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    SubjectRe: [patch V2 7/7] fs/jbd2: Free journal head outside of locked region
    On Thu 01-08-19 03:01:33, Thomas Gleixner wrote:
    > On PREEMPT_RT bit-spinlocks have the same semantics as on PREEMPT_RT=n,
    > i.e. they disable preemption. That means functions which are not safe to be
    > called in preempt disabled context on RT trigger a might_sleep() assert.
    >
    > The journal head bit spinlock is mostly held for short code sequences with
    > trivial RT safe functionality, except for one place:
    >
    > jbd2_journal_put_journal_head() invokes __journal_remove_journal_head()
    > with the journal head bit spinlock held. __journal_remove_journal_head()
    > invokes kmem_cache_free() which must not be called with preemption disabled
    > on RT.
    >
    > Jan suggested to rework the removal function so the actual free happens
    > outside the bit-spinlocked region.
    >
    > Split it into two parts:
    >
    > - Do the sanity checks and the buffer head detach under the lock
    >
    > - Do the actual free after dropping the lock
    >
    > There is error case handling in the free part which needs to dereference
    > the b_size field of the now detached buffer head. Due to paranoia (caused
    > by ignorance) the size is retrieved in the detach function and handed into
    > the free function. Might be over-engineered, but better safe than sorry.
    >
    > This makes the journal head bit-spinlock usage RT compliant and also avoids
    > nested locking which is not covered by lockdep.
    >
    > Suggested-by: Jan Kara <jack@suse.com>
    > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    > Cc: linux-ext4@vger.kernel.org
    > Cc: "Theodore Ts'o" <tytso@mit.edu>
    > Cc: Jan Kara <jack@suse.com>

    Looks mostly good. Just a small suggestion for simplification below:

    > @@ -2559,11 +2568,14 @@ void jbd2_journal_put_journal_head(struc
    > J_ASSERT_JH(jh, jh->b_jcount > 0);
    > --jh->b_jcount;
    > if (!jh->b_jcount) {
    > - __journal_remove_journal_head(bh);
    > + size_t b_size = __journal_remove_journal_head(bh);
    > +
    > jbd_unlock_bh_journal_head(bh);
    > + journal_release_journal_head(jh, b_size);
    > __brelse(bh);

    The bh is pinned until you call __brelse(bh) above and bh->b_size doesn't
    change during the lifetime of the buffer. So there's no need of
    fetching bh->b_size in __journal_remove_journal_head() and passing it back.
    You can just:

    journal_release_journal_head(jh, bh->b_size);

    > - } else
    > + } else {
    > jbd_unlock_bh_journal_head(bh);
    > + }
    > }
    >

    Honza
    --
    Jan Kara <jack@suse.com>
    SUSE Labs, CR

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