Re: [PATCH] ext4: fix deadlock allocating bio_post_read_ctx from mempool

["Theodore Y. Ts'o" <tytso@mit.edu>]

On Tue, Dec 31, 2019 at 12:12:22PM -0600, Eric Biggers wrote:
> From: Eric Biggers <ebiggers@google.com>
> 
> 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>

Thanks, applied.

						- Ted