| Message ID | 2eaf00e5-df91-a0be-8d00-801fb1823f7b@i-love.sakura.ne.jp (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | [v2,(resend)] fput: Allow calling __fput_sync() from !PF_KTHREAD thread. | expand |
Ping? On 2020/08/19 21:42, Tetsuo Handa wrote: > __fput_sync() was introduced by commit 4a9d4b024a3102fc ("switch fput to > task_work_add") with BUG_ON(!(current->flags & PF_KTHREAD)) check, and > the only user of __fput_sync() was introduced by commit 17c0a5aaffa63da6 > ("make acct_kill() wait for file closing."). However, the latter commit is > effectively calling __fput_sync() from !PF_KTHREAD thread because of > schedule_work() call followed by immediate wait_for_completion() call. > That is, there is no need to defer close_work() to a WQ context. I guess > that the reason to defer was nothing but to bypass this BUG_ON() check. > While we need to remain careful about calling __fput_sync(), we can remove > bypassable BUG_ON() check from __fput_sync(). > > If this change is accepted, racy fput()+flush_delayed_fput() introduced > by commit e2dc9bf3f5275ca3 ("umd: Transform fork_usermode_blob into > fork_usermode_driver") will be replaced by this raceless __fput_sync(). > > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> > --- > fs/file_table.c | 15 +++++---------- > 1 file changed, 5 insertions(+), 10 deletions(-) > > diff --git a/fs/file_table.c b/fs/file_table.c > index 656647f..7c41251 100644 > --- a/fs/file_table.c > +++ b/fs/file_table.c > @@ -359,20 +359,15 @@ void fput(struct file *file) > } > > /* > - * synchronous analog of fput(); for kernel threads that might be needed > - * in some umount() (and thus can't use flush_delayed_fput() without > - * risking deadlocks), need to wait for completion of __fput() and know > - * for this specific struct file it won't involve anything that would > - * need them. Use only if you really need it - at the very least, > - * don't blindly convert fput() by kernel thread to that. > + * synchronous analog of fput(); for threads that need to wait for completion > + * of __fput() and know for this specific struct file it won't involve anything > + * that would need them. Use only if you really need it - at the very least, > + * don't blindly convert fput() to __fput_sync(). > */ > void __fput_sync(struct file *file) > { > - if (atomic_long_dec_and_test(&file->f_count)) { > - struct task_struct *task = current; > - BUG_ON(!(task->flags & PF_KTHREAD)); > + if (atomic_long_dec_and_test(&file->f_count)) > __fput(file); > - } > } > > EXPORT_SYMBOL(fput); >
diff --git a/fs/file_table.c b/fs/file_table.c index 656647f..7c41251 100644 --- a/fs/file_table.c +++ b/fs/file_table.c @@ -359,20 +359,15 @@ void fput(struct file *file) } /* - * synchronous analog of fput(); for kernel threads that might be needed - * in some umount() (and thus can't use flush_delayed_fput() without - * risking deadlocks), need to wait for completion of __fput() and know - * for this specific struct file it won't involve anything that would - * need them. Use only if you really need it - at the very least, - * don't blindly convert fput() by kernel thread to that. + * synchronous analog of fput(); for threads that need to wait for completion + * of __fput() and know for this specific struct file it won't involve anything + * that would need them. Use only if you really need it - at the very least, + * don't blindly convert fput() to __fput_sync(). */ void __fput_sync(struct file *file) { - if (atomic_long_dec_and_test(&file->f_count)) { - struct task_struct *task = current; - BUG_ON(!(task->flags & PF_KTHREAD)); + if (atomic_long_dec_and_test(&file->f_count)) __fput(file); - } } EXPORT_SYMBOL(fput);
__fput_sync() was introduced by commit 4a9d4b024a3102fc ("switch fput to task_work_add") with BUG_ON(!(current->flags & PF_KTHREAD)) check, and the only user of __fput_sync() was introduced by commit 17c0a5aaffa63da6 ("make acct_kill() wait for file closing."). However, the latter commit is effectively calling __fput_sync() from !PF_KTHREAD thread because of schedule_work() call followed by immediate wait_for_completion() call. That is, there is no need to defer close_work() to a WQ context. I guess that the reason to defer was nothing but to bypass this BUG_ON() check. While we need to remain careful about calling __fput_sync(), we can remove bypassable BUG_ON() check from __fput_sync(). If this change is accepted, racy fput()+flush_delayed_fput() introduced by commit e2dc9bf3f5275ca3 ("umd: Transform fork_usermode_blob into fork_usermode_driver") will be replaced by this raceless __fput_sync(). Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp> --- fs/file_table.c | 15 +++++---------- 1 file changed, 5 insertions(+), 10 deletions(-)