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![Eric Biggers](https://secure.gravatar.com/avatar/a115d165abc47f88e8112f94bc25b6bd.jpg?s=96&d=mm)
Eric Biggers committed 2d7fa7564bc
ext4: fix deadlock allocating bio_post_read_ctx from mempool [ Upstream commit 68e45330e341dad2d3a0a3f8ef2ec46a2a0a3bbc ] Without any form of coordination, any case where multiple allocations from the same mempool are needed at a time to make forward progress can deadlock under memory pressure. This is the case for struct bio_post_read_ctx, as one can be allocated to decrypt a Merkle tree page during fsverity_verify_bio(), which itself is running from a post-read callback for a data bio which has its own struct bio_post_read_ctx. Fix this by freeing the first bio_post_read_ctx before calling fsverity_verify_bio(). This works because verity (if enabled) is always the last post-read step. This deadlock can be reproduced by trying to read from an encrypted verity file after reducing NUM_PREALLOC_POST_READ_CTXS to 1 and patching mempool_alloc() to pretend that pool->alloc() always fails. Note that since NUM_PREALLOC_POST_READ_CTXS is actually 128, to actually hit this bug in practice would require reading from lots of encrypted verity files at the same time. But it's theoretically possible, as N available objects isn't enough to guarantee forward progress when > N/2 threads each need 2 objects at a time. Fixes: 22cfe4b48ccb ("ext4: add fs-verity read support") Signed-off-by: Eric Biggers <ebiggers@google.com> Link: https://lore.kernel.org/r/20191231181222.47684-1-ebiggers@kernel.org Signed-off-by: Theodore Ts'o <tytso@mit.edu> Signed-off-by: Sasha Levin <sashal@kernel.org>