $(( $# - 1 ))
	OPTIONS=$1
else
    ONLINE=0
    FLAGS="-very_quiet"
    if [ "$KBUILD_EXTMOD" = "" ] ; then
        OPTIONS="-dir $srctree"
    else
        OPTIONS="-dir $KBUILD_EXTMOD -patch $srctree -I $srctree/include -I $KBUILD_EXTMOD/include"
    fi
fi

if [ ! -x "$SPATCH" ]; then
    echo 'spatch is part of the Coccinelle project and is available at http://coccinelle.lip6.fr/'
    exit 1
fi

if [ "$MODE" = "" ] ; then
    if [ "$ONLINE" = "0" ] ; then
	echo 'You have not explicitly specified the mode to use. Using default "chain" mode.'
	echo 'All available modes will be tried (in that order): patch, report, context, org'
	echo 'You can specify the mode with "make coccicheck MODE=<mode>"'
    fi
    MODE="chain"
elif [ "$MODE" = "report" -o "$MODE" = "org" ] ; then
    FLAGS="$FLAGS -no_show_diff"
fi

if [ "$ONLINE" = "0" ] ; then
    echo ''
    echo 'Please check for false positives in the output before submitting a patch.'
    echo 'When using "patch" mode, carefully review the patch before submitting it.'
    echo ''
fi

coccinelle () {
    COCCI="$1"

    OPT=`grep "Option" $COCCI | cut -d':' -f2`

#   The option '-parse_cocci' can be used to syntactically check the SmPL files.
#
#    $SPATCH -D $MODE $FLAGS -parse_cocci $COCCI $OPT > /dev/null

    if [ "$ONLINE" = "0" ] ; then

	FILE=`echo $COCCI | sed "s|$srctree/||"`

	echo "Processing `basename $COCCI`"
	echo "with option(s) \"$OPT\""
	echo ''
	echo 'Message example to submit a patch:'

	sed -ne 's|^///||p' $COCCI

	if [ "$MODE" = "patch" ] ; then
	    echo ' The semantic patch that makes this change is available'
	elif [ "$MODE" = "report" ] ; then
	    echo ' The semantic patch that makes this report is available'
	elif [ "$MODE" = "context" ] ; then
	    echo ' The semantic patch that spots this code is available'
	elif [ "$MODE" = "org" ] ; then
	    echo ' The semantic patch that makes this Org report is available'
	else
	    echo ' The semantic patch that makes this output is available'
	fi
	echo " in $FILE."
	echo ''
	echo ' More information about semantic patching is available at'
	echo ' http://coccinelle.lip6.fr/'
	echo ''

	if [ "`sed -ne 's|^//#||p' $COCCI`" ] ; then
	    echo 'Semantic patch information:'
	    sed -ne 's|^//#||p' $COCCI
	    echo ''
	fi
    fi

    if [ "$MODE" = "chain" ] ; then
	$SPATCH -D patch   $FLAGS -sp_file $COCCI $OPT $OPTIONS               || \
	$SPATCH -D report  $FLAGS -sp_file $COCCI $OPT $OPTIONS -no_show_diff || \
	$SPATCH -D context $FLAGS -sp_file $COCCI $OPT $OPTIONS               || \
	$SPATCH -D org     $FLAGS -sp_file $COCCI $OPT $OPTIONS -no_show_diff || exit 1
    else
	$SPATCH -D $MODE   $FLAGS -sp_file $COCCI $OPT $OPTIONS || exit 1
    fi

}

if [ "$COCCI" = "" ] ; then
    for f in `find $srctree/scripts/coccinelle/ -name '*.cocci' -type f | sort`; do
	coccinelle $f
    done
else
    coccinelle $COCCI
fi
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