anchao commented on PR #11177:
URL: https://github.com/apache/nuttx/pull/11177#issuecomment-1811808812
> This should be fine from the kernel mode point of view now. I'm just
wondering will we lose the benefit from your optimizations because of the
copying ?
This may depend on the application implement, In the previous
implementation, all file descriptors would be dup in files_duplist and closed
after nxtask_init in files_close_onexec.
```
exec_module
|
|->nxtask_init
| |
| ->group_setuptaskfiles
| |
| ->files_duplist /* all files will be duplicate from parent */
|
|->spawn_file_actions /* do file actions */
->files_close_onexec /* close exec files */
```
In the current implementation,
1. if the file actions has ACTION_CLOSE/OPEN operation, no needs to dup, and
also no needs to close;
2. if the file descriptor has the O_CLOEXEC flag, it will not perform
additional dup/close operations.
```
exec_module
|
->nxtask_init
|
->group_setuptaskfiles
|
->files_duplist /* pass the file actions to avoid */
|
|->spawn_file_is_duplicateable /* 1. SPAWN_FILE_ACTION_CLOSE: will
be close, do not dup (*)
| * 2. SPAWN_FILE_ACTION_OPEN: will
be open, no needs to dup (*)
| * 3. SPAWN_FILE_ACTION_DUP2: could
be dup
| */
->(filep->f_oflags & O_CLOEXEC) /* skip dup */
```
Of course, this improvement is obvious in flat build, but if it is in kernel
mode, it will depend on the ratio of action/cloexec in the file list.
```
exec_module
|
|->binfmt_copyactions
|
|->nxtask_init
| |
| ->group_setuptaskfiles
| |
| ->files_duplist /* pass the file actions to avoid */
| |
| |->spawn_file_is_duplicateable /* 1. SPAWN_FILE_ACTION_CLOSE: will
be close, do not dup (*)
| | * 2. SPAWN_FILE_ACTION_OPEN: will
be open, no needs to dup (*)
| | * 3. SPAWN_FILE_ACTION_DUP2: could
be dup
| | */
| ->(filep->f_oflags & O_CLOEXEC) /* skip dup */
|
->binfmt_freeactions
```
In addition, not re-dup the file in nxtask_init could also avoid potential
problems caused by some files/drivers that do not support multiple instances.
If the driver sets the O_CLOEXEC flag after open, but the driver will be dup
multiple times after exec, Then it is also unfriendly to the driver developer.
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