[ Upstream commit 45c222468d33202c07c41c113301a4b9c8451b8f ]
After setting the BTRFS_ROOT_FORCE_COW flag on the root we are doing a
full write barrier, smp_wmb(), but we don't need to, all we need is a
smp_mb__after_atomic(). The use of the smp_wmb() is from the old days
when we didn't use a bit and used instead an int field in the root to
signal if cow is forced. After the int field was changed to a bit in
the root's state (flags field), we forgot to update the memory barrier
in create_pending_snapshot() to smp_mb__after_atomic(), but we did the
change in commit_fs_roots() after clearing BTRFS_ROOT_FORCE_COW. That
happened in commit 27cdeb7096 ("Btrfs: use bitfield instead of integer
data type for the some variants in btrfs_root"). On the reader side, in
should_cow_block(), we also use the counterpart smp_mb__before_atomic()
which generates further confusion.
So change the smp_wmb() to smp_mb__after_atomic(). In fact we don't
even need any barrier at all since create_pending_snapshot() is called
in the critical section of a transaction commit and therefore no one
can concurrently join/attach the transaction, or start a new one, until
the transaction is unblocked. By the time someone starts a new transaction
and enters should_cow_block(), a lot of implicit memory barriers already
took place by having acquired several locks such as fs_info->trans_lock
and extent buffer locks on the root node at least. Nevertlheless, for
consistency use smp_mb__after_atomic() after setting the force cow bit
in create_pending_snapshot().
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.
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