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    Subject[PATCH 5.15 813/917] mm/zsmalloc.c: close race window between zs_pool_dec_isolated() and zs_unregister_migration()
    Date
    From: Miaohe Lin <linmiaohe@huawei.com>

    [ Upstream commit afe8605ca45424629fdddfd85984b442c763dc47 ]

    There is one possible race window between zs_pool_dec_isolated() and
    zs_unregister_migration() because wait_for_isolated_drain() checks the
    isolated count without holding class->lock and there is no order inside
    zs_pool_dec_isolated(). Thus the below race window could be possible:

    zs_pool_dec_isolated zs_unregister_migration
    check pool->destroying != 0
    pool->destroying = true;
    smp_mb();
    wait_for_isolated_drain()
    wait for pool->isolated_pages == 0
    atomic_long_dec(&pool->isolated_pages);
    atomic_long_read(&pool->isolated_pages) == 0

    Since we observe the pool->destroying (false) before atomic_long_dec()
    for pool->isolated_pages, waking pool->migration_wait up is missed.

    Fix this by ensure checking pool->destroying happens after the
    atomic_long_dec(&pool->isolated_pages).

    Link: https://lkml.kernel.org/r/20210708115027.7557-1-linmiaohe@huawei.com
    Fixes: 701d678599d0 ("mm/zsmalloc.c: fix race condition in zs_destroy_pool")
    Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
    Cc: Minchan Kim <minchan@kernel.org>
    Cc: Sergey Senozhatsky <senozhatsky@chromium.org>
    Cc: Henry Burns <henryburns@google.com>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
    Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    mm/zsmalloc.c | 7 ++++---
    1 file changed, 4 insertions(+), 3 deletions(-)

    diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
    index 68e8831068f4b..b897ce3b399a1 100644
    --- a/mm/zsmalloc.c
    +++ b/mm/zsmalloc.c
    @@ -1830,10 +1830,11 @@ static inline void zs_pool_dec_isolated(struct zs_pool *pool)
    VM_BUG_ON(atomic_long_read(&pool->isolated_pages) <= 0);
    atomic_long_dec(&pool->isolated_pages);
    /*
    - * There's no possibility of racing, since wait_for_isolated_drain()
    - * checks the isolated count under &class->lock after enqueuing
    - * on migration_wait.
    + * Checking pool->destroying must happen after atomic_long_dec()
    + * for pool->isolated_pages above. Paired with the smp_mb() in
    + * zs_unregister_migration().
    */
    + smp_mb__after_atomic();
    if (atomic_long_read(&pool->isolated_pages) == 0 && pool->destroying)
    wake_up_all(&pool->migration_wait);
    }
    --
    2.33.0


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