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Date: Tue, 21 Jul 2015 10:38:42 +0200
From: Jan Kara <jack@...e.cz>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Al Viro <viro@...iv.linux.org.uk>,
Dave Chinner <david@...morbit.com>,
Dave Hansen <dave.hansen@...ux.intel.com>,
Jan Kara <jack@...e.cz>, linux-fsdevel@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/4] fix the broken lockdep logic in __sb_start_write()
On Mon 20-07-15 19:01:03, Oleg Nesterov wrote:
> 1. wait_event(frozen < level) without rwsem_acquire_read() is just
> wrong from lockdep perspective. If we are going to deadlock
> because the caller is buggy, lockdep detect this problem.
>
> 2. __sb_start_write() can race with thaw_super() + freeze_super(),
> and after "goto retry" the 2nd acquire_freeze_lock() is wrong.
>
> 3. The "tell lockdep we are doing trylock" hack doesn't look nice.
>
> I think this is correct, but this logic should be more explicit.
> Yes, the recursive read_lock() is fine if we hold the lock on a
> higher level. But we do not need to fool lockdep. If we can not
> deadlock in this case then try-lock must not fail and we can use
> use wait == F throughout this code.
>
> Note: as Dave Chinner explains, the "trylock" hack and the fat comment
> can be probably removed. But this needs a separate change and it will
> be trivial: just kill __sb_start_write() and rename do_sb_start_write()
> back to __sb_start_write().
The patch looks good. Did you test this BTW? You can add:
Reviewed-by: Jan Kara <jack@...e.com>
Honza
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>
> ---
> fs/super.c | 73 ++++++++++++++++++++++++++++++++---------------------------
> 1 files changed, 40 insertions(+), 33 deletions(-)
>
> diff --git a/fs/super.c b/fs/super.c
> index 928c20f..d0fdd49 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1158,38 +1158,11 @@ void __sb_end_write(struct super_block *sb, int level)
> }
> EXPORT_SYMBOL(__sb_end_write);
>
> -#ifdef CONFIG_LOCKDEP
> -/*
> - * We want lockdep to tell us about possible deadlocks with freezing but
> - * it's it bit tricky to properly instrument it. Getting a freeze protection
> - * works as getting a read lock but there are subtle problems. XFS for example
> - * gets freeze protection on internal level twice in some cases, which is OK
> - * only because we already hold a freeze protection also on higher level. Due
> - * to these cases we have to tell lockdep we are doing trylock when we
> - * already hold a freeze protection for a higher freeze level.
> - */
> -static void acquire_freeze_lock(struct super_block *sb, int level, bool trylock,
> +static int do_sb_start_write(struct super_block *sb, int level, bool wait,
> unsigned long ip)
> {
> - int i;
> -
> - if (!trylock) {
> - for (i = 0; i < level - 1; i++)
> - if (lock_is_held(&sb->s_writers.lock_map[i])) {
> - trylock = true;
> - break;
> - }
> - }
> - rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, trylock, ip);
> -}
> -#endif
> -
> -/*
> - * This is an internal function, please use sb_start_{write,pagefault,intwrite}
> - * instead.
> - */
> -int __sb_start_write(struct super_block *sb, int level, bool wait)
> -{
> + if (wait)
> + rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, 0, ip);
> retry:
> if (unlikely(sb->s_writers.frozen >= level)) {
> if (!wait)
> @@ -1198,9 +1171,6 @@ retry:
> sb->s_writers.frozen < level);
> }
>
> -#ifdef CONFIG_LOCKDEP
> - acquire_freeze_lock(sb, level, !wait, _RET_IP_);
> -#endif
> percpu_counter_inc(&sb->s_writers.counter[level-1]);
> /*
> * Make sure counter is updated before we check for frozen.
> @@ -1211,8 +1181,45 @@ retry:
> __sb_end_write(sb, level);
> goto retry;
> }
> +
> + if (!wait)
> + rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, 1, ip);
> return 1;
> }
> +
> +/*
> + * This is an internal function, please use sb_start_{write,pagefault,intwrite}
> + * instead.
> + */
> +int __sb_start_write(struct super_block *sb, int level, bool wait)
> +{
> + bool force_trylock = false;
> + int ret;
> +
> +#ifdef CONFIG_LOCKDEP
> + /*
> + * We want lockdep to tell us about possible deadlocks with freezing
> + * but it's it bit tricky to properly instrument it. Getting a freeze
> + * protection works as getting a read lock but there are subtle
> + * problems. XFS for example gets freeze protection on internal level
> + * twice in some cases, which is OK only because we already hold a
> + * freeze protection also on higher level. Due to these cases we have
> + * to use wait == F (trylock mode) which must not fail.
> + */
> + if (wait) {
> + int i;
> +
> + for (i = 0; i < level - 1; i++)
> + if (lock_is_held(&sb->s_writers.lock_map[i])) {
> + force_trylock = true;
> + break;
> + }
> + }
> +#endif
> + ret = do_sb_start_write(sb, level, wait && !force_trylock, _RET_IP_);
> + WARN_ON(force_trylock & !ret);
> + return ret;
> +}
> EXPORT_SYMBOL(__sb_start_write);
>
> /**
> --
> 1.5.5.1
>
--
Jan Kara <jack@...e.cz>
SUSE Labs, CR
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