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SubjectRe: [BUG] fs/dcache: might_sleep is called under a spinlock
Thanks for your detailed explanation :)
I will improve my static analysis tool.

Jia-Ju Bai

On 2017/10/3 11:19, Al Viro wrote:
> On Tue, Oct 03, 2017 at 10:38:25AM +0800, Jia-Ju Bai wrote:
>> According to fs/dcache.c, might_sleep is called under a spinlock,
>> and the function call path is:
>> d_prune_aliases (acquire the spinlock)
>> dput
>> might_sleep
>> This bug is found by my static analysis tool and my code review.
>> A possible fix is to remove might_sleep in dput.
> ... or to fix your static analysis tool. First of all, that call
> of dput() really *can* block and if we had inode->i_lock or dentry->d_lock
> still held at that point we'd have a real bug. However, __dentry_kill()
> there is called with dentry->d_inode == inode and inode->i_lock held,
> so dentry->d_inode is stable until inode->i_lock is dropped. Said
> __dentry_kill() contains
> if (dentry->d_inode)
> dentry_unlink_inode(dentry);
> with inode->i_lock held until that point. dentry_unlink_inode() starts
> with
> struct inode *inode = dentry->d_inode;
> bool hashed = !d_unhashed(dentry);
> if (hashed)
> raw_write_seqcount_begin(&dentry->d_seq);
> __d_clear_type_and_inode(dentry);
> hlist_del_init(&dentry->d_u.d_alias);
> if (hashed)
> raw_write_seqcount_end(&dentry->d_seq);
> spin_unlock(&dentry->d_lock);
> spin_unlock(&inode->i_lock);
> so
> 1) inode in there is guaranteed to be equal to the argument of
> d_prune_aliases() and
> 2) both dentry->d_lock and inode->i_lock are dropped before
> dentry_unlink_inode() returns. inode->i_lock is not regained in the
> rest of __dentry_kill(); dentry->d_lock is regained and dropped before
> __dentry_kill() returns. IOW, we are fine - dput() in d_prune_aliases()
> is called without any spinlocks held.
> That, BTW, is the reason for
> goto restart;
> in there, instead of just continuing the loop - if we get to that point,
> the list of aliases might have changed.
> Removing might_sleep() in dput() would've been wrong - it really might
> sleep when called from that point. Here's how: we used to have two
> links to the same file - foo/bar and baz/barf. baz/barf used to be
> opened, then rm -rf baz happened and later we'd called d_prune_aliases()
> on the inode of foo/bar. And as the loop had been executed on one CPU,
> on another the opened file got closed, dropping the last reference to
> dentry that used to be baz/barf. Note that its parent (the thing that
> used to be dentry of baz) is unhashed and the only contributor to its
> refcount is our dentry, so dput(parent) *does* drop the last remaining
> reference, triggering the final iput() on inode of baz, along with
> freeing on-disk inode, doing disk IO, etc.
> Again, it's not that we can't block in that dput() - it's that __dentry_kill()
> drops all spinlocks.

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