CVE-2024-42149: fs: don't misleadingly warn during thaw operations

In the Linux kernel, the following vulnerability has been resolved:

fs: don't misleadingly warn during thaw operations

The block device may have been frozen before it was claimed by a filesystem. Concurrently another process might try to mount that frozen block device and has temporarily claimed the block device for that purpose causing a concurrent fsbdevthaw() to end up here. The mounter is already about to abort mounting because they still saw an elevanted bdev->bdfsfreezecount so getbdevsuper() will return NULL in that case.

For example, P1 calls dmsuspend() which calls into bdevfreeze() before the block device has been claimed by the filesystem. This brings bdev->bdfsfreezecount to 1 and no call into fsbdevfreeze() is required.

Now P2 tries to mount that frozen block device. It claims it and checks bdev->bdfsfreezecount. As it's elevated it aborts mounting.

In the meantime P3 called dmresume(). P3 sees that the block device is already claimed by a filesystem and calls into fsbdevthaw().

P3 takes a passive reference and realizes that the filesystem isn't ready yet. P3 puts itself to sleep to wait for the filesystem to become ready.

P2 now puts the last active reference to the filesystem and marks it as dying. P3 gets woken, sees that the filesystem is dying and getbdevsuper() fails.