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fix the broken lockdep logic in __sb_start_write()

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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 can't 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().

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Jan Kara <jack@suse.com>
This commit is contained in:
Oleg Nesterov 2015-07-20 00:50:55 +02:00
parent bee9182d95
commit f4b554af99
1 changed files with 40 additions and 33 deletions
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73
fs/super.c
View File

@ -1158,38 +1158,11 @@ void __sb_end_write(struct super_block *sb, int level)
} }
EXPORT_SYMBOL(__sb_end_write); EXPORT_SYMBOL(__sb_end_write);
#ifdef CONFIG_LOCKDEP static int do_sb_start_write(struct super_block *sb, int level, bool wait,
/*
* 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,
unsigned long ip) unsigned long ip)
{ {
int i; if (wait)
rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, 0, ip);
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)
{
retry: retry:
if (unlikely(sb->s_writers.frozen >= level)) { if (unlikely(sb->s_writers.frozen >= level)) {
if (!wait) if (!wait)
@ -1198,9 +1171,6 @@ retry:
sb->s_writers.frozen < level); 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]); percpu_counter_inc(&sb->s_writers.counter[level-1]);
/* /*
* Make sure counter is updated before we check for frozen. * Make sure counter is updated before we check for frozen.
@ -1211,8 +1181,45 @@ retry:
__sb_end_write(sb, level); __sb_end_write(sb, level);
goto retry; goto retry;
} }
if (!wait)
rwsem_acquire_read(&sb->s_writers.lock_map[level-1], 0, 1, ip);
return 1; 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); EXPORT_SYMBOL(__sb_start_write);
/** /**