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block: fix sector comparism in multiwrite_req_compare 

The difference between the start sectors of two requests can be larger
than the size of the "int" type, which can lead to a not correctly
sorted multiwrite array and thus spurious I/O errors and filesystem
corruption due to incorrect request merges.

So instead of doing the cute sector arithmetics trick spell out the
exact comparisms.

Spotted by Kevin Wolf based on a testcase from Michael Tokarev.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Christoph Hellwig 2010-05-19 20:53:10 +02:00 committed by Kevin Wolf
parent 618fbb8429
commit 77be4366ba
1 changed files with 13 additions and 1 deletions
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14
block.c
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@ -1929,7 +1929,19 @@ static void multiwrite_cb(void *opaque, int ret)
static int multiwrite_req_compare(const void *a, const void *b)
{
return (((BlockRequest*) a)->sector - ((BlockRequest*) b)->sector);
const BlockRequest *req1 = a, *req2 = b;
/*
* Note that we can't simply subtract req2->sector from req1->sector
* here as that could overflow the return value.
*/
if (req1->sector > req2->sector) {
return 1;
} else if (req1->sector < req2->sector) {
return -1;
} else {
return 0;
}
}
/*