Re: [PATCH v2] mm: zswap: fix crypto_free_acomp() deadlock in zswap_cpu_comp_dead()

[Nhat Pham]

On Wed, Feb 26, 2025 at 10:56 AM Yosry Ahmed <yosry.ahmed@xxxxxxxxx> wrote:
>
> Currently, zswap_cpu_comp_dead() calls crypto_free_acomp() while holding
> the per-CPU acomp_ctx mutex. crypto_free_acomp() then holds scomp_lock
> (through crypto_exit_scomp_ops_async()).
>
> On the other hand, crypto_alloc_acomp_node() holds the scomp_lock
> (through crypto_scomp_init_tfm()), and then allocates memory.
> If the allocation results in reclaim, we may attempt to hold the per-CPU
> acomp_ctx mutex.
>
> The above dependencies can cause an ABBA deadlock. For example in the
> following scenario:
>
> (1) Task A running on CPU #1:
>     crypto_alloc_acomp_node()
>       Holds scomp_lock
>       Enters reclaim
>       Reads per_cpu_ptr(pool->acomp_ctx, 1)
>
> (2) Task A is descheduled
>
> (3) CPU #1 goes offline
>     zswap_cpu_comp_dead(CPU #1)
>       Holds per_cpu_ptr(pool->acomp_ctx, 1))
>       Calls crypto_free_acomp()
>       Waits for scomp_lock
>
> (4) Task A running on CPU #2:
>       Waits for per_cpu_ptr(pool->acomp_ctx, 1) // Read on CPU #1
>       DEADLOCK

Lolll I was scratching my head with this issue while stress-testing
some of my zswap patches. Beat me to it :)

>
> Since there is no requirement to call crypto_free_acomp() with the
> per-CPU acomp_ctx mutex held in zswap_cpu_comp_dead(), move it after the
> mutex is unlocked. Also move the acomp_request_free() and kfree() calls
> for consistency and to avoid any potential sublte locking dependencies
> in the future.
>
> With this, only setting acomp_ctx fields to NULL occurs with the mutex
> held. This is similar to how zswap_cpu_comp_prepare() only initializes
> acomp_ctx fields with the mutex held, after performing all allocations
> before holding the mutex.
>
> Opportunistically, move the NULL check on acomp_ctx so that it takes
> place before the mutex dereference.
>
> Fixes: 12dcb0ef5406 ("mm: zswap: properly synchronize freeing resources during CPU hotunplug")
> Reported-by: syzbot+1a517ccfcbc6a7ab0f82@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/67bcea51.050a0220.bbfd1.0096.GAE@xxxxxxxxxx/
> Cc: <stable@xxxxxxxxxxxxxxx>
> Co-developed-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
> Acked-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>

LGTM! Thanks for fixing it.
Reviewed-by: Nhat Pham <nphamcs@xxxxxxxxx>