xfs: fix buffer lookup vs release race

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Since commit 298f342245 ("xfs: lockless buffer lookup") the buffer
lookup fastpath is done without a hash-wide lock (then pag_buf_lock, now
bc_lock) and only under RCU protection.  But this means that nothing
serializes lookups against the temporary 0 reference count for buffers
that are added to the LRU after dropping the last regular reference,
and a concurrent lookup would fail to find them.

Fix this by doing all b_hold modifications under b_lock.  We're already
doing this for release so this "only" ~ doubles the b_lock round trips.
We'll later look into the lockref infrastructure to optimize the number
of lock round trips again.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Carlos Maiolino <cem@kernel.org>

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This commit is contained in:

Christoph Hellwig 2025-01-16 07:01:42 +01:00 • committed by Carlos Maiolino

parent 07eae0fa67

commit ee10f6fcdb

3 changed files with 54 additions and 51 deletions

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93

fs/xfs/xfs_buf.c

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2

fs/xfs/xfs_buf.h

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10

fs/xfs/xfs_trace.h

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